Download Flaxseed oil

Highlights
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Flax compounds; linatine, cyanogenic glycosides, Cd and mucilage are discussed.
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The difficulty in attributing bioactivity to flaxseed compounds is discussed.
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A systematic nomenclature of 21 flaxseed orbitides (cyclolinopeptides) is proposed.
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The biological activity and ribosomal origin of flaxseed orbitides are reviewed.
Flaxseed (Linum usitatissimum L.) is an oilseed used in industrial and natural health products.
Flaxseed accumulates many biologically active compounds and elements including linolenic
acid, linoleic acid, lignans, cyclic peptides, polysaccharides, alkaloids, cyanogenic
glycosides, and cadmium. Most biological and clinical studies of flaxseed have focused on
extracts containing α-linolenic acid or lignan. Other flaxseed compounds have received less
attention and their activity is not well described. The benefits of consumption of whole
flaxseed fractions such as oil, mucilage and protein indicate that consideration of the entire
portfolio of bioactives present is required to associate biological activity with specific
compounds.
Abbreviations














CLs, cyclolinopeptides;
ALA, α-linolenic acid;
TG, triglyceride;
LDL, low-density lipoprotein;
HDL, high-density lipoprotein;
FFAs, free fatty acids;
CsA, cyclosporine A;
MeOH, methanol;
HPLC, high-performance liquid chromatography;
NMR, nuclear magnetic resonance;
Met, methionine;
MetO, methionine S-oxide;
MetO2, methionine S,S-dioxide;
bw, body weight
Introduction
Flaxseed (Linum usitatissimum L.), one of the oldest cultivated crops, continues to be widely
grown for oil, fiber, and food ( Oomah, 2001). The average worldwide flaxseed production
between 2007 and 2011 was 1,862,449 tonnes (FAO, 2011). Flaxseed oil is an excellent
source of the omega-3 fatty acid linolenic acid with typical levels of 55% in the oil (Oomah,
2001) making it ideal for paints, varnishes, and inks due to its fast polymerization properties.
Increasing demand for edible oil sources with significant percentages of omega-3 fatty acids
is resulting in consumption of flaxseed as a functional food. Flaxseed is also added to animal
feed to improve animal reproductive performance and health ( Heimbach, 2009 and Turner
et al., 2014).
Mature flaxseed is oblong and flattened, comprising an embryo with two cotyledons
surrounded by a thin endosperm and a smooth, often shiny yellow to dark brown seed coat
(hull) (Fig. 1). The composition of flaxseed is presented in Table 1 (Hadley et al.,
1992 and Smith, 1958). Analysis of brown Canadian flaxseed conducted by the Canadian
Grain Commission showed the average composition of commercial seed was 41% fat, 20%
protein, 28% total dietary fiber, 7.7% moisture, and 3.4% ash (DeClercq, 2012). Minor
components included: cyanogenic glycosides, phytic acid, phenolics, trypsin inhibitor,
linatine, lignans (phytoestrogens), minerals, vitamins, cadmium, selenium and
cyclolinopeptides (CLs) (Bhatty, 1995 and Matsumoto et al., 2002) (Fig. 2).
Fig. 1.
Hand-cut sections of flaxseed (L. usitatissimum L., var. CDC Bethune) mounted in
distilled water showing anatomical structures. (A) The side of flaxseed (B) Hand-cut
section of flaxseed. Images were obtained (×1000 magnification) with a Canon Eos
300D digital camera mounted on a Zeiss Stemi SV 11 light microscope. The images
were subsequently processed in Photoshop 7.
Figure options
Table 1.
Flaxseed composition.
Cotyledons and embryo
Seed coata
Constituent (%) Whole seed
Moisture
Nitrogen
Oil
Fiber (soluble)
Fiber (insoluble)
Ash
Weight fraction
% of total oil
a
7.13
4.01
38.71
10.22
30.41
NRb
NRb
With fat
4.31
4.64
53.20
NRb
NRb
3.38
58.60
Without fat
NRb
10.92
NRb
NRb
NRb
7.95
40.40
96.70
With fat Without fat
7.89
NRb
3.18
3.52
1.84
NRb
NRb
NRb
NRb
NRb
2.99
3.31
41.40
59.96
3.30
Hull.
b
Not reported.
Data taken from Hadley et al., 1992 and Smith, 1958 and source papers cited therein.
Table options
Fig. 2.
Structures of four cyanogenic glycosides (mono-glycosides: linamarin, lotaustralin;
di-glycosides: linustatin and neolinustatin) and SDG in flaxseed.
Figure options
The reported protein content of flaxseed varies widely from 10 to 31% (Bajpai et al., 1985,
Morita et al., 1997, Oomah and Mazza, 1993a and Salunkhe and Desai, 1986).
Approximately 56–70% of the protein is found in cotyledons and about 30% in the seed coat
and endosperm (Dev et al., 1986 and Sosulski and Bakal, 1969). Flaxseed protein contains
higher amounts of arginine, aspartic acid, and glutamic acid than other amino acids (Table 2;
Bhatty and Cherdkiatgumchai, 1990 and Oomah and Mazza, 1993b). The essential amino
acids found in flaxseed meal are similar in concentration and composition to those in
soybean.
Table 2.
Amino acid compositions of flaxseed in comparison to soybean.
Brown flaxseed
(NorLin)1
Amino acid
Alanine (Ala)
Arginine (Arg)
Aspartic acid (Asp)
Cystine (Cys)
Glutamic acid (Glu)
Glycine (Gly)
Histidine (His)a
Isoleucine (Ile)a
Leucine (Leu)a
Lysine (Lys)a
Methionine (Met)a
Phenylalanine
(Phe)a
Proline (Pro)
Serine (Ser)
Threonine (Thr)a
Tryptophan (Trp)a
Tyrosine (Tyr)
Valine (Val)a
a
Yellow flaxseed
(Omega)1
g/100 g protein
Soybean2
4.4
9.2
9.3
1.1
19.6
5.8
2.2
4.0
5.8
4.0
1.5
4.5
9.4
9.7
1.1
19.7
5.8
2.3
4.0
5.9
3.9
1.4
4.1
7.3
11.7
1.1
18.6
4.0
2.5
4.7
7.7
5.8
1.2
4.6
4.7
5.1
3.5
4.5
3.6
1.83
2.3
4.6
3.5
4.6
3.7
NRb
2.3
4.7
5.2
4.9
3.6
NRb
3.4
5.2
Essential amino acids for humans.
b
Not reported.
Data taken from 1Oomah and Mazza (1993b), 2Friedman and Levin (1989), and
3
Bhatty and Cherdkiatgumchai (1990) source papers cited therein.
