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DRUG METABOLISM AND DISPOSITION
Copyright © 2001 by The American Society for Pharmacology and Experimental Therapeutics
DMD 29:513–516, 2001
Vol. 29, No. 4, Part 2
290111/893760
Printed in U.S.A.
INTOLERANCE TO LACTOSE AND OTHER DIETARY SUGARS
DALLAS M. SWALLOW, MARK POULTER,
AND
EDWARD J. HOLLOX
Medical Research Council Human Biochemical Genetics Unit, Galton laboratory, Department of Biology, London, United Kingdom
This paper is available online at http://dmd.aspetjournals.org
ABSTRACT:
cies. This article reviews this topic and discusses in more detail the
current state of our own research on lactase.
Dietary carbohydrates are of two kinds: polysaccharides, such as
starches, and sugars, that is, di- and monosaccharides. Starches from
plants make up three-quarters of dietary carbohydrates and are composed of two structurally different polysaccharides, amylose, which is
a linear ␣ 1-4-linked D-glucose polymer, and amylopectin, with additional ␣ 1-6 linkages, which give it a branched structure. Digestion
commences with salivary and pancreatic amylases (EC 3.2.1.1). Alpha-amylase produces linear maltose (glucose ␣ 1-4-glucose) and
isomaltose (glucose ␣ 1-6-glucose) oligosaccharides as well as some
larger oligomers. The final hydrolysis of these occurs in the small
intestine where the brush-border membrane maltase-glycoamylase
(EC 3.2.1.20, 3.2.1.3) and sucrase-isomaltase (EC 3.2.1.48, 3.2.1.10)
convert them to glucose, which is taken into the absorptive cells by the
sodium-dependent glucose transporter SGLT11. Sucrase-isomaltase
digests all the sucrose and 80% of the dietary maltose, while maltaseglucoamylase digests the remaining maltose and alone digests the
glucose oligomers.
Dietary disaccharides include lactose, the main carbohydrate in
human and other mammalian milks, sucrose from sugar beet and sugar
cane, and a rather rarer dietary component trehalose (glucose ␣
1-1-glucose), which is present in mushrooms, insects, and certain
seaweeds. These are hydrolyzed by lactase, sucrase, and trehalase,
respectively, before transport of the component monosaccharides by
the brush-border sugar transporters. The bulk of the molecule of each
of the hydrolases projects into the lumen of the intestine, although the
mode of anchorage into the membrane differs. The transporters in
contrast span the membrane.
Dietary monosaccharides are present principally in fruits and are
transported directly by the brush-border membrane transporters. Fructose,
which is also one of the products of sucrose digestion, is transported by
GLUT5, whereas glucose and galactose are transported by SGLT1.
There are thus several molecules at the apical surface of the small
intestinal absorptive cells, which are of critical importance to normal
carbohydrate digestion (Tables 1 and 2) (Levin, 1994). Reduced
function of any one of these leads to the passage of undigested
carbohydrate into the bowel where it is fermented by bacterial flora to
form short-chain fatty acids and gases. While fatty acid absorption
increased by colonic fermentation is thought by some to be beneficial,
excessive production of gases leads to flatulence, and the osmotic
effect of di- and monosaccharides in the luminal contents leads to
diarrhea.
Tables 1 and 2 list these hydrolases and transporters and the known
genetic deficiencies relating to them. These genetic deficiencies vary
dramatically in frequency. Congenital glucose/galactose malabsorption, due to deficiency of SGLT1, which transports both the glucose
and galactose produced by lactose digestion, is certainly rare. Congenital alactasia is very rare indeed, although a cluster of cases has
been reported in Finland, where this is one of the “Finnish recessive
disorders”. Both are very severe conditions, which can be fatal if not
diagnosed immediately, since there is onset of symptoms as soon as
milk is first consumed. In contrast, adult lactase deficiency or lactase
nonpersistence is more common than lactase persistence and represents a genetically determined polymorphism in human populations
that involves developmental regulation. It is uncertain how frequently
genetic deficiencies of sucrase-isomaltase, trehalase, and maltaseglucoamylase occur. In most cases half levels of the normal protein,
as present in heterozygotes, are sufficient to deal with the dietary load,
but there are several indications that this is not always the case.