Table options
Flaxseed contains substantial soluble and insoluble fiber. Cui (2001) reported the content of
insoluble and soluble fiber to be 20% and 9%, respectively, whereas Hadley et al. (1992)
reported 30% and 10%, respectively. The differences likely arise from seed and/or extraction
protocols used. Soluble fiber, also known as mucilage, occurs in the seed coat and is readily
extracted with hot (Cui, Mazza, Oomah, & Biliaderis, 1994) and less readily with cold water
(Paynel et al., 2013). This soluble fiber includes acidic [composed of l-rhamnose (25.3%), lgalactose (11.7%), l-fructose (8.4%), and d-xylose (29.1%)] and neutral polysaccharides [larabinose (20%) and d-xylose/d-galactose (76%)] (Anderson & Lowe, 1947). Insoluble fiber
is composed of cellulose (7–11%), lignin (2–7%), and acid detergent fiber (10–14%) (Cui
et al., 1994). Flaxseed mucilage arabinogalactans are associated with protein (Ray et al.,
2013).
Flaxseed oil content of ranges from 38 to 44% due to genotype and environmental parameters
(Oomah and Mazza, 1997 and van Uden et al., 1994). Microscopic membrane bound oil
bodies, known as oleosomes, are the main storage form of oil in the endosperm and
cotyledons. Oleosomes may be extracted from other seed components and they contain CLs
(Gui, Shim, & Reaney, 2012). Fatty acid composition varies among different flaxseed types
and cultivars. The majority of the flaxseed oil (75%) is found in cotyledons, with much of the
remaining oil (22%) present in the seed coat and endosperm (Dorrell, 1970). The oil is
primarily in the form of triacylglycerides with a fatty acid profile typically including linolenic
(52%), linoleic (17%), oleic (20%), palmitic (6%), and stearic (4%) acids (Green, 1990).
Minor lipids and lipid soluble compounds include monoacylglycerides, diacylglycerides,
tocopherols, sterols sterol-esters, phospholipids, waxes, CLs, free fatty acids (FFAs),
carotenoids, chlorophyll, and other compounds. The oxidative instability of renders flax less
desirable for use as cooking oil although flaxseed oil is used in northern China for cooking
(Choo, Birch, & Dufour, 2007). Australian scientists selected a new genetic variant; Linola™
with linoleic acid above 65% and α-linolenic acid (ALA) below 2% that has improved
oxidative stability (Green and Dribnenki, 1994 and Haumann, 1990) compared to
conventional flaxseed. Human metabolic pathways do not synthesize ALA, an essential
polyunsaturated fatty acid in flaxseed. It is an intermediate in the biosynthesis of hormonelike eicosanoids, which regulate inflammation and immune function in higher animals
(Mantzioris et al., 1994 and Mantzioris et al., 1995) and may contribute to effects of dietary
flaxseed oil.
Biological effects of flaxseed and flaxseed fractions
There are many flaxseed products that are consumed including: whole seed, ground whole
seed, flaxseed oil, partially defatted flaxseed meal (usually from expeller pressing), fully
defatted flaxseed meal (from solvent extraction), flaxseed mucilage extracts, flaxseed hulls,
flaxseed oleosomes and flaxseed alcohol extracts. Each of these products is associated with
specific beneficial health effects. Although each fraction contains more than one bioactive
component reports commonly ignore the presence of a plurality of bioactive compounds in
flaxseed fractions or attribute the effect of a component of the flaxseed on the observed
effect.
Whole flaxseed is widely accepted as a healthy food that has anticancer activity. Controlled
experimental diets have demonstrated numerous beneficial effects of flaxseed consumption
(Clark et al., 1995, Cunnane et al., 1993 and Jenkins et al., 1999). Dietary flaxseed flour (see
description below) reduces epithelial cell proliferation and nuclear aberrations in female rat
mammary glands. This finding indicates that flaxseed may reduce the growth rate of
mammary cancer (Serraino & Thompson, 1991). Additionally, it has been found that flaxseed
lignan reduces mammary tumor growth in the later stages of carcinogenesis (Thompson,
Seidl, Rickard, Orcheson, & Fong, 1996). Supplements of 14% flaxseed oil and 20% flaxseed
meal reduce the incidence of azoxymethane-induced aberrant crypt foci formation in Fisher
344 male rats (Williams et al., 2007a and Williams et al., 2007b). Similarly, it has been
shown that the substitution of corn meal with flaxseed meal (15%) or corn oil with flaxseed
oil (15%) in a basal diet, significantly decreased tumor multiplicity and size in the small
intestine and colon of Fisher 344 male rats. The authors concluded that flaxseed meal and oil
are effective chemo-preventive agents (Bommareddy et al., 2009).
Inclusion of flaxseed products has also been associated with improvement in blood lipids.
Inclusion of 20% flaxseed in diets of rats decreased total plasma cholesterol, triglyceride
(TG), and low-density lipoprotein (LDL) cholesterol by 21, 34, and 23%, respectively.
Supplementation with 30% flaxseed had a more pronounced effect, reducing the same factors
by 33, 67, and 23% (Ratnayake et al., 1992). In human studies, 15 g/d of flaxseed
administered for three months was associated with reduction in serum TG and LDL
cholesterol without any alteration of high-density lipoprotein (HDL) cholesterol
(Bierenbaum, Reichstein, & Watkins, 1993). It has also been reported that consumption of
50 g flaxseed/day for four weeks lowered the plasma LDL cholesterol by 8% in young
healthy adults (Cunnane et al., 1995). Indeed, a meta-analysis of studies published from
January 1990 to October 2008 revealed a consensus regarding the effect of flaxseed products
on blood lipids. All flaxseed interventions, included in the analysis, reduced both total
cholesterol [0.10 mmol/L (95% CI: −0.20, 0.00 mmol/L)] and LDL cholesterol [0.08 mmol/L
(95% CI: −0.16, 0.00 mmol/L)]. While flaxseed oil had no significant effect on either
cholesterol or LDL cholesterol. Interventions with whole flaxseed and lignan enriched
supplements (flaxseed alcoholic extracts) lowered total cholesterol (−0.21 and
−0.16 mmol/L) and LDL cholesterol (−0.28 and −0.16 mmol/L). Female subjects, and
individuals with elevated initial cholesterol concentrations, showed a greater response. These
experimental findings support the hypothesis that flaxseed consumption has a positive effect
on suppressing the development of atherosclerosis. Recently it was demonstrated that
consumption of one whole flaxseed product in the form of a bagel, muffin, bar or bun that
containing 30 g of flaxseed by a group of patients displaying pulmonary artery disease and
elevated blood pressure significantly reduced both systolic and diastolic blood pressure
(Rodriguez-Leyva et al., 2013). In this study the authors reported significant linear
correlations between blood pressure lowering effects and markers of flax consumption.