In the first part of the article current information on the proteins
involved in digestion and transport is summarized. In the second part
current progress on lactase is reviewed since lactase deficiency is
perhaps the best understood and is the subject of our own research.
1
Abbreviations used are: SGLT, sodium-dependent glucose transporter; SI,
sucrase-isomaltase.
Send reprint requests to: Dallas Swallow, MRC Human Biochemical Genetics
Unit, Galton Lab, Dept. of Biology, Wolfson House, 4 Stephenson Way, London
NW1 2HE, UK. E-mail: [email protected]
Properties of the Enzymes and Transporters
Sucrase-Isomaltase. The human sucrase-isomaltase gene (SI) on
chromosome 3 encodes an mRNA transcript of 5.48 kb and a 1827amino acid peptide precursor (Chantret et al., 1992). Sucrase-isomaltase is a two-active site molecule, synthesized as a single polypeptide
chain (pro sucrase-isomaltase), which is cleaved by a pancreatic
protease to form the two subunits with different substrate specificity
(Semenza et al., 2000). The two subunits remain associated, and only
one subunit of the dimer is attached to the brush-border membrane by
its N-terminal end. These heterodimers themselves dimerize to form a
tetramer, which is the active catalytic unit localized in the brushborder membrane.
Sucrase-isomaltase deficiency is probably not uncommon. Deficiency of SI has been found in about 2% of diagnostic biopsies in
London (A. D. Phillips, personal communication) and in the USA
(Ringrose et al., 1979). In the Inuit from Greenland it was detected in
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Intolerance of dietary carbohydrate and sugars can result from a
variety of genetically determined enzyme and transporter deficien-
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SWALLOW ET AL.
TABLE 1
Brush-border disaccharidases and their deficiencies
Di- and Polysaccharide
a
Gene Locusa
Enzyme
Deficiencies
Lactose
Lactase
LCT
Sucrose Maltose Isomaltose
Maltose
Terminal alpha 1-4 linkages on polysaccharide
Trehalose
Sucrase-isomaltase
Maltase-glucoamylase
SI
MGAM
Congenital alactasia
Lactase nonpersistence
Congenital sucrase deficiency
Congenital maltase—glucoamylase deficiency
Trehalase
TREH
Trehalase deficiency
For official gene symbol see http://www.gene.ucl.ac.uk/cgi-bin/nomenclature/searchgenes.pl for Online Mendelian Inheritance in Man (OMIM) and other literature and sequence links.
TABLE 2
Brush-border carbohydrate transporters and their deficiencies
Transporter
Gene Locusa
Deficiencies
Glucose
Galactose
Glucose
Fructose
SGLT1 sodium-dependent glucose transporter
SLC5A1
Glucose/galactose malabsorption
GLUT5 glucose transporter 5
SLC2A5
? Hereditary
Fructose malabsorption
For official gene symbol see http://www.gene.ucl.ac.uk/cgi-bin/nomenclature/searchgenes.pl for Online Mendelian Inheritance in Man (OMIM) and other literature and sequence links.
10% of the population (McNair et al., 1972), where it was a significant
clinical problem because of excessive feeding of sugar to babies;
furthermore, there are reports of reduced SI in black South Africans,
which suggests the possibility of activity polymorphism (Veitch et al.,
1998). Studies on the protein suggest that various different mutations
cause the deficiency, but so far only one exonic mutation has been
identified, which results in substitution of proline by a glutamine at
amino acid 1098 (Ouwendijk et al., 1996), but it is not known whether
it is this mutation that occurs at high frequency. Severity of the
condition depends directly on the dietary sucrose intake, and in the
case of the Inuit the severity led to failure to thrive before this
condition had been understood.