Unfortunately, none of the correlations were demonstrated to be causal. Based on the weight
of evidence from human clinical trials available prior to 2011 Health Canada's Food
Directorate concluded that a claim linking consumption of ground whole flaxseed and blood
cholesterol lowering was warranted (Health Canada, 2014).
While there are many known flaxseed varieties and flaxseed composition is known to be
affected by genotype and environmental conditions virtually none of the studies of flaxseed
effects on physiology have shown definitively that the active ingredient is associated with the
observed biological effect. Differential studies that vary just one metabolic constituent in the
diet are more useful but such studies are rare. In one such study researchers showed that
flaxseed with both high and negligible ALA content showed that flaxseed with very low ALA
content lowered hypercholesterolemic atherosclerosis in rabbits (Prasad, Mantha, Muir, &
Westcott, 1998). A second approach to differential analysis has been presented by Marambe,
Shand, and Wanasundara (2011). In their study the digestion of flaxseed protein was
compared for whole seed and for flaxseed meal after removal of mucilage. In vitro
digestibility of proteins was slowed by the presence of mucilage. No biological activity was
observed in this study but the approach could conclusively demonstrate the role of flaxseed
mucilage in flaxseed biological activity. In addition, researchers commonly associate
observed effects with metabolic differences that may be spurious and unintentionally
misleading. Would the low ALA flax studied by Prasad, Strauss, and Reichart (1999) have
evoked the same profound response of blood pressure on individual subjects with peripheral
artery disease observed by Rodriguez-Leyva et al. (2013)? Was the flaxseed used in the
Rodriguez-Leyva et al. (2013) study a representative variety or was it anomalous? Is one
component of the flaxseed or multiple components contributing to the biological effects?
While these questions are easily asked where whole flaxseed is studied it is important to note
that similar questions may be posed when the biological activity of enriched flaxseed
fractions is considered.
Flaxseed oil
Authors of a number of studies have suggested that the primary benefit of flaxseed oil
consumption is due to its ALA. Flaxseed oil consumption exerts several effects on
inflammatory mediators and markers depending on dose. Flaxseed oil given at 14 g/d to
human subjects over 4 weeks decreased the levels of tumor necrosis factor-α (TNF-α),
interleukin-6 (IL-6), and cytokines. A lower dose did not have this effect (Caughey et al.,
1996, Thies et al., 2001 and Wallace et al., 2003). Supplementation of the diet with 6% ALA
depressed the levels of IL-6 and IL-10 and increased the production of TNF-α in mice
(Chavali, Zhong, & Forse, 1998). A lower omega-6 to omega-3 fatty acid ratio decreased
atherosclerosis in comparison to a higher ratio in apolipoprotein E, in LDL receptor double
knockout mice. Feeding Golden Syrian hamsters 20 g/d ALA for six weeks reduced serum
cholesterol by 17–21% (Yang et al., 2005). However, no changes were found in serum total
cholesterol, LDL cholesterol or HDL cholesterol in healthy subjects or hyperlipidemic
patients (Freese and Mutanen, 1997, Kestin et al., 1990, Sanders and Roshanai,
1983 and Singer et al., 1990). David (1983) suggested that ALA might lower the growth rate
of breast and colon cancers. It is noteworthy that almost all the literature extolling the
beneficial functions of flaxseed oil fails to confirm that ALA itself, rather than other
bioactive compounds found in flaxseed oil produced the observed health benefits.
The quality of flaxseed oil is poorly described in most of the previously mentioned studies.
Flaxseed oil contains bioactive peptides that oxidize over the first few weeks of storage under
nitrogen (Brühl et al., 2007). In addition, flaxseed oil products were not examined during
most studies to determine if markers of oxidation accumulate in the oil. Oxidation of
vegetable oil produces potentially toxic biologically active compounds (Esterbauer,
1993 and Han and Csallany, 2009). It is interesting that Collins, Shand, and Drew (2011)
observed large differences in flaxseed oil metabolism of fish fed flaxseed oil that was
protected either by addition of an antioxidant or by a combination of antioxidant and
encapsulation when compared with an unprotected oil indicating that flaxseed oil quality may
determine ALA metabolism in fish. It is not known if the effect of oil quality on ALA
metabolism extends to other species but few studies verify the presence of oxidation products
in oil during the research. More recently, Randall, Reaney, and Drew (2013) observed that
linolenic acid conversion to eicosapentaenoic acid was enhanced in fish simultaneously fed
flaxseed and petroselenic acid (18:1 Δ6), in the form of coriander oil, a fatty acid that
potentially decreases linoleic acid conversion to arachidonic acid. Specific fatty acids
included in dietary studies of flaxseed oil may modify ALA metabolism.
Biological activity of flaxseed lignan and lignan complex
Secoisolariciresinol diglucoside (SDG, MW 686.7), is a phytochemical and antioxidant that
acts as a precursor of mammalian lignans and a phytoestrogen (Adolphe et al.,
2010 and Prasad, 2004). Extraction of flaxseed with aqueous alcohol yields a solution
containing SDG, a form of lignan, in a polymer with phenylpropanoids and
hydroxymethylglutaric acid referred to as flaxseed lignan complex (FLC). The oil-insoluble
complex is reported to exert multiple physiological effects in animals and humans. Westcott
and Paton (2001) reported a FLC is composed of 34–38% SDG, 15–21% cinnamic acid
glucoside and 9.6–11% hydroxymethylglutaric acid. Consumption of the lignans or the FLC
is reported to slow the progression of atherosclerosis in humans and other mammals (Prasad,
2005, Prasad, 2009a, Prasad, 2009b and Zhang et al., 2008). Treatment of subjects with FLC
[40 mg/kg body weight (bw)/d] for eight weeks suppressed the development of
hypercholesterolemic atherosclerosis by 34% in rabbits (Prasad, 2005). Hypercholesterolemic
humans were treated with 300 mg or 600 mg of FLC for eight weeks. The 300 mg dose
reduced total cholesterol and LDL cholesterol by 15 and 17%, respectively, without any
change in the ratio of total cholesterol/HDL cholesterol. A higher dose of 600 mg reduced the
serum total cholesterol and LDL cholesterol by 24 and 22%, respectively, with a decrease in
the total cholesterol/HDL cholesterol ratio (Zhang et al., 2008). Prasad (2009b) also found
that FLC was effective in slowing the progression of atherosclerosis by 31% in
hyperlipidemic rabbits, as well as reducing oxidative stress.