Maltase-Glucoamylase. Maltase-glucoamylase is encoded by a
gene, MGAM, located on chromosome 7. The mRNA transcript is
6513 bp, and the calculated molecular weight of the protein (before
glycosylation) is 209,702 (Nichols et al., 1998). This protein probably
forms homodimers, and the complex is anchored to the brush-border
membrane by the N terminus of the polypeptides.
Analysis of the human maltase-glucoamylase sequence shows that
the catalytic sites are identical to those in sucrase-isomaltase, but
overall the protein sequence is only 59% identical.
Lactase. Lactase-phlorizin hydrolase (EC 3.2.1.23, 3.2.1.62) encoded by a gene LCT on chromosome 2 is present in the small
intestine of most mammals and is responsible for the hydrolysis of the
disaccharide lactose into its two constituent monosaccharides glucose
and galactose (Swallow and Hollox, 2000). Lactose is only present in
high quantities in milk, but is thus essential for suckling mammals.
Lactase has two activities: a ␤-galactosidase activity hydrolyzing
lactose and a ␤-glucosidase activity (Wacker et al., 1992) responsible
for hydrolyzing phlorizin, a disaccharide found in roots and bark of
plants of the family Rosaceae and some seaweeds. Lactase is synthesized as a pro-polypeptide of Mr 245,000 that is glycosylated and is
proteolytically cleaved inside the cell to form the mature enzyme with
an apparent molecular weight of 150,000. It dimerizes on the brushborder membrane to form the active enzyme. The cleaved polypeptide
has no apparent enzymatic function, but it may function as a molecular chaperone (Oberholzer et al., 1993; Naim et al., 1994). Lactase
has a C-terminal membrane-spanning domain.
Trehalase. A human mRNA transcript of 2 kb encodes a trehalase
(EC 3.2.1.28) protein with a calculated molecular weight of 66,595,
which is expressed in kidney and liver as well as intestine (Ishihara et
al., 1997). The gene TREH has not been mapped to a human chro-
mosome to date. Studies using in vitro transcribed rabbit trehalase
mRNA injected into Xenopus laevis oocytes and treatment with phospholipase C suggested that, unlike lactase, sucrase-isomaltase, and
maltase-glucoamylase, trehalase is attached to the brush-border membrane by a phosphatidylinositol anchor (Ruf et al., 1990). Trehalase
deficiency is thought to be an autosomal recessive trait and has been
reported at a frequency of 8% in Greenland (McNair et al., 1972), but
has been reported in a parent and child and three other close relatives
(Madzarovova-Nohejlova, 1973). Given the unusual distribution of
the disaccharide, deficiency has not been of any nutritional consequence until recently, but it may become a problem because of its
introduction as a sweetener in foods.
Sodium-Dependent Glucose Transporter (SGLT1). SGLT1 is
encoded by the gene SLC5A1 located on chromosome 22. It is a
protein with 13 membrane spanning domains and is the major apical
transporter of glucose and galactose. Deficiency of SGLT1 leads to
congenital glucose/galactose malabsorption. Many different mutations have been found.
Glucose Transporter GLUT5. GLUT5 is an apical glucose transporter whose main role seems to be the transport of fructose (Levin,
1994). It also has many trans-membrane domains, and it is encoded
by the gene SLC2A5.
Glycosylation. The brush-border hydrolases and transporters are all
glycosylated, and in the case of lactase and sucrase-isomaltase there is
not only evidence for both N- and O-glycosylation, but also person to
person difference in terminal glycosylation since both proteins carry
the ABH blood group antigens (Green et al., 1988).
Hereditary Fructose Intolerance
Intolerance of dietary fructose shows two quite different clinical
presentations. Fructose malabsorption with the classical symptoms of
diarrhea and flatulence was thought to be due to a transporter defect,
but sequencing revealed no mutations in the coding sequence of
GLUT5 (SLC2A5) (Wasserman et al., 1996). A somewhat more
common condition manifests as recurrent hypoglycemia and vomiting, failure to thrive, and kidney and liver involvement. In this
condition, deficiency of fructose phosphate aldolase (aldolase B) has
been found. Aldolase catalyzes the conversion of fructose-1-phosphate to dihydroxyacetone phosphate and D-glyceraldehyde, and is
thus essential for the metabolism of dietary fructose, which is transported to the liver from the enterocytes via the basolateral glucose
transporter GLUT2. A number of different mutations leading to amino
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a
Monosaccharide
INTOLERANCE TO LACTOSE AND OTHER DIETARY SUGARS
FIG. 1. Diagrammatic representation of the level of lactase expression at
different stages of development and in lactase persistent and
nonpersistent adults.
The levels of activity are mainly regulated at the RNA level.
The Molecular Basis of the Lactase Persistence/Nonpersistence
Polymorphism
The expression of lactase is polymorphic in adult humans (Swallow
and Hollox, 2000). This is an unusual genetically determined regulatory polymorphism with large differences in allele frequency in human populations. Lactase activity is necessary for obtaining full
nutritional benefit from both human and animal milk because it is
needed for digestion of the lactose, which is the major carbohydrate in
milk. Lactase activity is thus vital in baby mammals. It shows tight
control of developmental expression, being expressed at low levels in
fetal life and increasing around birth, and it is only expressed in
small-intestinal enterocytes (Wang et al., 1998). Lactase declines after
weaning in most adults but persists into adult life in many others (Fig.
1). Lactase persistence tends to be the most frequent phenotype in
populations where fresh milk forms a significant part of the adult diet;
for example, Northern Europeans and pastoral nomadic tribes. Lactase-nonpersistent adults can usually consume only limited amounts
of fresh milk without experiencing flatulence and diarrhea. The very
high frequency of the lactase persistence allele in certain populations
probably results from selection for milk drinking, due the nutritional
value of milk.
We have shown that the lactase persistence/nonpersistence polymorphism is controlled by a cis-acting regulatory element (Fig. 2) and
have provided direct evidence of down-regulation of one transcript in
heterozygous children (Wang et al., 1995, 1998) by examining the
level of expression of both transcripts in individuals heterozygous for
exonic polymorphisms. Our hypothesis is that genetic differences in a
sequence element, in the vicinity of the lactase gene, cause it to
interact differentially with a developmentally regulated trans-acting
protein(s).
We have studied 70-kb haplotypes across the lactase gene in
different human populations (Hollox et al., 2001). The results show
marked linkage disequilibrium across this region, although with population differences and much greater haplotype diversity in African
populations. One particular haplotype, A, is associated with lactase
persistence and is at very much higher frequency in Northern Europe
than any other population (Harvey et al., 1998) (Fig. 3), suggesting a
selective sweep by the linked alleles responsible for the phenotypic
polymorphism. We have characterized a hypervariable region immediately upstream from the lactase gene and have shown evidence of
variable binding to a possible trans-acting protein (Hollox et al.,
1999), although this region does not correspond to the critical causative element.
Analysis of 12 kb of upstream sequence showed no polymorphism
that associates exclusively with persistence (Boll et al., 1991; Lloyd et
al., 1992; M. Poulter, E. J. Hollox, and D. M. Swallow, unpublished
observations). Thus, to search the sequence responsible for this novel
cis-acting mechanism, we have constructed a 700-kb contig (700-kb
region covered by overlapping clones) centered on the lactase gene,
and the sequences at the end of each clone or “sequence tag connectors” are being used to search for single nucleotide polymorphisms.
These are being used to define the region of maximal association and
subdivide the haplotypes, which will allow eventual identification of
the causal element.
The locus responsible for Finnish congenital alactasia has also been
genetically mapped 5⬘ of LCT (Jarvela et al., 1998). It is possible that
this disorder involves disruption of the same element that controls the
developmental down-regulation of lactase.