SDG produced by chemical hydrolysis and isolated from flax alcohol extracts can present
biological activity that is similar to the complex. SDG can prevent the development of
atherosclerosis and diabetes (Fukumitsu et al., 2008, Prasad et al., 1999 and Prasad, 2009b),
and additional benefits include modification of blood lipids and cholesterol levels
(Fukumitsu, Aida, Shimizu, & Toyoda, 2010). As substantial recent reviews of SDG activity
have been presented it is not reviewed further here. In some studies alcoholic extracts that
contain SDG have been used without further separation. These fractions may contain a range
of ethanol soluble compounds including linatine, cyanogenic glycosides, CLs etc.
Interpretation of these studies requires caution.
Biological activity of selected flaxseed components
Based on the complexity of the flaxseed fractions used for much of the research discussed
above it is not possible to attribute the health benefits of flaxseed consumption to a sole
bioactive component present in flaxseed. The exploration of the biological roles of flaxseed
polyunsaturated fatty acids and lignan has been substantial in contrast to the modest efforts
made on CLs and other components. Others have recently reviewed much of this research
(Carayol et al., 2010, Landete, 2012, Lane et al., 2014, Katare et al., 2012, Rabetafika et al.,
2011 and Rodriguez-Leyva et al., 2010).
Protein and hydrolysates
Flaxseed protein products are rich in arginine, an amino acid that, when present in the
vascular endothelia, can lower blood pressure (Udenigwe et al., 2012). Flaxseed protein
isolates (FPI) fed at 200 mg/kg bw to spontaneously hypertensive rats (SHR) effectively
lowered blood pressure four hours after administration. Hydrolysis of the FPI followed by
isolation of a peptide cation rich fraction produced peptides with highly elevated arginine.
This fraction produced a marked decrease in blood pressure in the same SHR system in just
2 h. Flaxseed protein, like protein from other sources, produces bioactive linear peptide
fractions when hydrolyzed by proteases. These fractions have been tested for a number of
modes of action. For example, flaxseed proteins digested with Flavourzyme® inhibited
angiotensin converting enzyme (Marambe, Shand, & Wanasundara, 2008). The same protein
fraction is also effective at scavenging hydroxyl radicals. Flaxseed peptides produced by FPI
hydrolysis by thermolysin and pronase (Udenigwe & Aluko, 2010) or alcalase (Omoni &
Aluko, 2006) were biologically active in a number of assays. A recent review of flaxseed
proteins concluded that the association of flaxseed proteins with mucilage was an advantage
in their applications in food formulations. However, mucilage increases the viscosity of
aqueous solutions making the separation of protein difficult. It was noted that preparing
protein isolates that were free from mucilage was technically difficult due to viscosity
(Omoni and Aluko, 2006, Udenigwe and Aluko, 2010 and Udenigwe et al., 2009).
Flaxseed protein hydrolysis products may inhibit angiotensin I-converting enzyme but only a
few studies have focused on the hydrolysis of flaxseed protein as would occur under gut
conditions (Marambe et al., 2008 and Marambe et al., 2011) investigated the in vitro
digestion of flaxseed protein and found that whole ground flaxseed resisted digestion in both
the modeled stomach and intestinal digestion conditions but that treatments that would
inactivated flaxseed protease inhibitors or removed mucilage or both increased total
digestion. The amount of undigested protein in whole ground seed was just 12%.
Flaxseed mucilage
Effects of flaxseed mucilage on the digestive tract have been investigated recently in human
and animal trials. The products were supplied in solution or incorporated into food. As would
be expected for consumption of fiber, feelings of satiety and fullness were expressed by
subjects consuming 2.5 g of soluble fiber in contrast to control subjects. A significant
decrease in subsequent energy intake was observed after the flaxseed drink compared to the
control (2937 vs. 3214 kJ). There was no difference in either appetite ratings or energy intake
by subjects that consumed soluble flaxseed fiber in a drink or as a tablet. Consuming flaxseed
mucilage liquid three times a day was compared to consuming the same amount in the form
of bread ( Kristensen et al., 2012). Fiber consumed by both means lowered total fasting
cholesterol and LDL cholesterol after seven days. Fecal fat excretion increased with both
treatments while the liquid form of the mucilage, and not the bread product, increased energy
excretion in feces. The authors concluded that soluble flaxseed fiber might be used to reduce
cholesterol and may be useful in controlling energy balance. Similar observations were made
for rats provided soluble flaxseed fiber as part of their diet. Weight gain was also limited in
rats fed soluble fiber and dietary linseed in contrast to controls (Kristensen, Knudsen, et al.,
2013). The effects of soluble flaxseed fiber on appetite, blood triglycerides and appetite
regulating hormones were observed for seven hours after a test meal (Kristensen, Savorani,
et al., 2013). Higher levels of dietary fiber served with the meal (3.4 g/MJ from flaxseed)
were associated with decreased blood triglyceride (18%) after the meal. Additionally, satiety
and fullness ratings increased while insulin decreased with the higher dietary fiber treatment.
The concentrations of hormones ghrelin, cholecystokinin and glucagon-like peptide 1 were
not affected by the treatment.
Polyunsaturated fatty acids, the lignan complex, polysaccharides, flaxseed proteins and CLs
are major functional classes of compounds that might impart some or all of the observed
experimental results of flaxseed products. While compounds associated with health benefits
were consumed, compounds that are also bioactive but potentially toxic were also present in
many of the aforementioned studies. These compounds include: linatine, cyanogenic
glycosides, cadmium, phytate, selenium, and SDG (Fig. 3). Paradoxically, these compounds
may also have contributed to the observed health benefits.
Fig. 3.
Classification of plant cyclopeptides modified from Tan and Zhou (2006).
Figure options
Linatine
When fed flaxseed meal as a major portion of the diet poultry suffer symptoms similar to
vitamin B deficiency. Addition of vitamin B in the form of pyridoxine overcame this
deficiency. Linatine, a component of flaxseed meal, may cause the decreased growth
(Klosterman, Lamoureux, & Parsons, 1967). Most studies have not demonstrated direct
effects of linatine on health but rather have shown weight gain responses to vitamin B
supplementation. For example it was concluded that linatine did not affect the performance of
swine fed meal from low linolenic acid flaxseed as their growth rate was unaffected by the
inclusion of pyridoxine (Batterham, Andersen, & Green, 1994). Another route to showing the
potential impact of linatine is to determine the vitamin B status. Bishara and Walker (1977)
fed pigs a diet containing 30% flaxseed meal by dry weight and challenged the test animals
with tryptophan to determine vitamin B status. An observable change in tryptophan
metabolites was reported suggesting that the animals were experiencing marginal vitamin B6
deficiency (Bishara & Walker, 1977). While there are no reports of vitamin B deficiency in
humans that have consumed flaxseed this is potentially related to the relatively lower
consumption of flaxseed products as a portion of diet. The risk of flaxseed causing a
deficiency could be related to the level of consumption and the vitamin B status of the
individual consuming the seed. The elderly, vegetarians, and vegans have been noted as
groups that are more likely to have a deficiency in vitamin B (Allen, 2009 and Gilsing et al.,
2010).
Cyanogenic glycosides
Cyanogenic compounds have profound biological effects. Flaxseed contains linamarin,
linustatin and neolinustatin with the potential to release 7.8 μM of cyanogenic compound/g of
flaxseed. A thirty-gram dose of flaxseed could release 240 μM of cyanide. Researchers have
found no effect of prolonged consumption of flaxseed.
Cyanogenic glycoside consumption may not always produce undesirable effects.
Consumption of 20% flaxseed meal by weight of diet counteracted the toxicity of selenium
fed in the form of sodium selenite (Olson & Halverson, 1954). Furthermore, it was
discovered that the protective factor could be extracted from flaxseed meal by aqueous
methanol (MeOH). Chromatography of these protective extracts revealed that fractions that
protected against selenium toxicity were also rich in the cyanogenic glycosides linustatin and
neolinustatin. HPLC peaks that contained these compounds and pure linamarin both provided
some protective effect against selenium toxicity (Palmer, Olson, Halverson, Miller, & Smith,
1980).
The capacity of individuals to detoxify cyanide is related to the presence of sulfur containing
amino acids in the diet. Most research into the toxicity of cyanogenic compounds to humans
is related to the consumption of cassava as a staple in the diet. The toxicity of cassava is
exacerbated due to the typical high levels of consumption and the low availability of dietary
protein to those that rely on this staple (World Health Organization, 2004). The toxicity of
flaxseed cyanogenic glycosides is likely to be rare except in cases where flaxseed fractions
are consumed in relatively large amounts in low protein diets.
Cadmium
Many plants, including flaxseed, absorb the toxic heavy metal cadmium from the soil which
then accumulates in the seed. Plant products contribute about two thirds of dietary cadmium
(Satarug, Garrett, Sens, & Sens, 2010). Of this, whole oilseeds and oilseed meal including
products of sunflower, peanut and flaxseed represent the richest cadmium sources. Cadmium
toxicity is typically observed as the loss of kidney function and bone density although many
other symptoms are recognized. These effects are cumulative and may require decades of
exposure. For most of the population exposure to cadmium through food occurs at a low
level. For example, it has been estimated that cadmium exposure through diet in European
countries was 2.3 μg/kg bw/week (range from 1.9 to 3.0 μg/kg bw/week) though vegetarians
consumed higher levels of cadmium (up to 5.4 μg/kg bw/week) (Alexander et al., 2009).
A Proposed Tolerable Weekly Intake (PTWI) was established to be 7 μg/kg bw/week (WHO
Food Additives, 1989). The choice of weekly dietary exposure was chosen over the daily
exposure due to the high levels of cadmium in specific foods and the slow adsorption and
excretion characteristics of cadmium. Cadmium has a very long half-life in the human
system. For a 62 kg human this amounts to 434 μg/per week (Walpole et al., 2012). Thirty
grams of flaxseed consumption (Leyva et al., 2011) containing 500 μg cadmium/kg
(Saastamoinen et al., 2013) on a daily basis would contribute 105 μg/per week to the diet or
1.69 μg/kg bw/week. This level of cadmium consumption is below the PTWI and normal
exposure levels for Europeans. However, the Contaminants in the Food Chain (CONTAM)
panel (Alexander et al., 2009) advised that the cadmium TWI recommendation be lowered to
protect more sensitive individuals in the population. Based on a meta-analysis of kidney
function and exposure to cadmium it was established that the formerly established PTWI for
cadmium of 7 μg/kg bw/week may place specific individuals at risk. The panel recommended
lowering the TWI to just 2.5 μg/kg bw. Consumption of 30 g of flaxseed/day would
contribute 70% of the newly proposed TWI. It is possible that consuming larger amounts of
flaxseed over longer periods of time may place a significant cadmium burden on susceptible
individuals.
Cadmium is toxic to specific cancer cell lines and has been considered for use in
chemotherapy (Waalkes & Diwan, 1999). However, chronic exposure to cadmium increases
the risks of cancer (Satarug et al., 2010).
Cyclolinopeptides
Although CLs have been known for more than half a century research has largely been
restricted to structural and physical characterization. Most of what is known about biological
activities is anecdotal or based on small animal studies with modest numbers of specimens or
in vitro analysis. CLs occur as a significant seed component in flaxseed, but the role of these
compounds in planta is largely unknown. In vitro studies of CL biological activity have been
described in various publications ( Gaymes et al., 1997, Górski et al., 2001, Kessler et al.,
1986a, Kessler et al., 1986b, Siemion et al., 1999 and Wieczorek et al., 1991).
Kessler, Klein, et al. (1986) reported that CLA (1) inhibits cholate uptake into hepatocytes.
Later, the tripeptide -Phe-Phe-Pro- observed in isolated form from 1, which is similar to
structures in antamanide and somatostatin, was proved to suppress the hepatocyte cell
transport system. It is possible that this peptide sequence imparts the observed cytoprotective
effects of 1 on hepatocytes (Rossi & Bianchini, 1996). Immunomodulatory activity of 1 was
studied using: Jerne's plaque-forming cell number determination test for the primary and
secondary humoral immune response, delayed type hypersensitivity reaction, the skinallograft rejection, the graft-versus-host reaction for the cellular immune response in mice,
human lymphocyte proliferation test in vitro and the post-adjuvant polyarthritis test in rats
and hemolytic anemia test in New Zealand black mice ( Wieczorek et al., 1991). The results
show 1 affected both the humoral and cellular immune responses. An increased skin-allograft
rejection time and reduced graft-versus-host reaction index was also observed. Human
lymphocyte proliferation was inhibited in vitro by 1via phytohemagglutinin ( Wieczorek
et al., 1991). The symptoms associated with two immune diseases, post-adjuvant polyarthritis
in rats and hemolytic anemia of New Zealand black mice, were alleviated. In the research of
Górski et al. (2001), the immunosuppressive effects of 1 were compared with cyclosporine A
(CsA), a known immunosuppressant. Both 1 and CsA function by inhibiting the action of
Interleukin-1-α and Interleukin-2. This finding strongly indicates that 1 shares the same
mechanism as CsA in the plaque-forming cells test and the autologous rosette-forming cells
test. This study also compared the effects of both compounds on human lymphocytes in vitro.
It was found that at very low concentrations, 1 induced the same effects as CsA on T- and Bcell proliferation, acquisition of activation antigens and immunoglobulin synthesis (Górski
et al., 2001). Overall, these studies demonstrated that 1 had similar biological effects to CsA.
The toxicity of 1 was evaluated by intravenous and oral administration in rats and mice
(Wieczorek et al., 1991). Oral administration of 4 g/kg CL 1 in olive oil, 2% gelatin solution
did not harm mice while a concentration of 3 g/kg in rats was also well tolerated. Intravenous
administration of 1 at 230 mg/kg is non-toxic to mice. The combined strong
immunosuppressive activity and low toxicity at relatively large doses of 1 indicates a
potential as an immunosuppressive drug.
Other CLs and their analogs were also investigated for immunosuppressive activities.
According to the research of Morita et al. (1997), CL 2 inhibits concanavalin-A induced
proliferation of human peripheral blood lymphocytes at treatment levels comparable to that of
CsA. CLs 2 and 9 also manifested a moderate inhibitory effect on concanavalin-A induced
mouse lymphocyte proliferation (Morita et al., 1997). Many chemical analogs of 1 were
tested for their effects on the immune response (Benedetti and Pedone, 2005, Picur et al.,
2006 and Siemion et al., 1999). Many of these compounds including a -Pro-Xxx-Phesequence (where Xxx means a hydrophobic, aliphatic, or aromatic residue) were found to
exert immunosuppressive activity although none of them were greater than CL 1 (Picur et al.,
2006). The immunosuppressive activity of CLs and analogs make them potential value-added
natural products of flaxseed and should lead to further investigation of the biological
activities of CLs.
Flaxseed CLs belong to the peptide class of orbitides that are plant cyclic peptides produced
from ribosomal precursors (Arnison et al., 2013). These natural products occur in significant
quantities in some plant species. Databases containing expressed sequence tags and genome
sequences were investigated for information about orbitide biosynthesis. This revealed genes
that appeared to encode precursors, which are subsequently cyclized to mature orbitides
(Covello et al., 2012). Moreover, the presence of new orbitides, which were predicted by
sequence analysis, has been confirmed by mass spectrometry (MS) (Okinyo-Owiti, Young,
Burnett, & Reaney, 2014). Sequence analysis also predicts the presence of many similar
precursor genes in Euphorbiaceae, Caryophyllaceae, Linnaceae, and Rutaceae (Arnison et al.,
2013). These peptides are made of proteinogenic amino acids and are produced by ribosomal
synthesis. A more thorough discussion of CLs is provided below.
Flaxseed CLs are orbitides
Plant cyclopeptides are cyclic compounds found in higher plants. They typically comprise 2
to 37 amino acids. The cyclic peptides of flaxseed specifically contain proteinogenic amino
acids and their oxidized products. Tan and Zhou (2006) reviewed the chemistry of plant
cyclopeptides and reported structures of 455 cyclopeptides in 24 plant families. In their
review they divided plant cyclopeptides into two classes, five subclasses, and eight types
according to their bonding structures (Fig. 3). Individual classes were named according to
taxonomic distribution. Cyclopeptide alkaloids (Type I), Caryophyllaceae-type cyclopeptides
(Type VI), and Cyclotides (Type VIII) are the three largest groups numbering 185, 168, and
51 members, respectively. Flaxseed cyclic peptides belong to type VI peptides. Although Tan
and Zhou (2006) classified all cyclopeptides found in plants calling Type VI cyclopeptides
Caryophyllaceae-type homomonocyclopeptides. This name was cumbersome and Arnison
et al. (2013) suggested calling homodetic plant cyclic peptides, comprising those peptides
arising from ribosomal synthesis that do not contain cysteine knots and having 5–12 amino
acids, orbitides (Arnison et al., 2013). This distinguishes orbitides from the cyclotides that
contain cysteine knots. CLs are orbitides. Both orbitides and cyclotides are homodetic cyclic
peptides as the ring consists solely of amino-acid residues with N- and C-terminals linked
through a peptide bond.
Proposed nomenclature for CLs
CLs were classified as members of the “Caryophyllaceae Type VI homomonocyclopeptides”
that contain eight or nine amino acid residues with molecular masses of approximately 1 kDa.
The first CL (first orbitide CL 1), was identified after it was isolated from sediments
recovered from crude flaxseed oil (Kaufmann & Tobschirbel, 1959). In 1968, Weygand
discovered a similar cyclic nonapeptide, CL 2. Between 1999 and 2014, 17 CLs (3–6, 8–12,
and 14–21) were identified from the seed and root of flax (Matsumoto et al., 2001a,
Matsumoto et al., 2002, Matsumoto et al., 2001b, Morita et al., 1999, Okinyo-Owiti et al.,
2014, Stefanowicz, 2001 and Stefanowicz, 2004). In addition, another cyclic peptide with a
non-proteinaceous amino acid residue (N-methyl-4-aminoproline) was isolated from Linum
album in 1998 ( Picur, Lisowski, & Siemion, 1998). Subsequently, cyclic peptides from
flaxseed were named according to the date of their discovery with each newly discovered
peptide being ascribed the next letter in the alphabet (old name, Table 3). Type VI
nomenclature and flaxseed orbitide nomenclature are highly irregular with publications
defining novel designations for oxidation products and authors using a single name for two
distinct compounds. International Union of Pure and Applied Chemists (IUPAC)
recommendation relating to methionine sulfur oxidation products is clear. “Indication of the
modification of this sulfur should not suggest the addition of further sulfur. Hence calling…
methionine S-oxide by the name methionine sulfoxide, and methionine S,S-dioxide by the
name methionine sulfone may be confusing and is not recommended.” Therefore methionine
S-oxide and methionine S,S-dioxide are designated according to notes 1 and 2 ( Giles,
1999 and IUBMB/IUPAC, 1993).
equation(1)
MethionineSoxide(methionineoxide)MetOorMetO
Turn MathJax on
equation(2)
MethionineS,S-dioxide(methioninedioxide)MetO2orMetO2
Turn MathJax on
Table 3.
CLs in L. usitatissimum L.
Code
New
namea
Lit.
nameb
1
[1–9NαC]CLA
CLA
2
[1–9NαC]CLB
CLB
3
[1–9NαC],[1-
CLC
Amino
acid
Chemical
MW
References
sequence
formula
(Da)
(NαC-)c
Ile-LeuVal-ProKaufmann &
Pro-Phe- C57H85N9O9 1040.34
Tobschirbel, 1959
Phe-LeuIle
Met-LeuIle-ProWeygand,
Pro-Phe- C56H83N9O9S 1058.38 1968 and Morita et al.,
Phe-Val1997
Ile
MetOC56H83N9O10S 1074.38 Morita et al., 1999
Leu-Ile-
Amino
Lit.
acid
Code
b
name sequence
(NαC-)c
MetO]Pro-ProCLB
Phe-PheVal-Ile
MetO2[1–9Leu-IleNαC],[14
CLK
Pro-ProMetO2]Phe-PheCLB
Val-Ile
Met-Leu[1–8CLD',
Leu-Pro5
NαC]CLK,d
Phe-PheCLD
CLOe
Trp-Ile
MetO[1–8Leu-LeuNαC],[16
CLD
Pro-PheMetO]Phe-TrpCLD
Ile
MetO2[1–8Leu-LeuNαC],[1- 1-Msn7
Pro-PheMetO2]- CLD
Phe-TrpCLD
Ile
Met-LeuCLE',
[1–8Val-Phe8
CLJ,d
NαC]-CLE
Pro-LeuCLPe
Phe-Ile
MetO[1–8Leu-ValNαC],[19
CLE
Phe-ProMetO]Leu-PheCLE
Ile
MetO2[1–8Leu-ValNαC],[110
CLJ
Phe-ProMetO2]Leu-PheCLE
Ile
Met-Leu[1–8CLF,
Met-Pro11
NαC]-CLF CLL
Phe-PheTrp-Val
[1–8Met-LeuNαC],[3MetO12
CLI
MetO]Pro-PheCLF
Phe-TrpNew
namea
Chemical
formula
MW
(Da)
C56H83N9O11S 1090.38
References
Matsumoto, Shishido,
and Takeya (2001)
Stefanowicz, 2001,
Stefanowicz,
C57H77N9O8S 1048.34
2004 and OkinyoOwiti et al., 2014
C57H77N9O9S 1064.34 Morita et al., 1999
C57H77N9O10S 1080.34 Olivia, 2013
Stefanowicz, 2001,
Stefanowicz,
C51H76N8O8S 961.26
2004 and OkinyoOwiti et al., 2014
C51H76N8O9S 977.26 Morita et al., 1999
C51H76N8O10S 993.26
Matsumoto, Shishido,
and Takeya (2001)
Stefanowicz,
C55H73N9O8S2 1051.50 2001 and Stefanowicz,
2004
C55H73N9O9S2 1068.35
Matsumoto, Shishido,
Morita, et al., 2001
Amino
Lit.
acid
Code
b
name sequence
(NαC-)c
Val
MetO[1–8Leu-MetNαC],[1- (Mso)13
Pro-PheMetO]LCP-3
Phe-TrpCLF
Val
MetO[1–8LeuNαC],[1,3MetO14
CLF
MetO]Pro-PheCLF
Phe-TrpVal
Met-Leu[1–8CLG,
Met-Pro15 NαC]CLM
Phe-PheCLG
Trp-Ile
Met-Leu[1–8MetONαC],[316
CLN
Pro-PheMetO]Phe-TrpCLG
Ile
MetO[1–8Leu-MetNαC],[117
CLH
Pro-PheMetO]Phe-TrpCLG
Ile
MetO[1–8LeuNαC],[1,3MetO18
CLG
MetO]Pro-PheCLG
Phe-TrpIle
Met-Leu[1–9[1–9Lys-Pro19 NaC]NαC]- Phe-PheCLQ
CLQ
Phe-TrpIle
MetO[1–9[1–9Leu-LysNαC],[1- NαC],[120
Pro-PheMetO]MetO]Phe-PheCLQ
CLQ
Trp-Ile
[1–9[1–9Gly-Ile21
NaC]-CLR NaC]- Pro-ProNew
namea
Chemical
formula
MW
(Da)
References
C55H73N9O9S2 1068.35 Reaney et al., 2013,
C55H73N9O10S2 1084.35
Matsumoto, Shishido,
Morita, et al., 2001
Stefanowicz,
C56H75N9O8S2 1066.38 2001 and Stefanowicz,
2004
C56H75N9O9S2 1082.38 Stefanowicz, 2004
C56H75N9O9S2 1082.38
Matsumoto, Shishido,
Morita, et al., 2001
C56H75N9O10S2 1098.38
Matsumoto, Shishido,
Morita, et al., 2001
C66H87N11O9S 1210.53
Okinyo-Owiti et al.,
2014
C66H87N11O10S 1226.52
Okinyo-Owiti et al.,
2014
C54H76N10O10 1025.24
Okinyo-Owiti et al.,
2014
Code
New
namea
Amino
Lit.
acid
b
name sequence
(NαC-)c
CLS
Phe-TrpLeu-ThrLeu
Chemical
formula
MW
(Da)
References
a
Post-translational modifications of all ribosomally synthesized and posttranslationally modified peptides (RiPPs) are noted according to Arnison et al. (2013).
All CLs listed are either a fundamental parent structure or oxidized products of a
fundamental parent structure. The first recognized compound of each fundamental
parent structure is used for all structure names. Fundamental parent structures are
provided for CLA, CLB, CLD, CLE, CLF, CLG, CLQ, and CLR.
b
Name used in first literature description (name used in subsequent literature
description).
c
The methionine residues of amino acid sequences are highlighted in Fig. 4.
Abbreviations are MetO for methionine S-oxide and MetO2 for methionine S,Sdioxide.
d
Stefanowicz (2004) reused CLJ and CLK for 8 and 5 after these names were used
originally for 10 and 4 respectively by Matsumoto, Shishido, and Takeya (2001).
e
Okinyo-Owiti et al. (2014) proposed the use of new naming convention of CLO and
CLP for Met-containing CLPs to distinguish them from MetO containing forms CLD
and CLE, respectively.
Table options
Designation of the first amino acid in a homodetic cyclic peptide is problematic and not
described by IUPAC. Mass spectrometry fragmentation has allowed analysts to choose the
preferred first amino acid in each sequence as the N-terminal of the first linear cleavage
product in MS/MS analysis. Now that it is known that mRNA directs orbitide synthesis it is
appropriate that the linear sequence presented for flaxseed cyclic peptides matches the linear
sequence of the DNA and RNA that gave rise to the cyclic peptide preprotein and peptide.
This designation is recommended in a recent review of all cyclic peptides synthesized from
ribosomal precursors (Arnison et al., 2013). The DNA and amino acid sequences of the CL
precursor protein are provided by Shim, Gui, Arnison, Wang, and Reaney (2014) and
Okinyo-Owiti et al., 2014. Sequences of DNA bases and amino acids that are processed to
CLs are highlighted (Fig. 4).
Fig. 4.
Structures of CLs. Abbreviations are Ile for Isoleucine, Gly for Glycine, Met for
methionine, MetO for methionine S-oxide, and MetO2 for methionine S,S-dioxide.
Predicted products of NCBI genes AFSQ01016651.1 (1, 2, 8, and 19),
AFSQ01025165.1 (5, 11, and 15), AFSQ01011783.1 (21), and AFSQ01011783.1 (21)
are highlighted in green, blue, yellow, and yellow, respectively. (For interpretation of
the references to color in this figure legend, the reader is referred to the web version
of this article.)
Figure options
The structures proposed nomenclature for CLs (1–21) are shown in Table 3 and Fig. 5.
Linkage [1–#-NαC] occurs between amino acid 1 and amino acid “#” through the alpha
amino group that forms a N–C cyclization of the core peptide. The en dash (–) is used and
placed in square brackets. Identical amino acids substituents are numbered and grouped as
shown. Modifications of amino acids use conventions of UniProt and IUPAC in square
brackets. Authors only indicate when there is post-translational modification, for example,
oxidation of the Met residue to MetO and MetO2 (omitted Met). CL name (CLX) is
recognized by 3 capital letters. There is no provision for a nomenclature that involves more
than 26 CLs.
Fig. 5.
Proposed nomenclature for CLs.
Figure options
Fresh aqueous methanolic (70%) extracts of flaxseed comprise as major CL constituents, 1
and methionine (Met) containing 2 (Morita et al., 1997 and Weygand, 1968), 5, 8, 11, and 15
(Stefanowicz, 2001) (curiously, Stefanowicz (2004) renamed these same peptides three years
later citing the structures as being “new”. The latter publication did not cite the earlier
report). On the other hand, aged flaxseed oil contains mainly 1 and methionine S-oxide
(MetO) possessing CLs such as 3, 6, 9, 14, and 18 (Reaney et al., 2013). The MetO residues
derive from Met oxidation, with further oxidation of these residues leading to methionine S,Sdioxide (MetO2) containing CLs such as 4 and 10 (Fig. 6) (Morita & Takeya, 2010). CL 1
does not contain Met and is, therefore, not prone to this form of oxidation. Met oxidation to
MetO/MetO2 residue in a given CL could be a good indicator of flaxseed oil storage duration
since these processes occur over a period of time ( Brühl et al., 2007 and Jadhav et al., 2013).
For instance 9 confers a bitter flavor to linseed oil and is a product of oxidation of 8. The
oxidative transformation of 8 to 9 in freshly prepared oil begins after 2 days at room
temperature and the intensity of bitterness increases over a period of 20 weeks, corresponding
with increasing accumulation of 9 in the oil (Brühl et al., 2007). Met amino acids present in
the CLs present in partially defatted flaxseed products were resistant to oxidation
(Aladedunye, Sosinska, & Przybylski, 2013).
Fig. 6.
Transformation of CL by the chemical oxidation of Met to MetO and MetO2 modified
from Morita and Takeya (2010).
Figure options
Crystal structure of CLs
The biological activity of orbitides is a product of their 3D shape and primary amino acid
composition. The genome of flax is published but the primary sequence will provide only
partial information regarding the mechanism of protein function. Detailed 3D structure of
each gene product is vital to understand its full function. The structures of nine different CLs
(1–3, 6, 9, 12, 14, 17, and 18) have been elucidated by 2D FT-NMR spectroscopy
(Matsumoto et al., 2001a, Matsumoto et al., 2002 and Morita et al., 1999). Structures of 1
with different co-crystallized solvent molecules have been determined by single-crystal X-ray
diffraction (Di Blasio et al., 1987, Di Blasio et al., 1989, Matsumoto et al., 2002 and Quail
et al., 2009). Recently, Reaney et al. (2013) have reported the solid state and solution
structures of four CLs using X-ray diffraction and 2D-NMR. 1, 2, and 4 were readily
crystallized; two adjacent prolines form a cis bond that contributes rigidity to the molecule (
Jadhav et al., 2013). In contrast, 9 and 10, that contain only one proline residue, do not
crystallize readily. Difficulty in crystallization of compounds containing MetO may be
caused, in part, by the optical activity of the MetO moiety. It is typically more difficult or
impossible to form crystals of diastereomers.
Conclusions
Flaxseed oil is a rich source of ALA but this oil also contains cyclic peptides. Most studies of
ALA acid are truly studies of cold pressed flaxseed oil. These studies may erroneously
attribute biological activity to ALA that is contributed by cyclic peptides. Additionally the
oxidative state of the oil in many studies is not known. Flaxseed coat materials are a rich
source of lignans and the polysaccharide mucilage. The latter has profound effects on
digestive health. Studies that attribute biological activity of flaxseed hull and flaxseed hull
extracts to either lignans or polysaccharides alone may also be in error. Alcohol extracts of
flaxseed meal contain lignans, cyanogenic glycosides and cyclic peptides. These materials
may all contribute biological activity.
Flaxseed cyclic peptide nomenclature has not been applied with rigor. It is recommended that
peptides be named as described in Table 3 for future publications.
The risk of toxicity from flaxseed consumption due to linatine, cyanogenic glycosides and
cadmium appears to be negligible for most individuals when flaxseed products are consumed
in moderation. Regular consumption of flaxseed or flaxseed meal products could place a
significant portion of the “cadmium burden” on individuals. However, current
recommendations for maximum weekly cadmium consumption are not likely to be exceeded
with reasonable flaxseed product consumption levels. The cyanogenic glycoside levels in
flaxseed do not appear to be sufficiently concentrated to contribute any biological effect.
There is a reported interaction of cyanogenic glycosides with selenium toxicity. This
interaction has not been studied in sufficient detail to use flaxseed as a treatment for selenium
poisoning. The level of the vitamin B antagonist, linatine, in flaxseed has never been
associated with toxicity in humans but the consumption of large amounts of flaxseed can lead
to evidence of limited vitamin B availability in swine. It is not known if individuals with
compromised vitamin B might become deficient when consuming flaxseed. The level of
linatine is not known for most current flaxseed varieties and thus it is not possible to suggest
that the dose of this compound is acceptable in untested varieties of flax.
Acknowledgments
This work was supported by the Saskatchewan Agriculture Development Fund (Project
20080205) and Genome Canada.
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