FIG. 2. Diagram showing the likely mechanism of the cis-acting polymorphism in determining lactase expression in persistent and nonpersistent adults.
A developmentally regulated DNA binding protein may interact with a cis-element present on one allele and not the other (due to sequence differences within the
elements).
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acid substitutions have been found, but a single mutation is found at
a heterozygosity of more that 0.01 in the UK, for example (Ali et al.,
1998). The high incidence of the deficiency alleles suggests that
before the recent increase in intake of dietary fructose there was little
selective disadvantage for this allele. The condition varies in its
clinical severity according to the dietary intake of fructose, and
removal of fructose from the diet prevents the disease. It is of interest
to note that in patients who have consciously or unconsciously
avoided fructose there is a remarkable absence of dental caries.
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The numbers represent the published lactase persistence allele frequencies (see Swallow and Hollox, 2000).
Conclusion
There are many examples of intolerance of dietary saccharides.
These intolerances are not usually life threatening because symptoms
can be avoided by removal of the offending sugar from the diet,
although congenital lactase deficiency and glucose-galactose malabsorption can be fatal if not diagnosed quickly. The presence of some
these deficiencies at relatively high frequencies in some populations is
of evolutionary interest in relation to changes in human diet, and the
significance of homozygosity or heterozygosity for deficiency alleles
to human nutrition and health is an area that is relatively unexplored.
Acknowledgments. We thank M. Zvarik and V. Ferak (Bratislava);
A. Krause and T. Jenkins (Witwatersrand, South Africa); A. Koslov
(Moscow); and N. Saha, Singapore, for their contribution to the work
on population samples, which will be published in detail elsewhere.
References
Ali M, Rellos P, and Cox TM (1998) Hereditary fructose intolerance. J Med Genet 35:353–365.
Boll W, Wagner P and Mantei N (1991) Structure of the chromosomal gene and cDNAs coding
for lactase-phlorizin hydrolase in humans with adult-type hypolactasia or persistence of
lactase. Am J Hum Genet 48:889 –902.
Chantret I, Lacasa M, Chevalier G, Ruf J, Islam I, Mantei N, Edwards Y, Swallow D and Rousset
M (1992) Sequence of the complete cDNA and the 5⬘ structure of the human sucraseisomaltase gene. Possible homology with a yeast glucoamylase. Biochem J 285:915–923.
Green FR, Greenwell P, Dickson L, Griffiths B, Noades J, and Swallow DM (1988) Expression
of the ABH, Lewis and related antigens on the glycoproteins of the human jejunal brush
border, in Subcellular Biochemistry (Harris JR ed) pp 119 –153, Plenum Press, New York.
Harvey CB, Hollox EJ, Poulter M, Wang Y, Rossi M, Auricchio S, Iqbal TH, Cooper BT, Barton
R, Sarner M, et al. (1998) Lactase haplotype frequencies in Caucasians: Association with the
lactase persistence/non-persistence polymorphism. Ann Hum Genet 62:215–223.
Hollox EJ, Poulter M, Wang Y, Krause A, and Swallow DM (1999) Common polymorphism in
a highly variable region upstream of the human lactase gene affects DNA-protein interactions.
Eur J Hum Genet 7:791– 800.
Ishihara R, Taketani S, Sasai-Takedatsu M, Kino M, Tokunaga R and Kobayashi Y (1997)
Molecular cloning, sequencing and expression of cDNA encoding human trehalase. Gene
202:69 –74.
Jarvela I, Sabri Enattah N, Kokkonen J, Varilo T, Savilahti E and Peltonen L (1998) Assignment
of the locus for congenital lactase deficiency to 2q21, in the vicinity of but separate from the
lactase-phlorizin hydrolase gene. Am J Hum Genet 63:1078 –1085.
Levin RJ (1994) Digestion and absorption of carbohydrates—from molecules and membranes to
humans. Am J Clin Nutr 59:690S– 698S.
Lloyd M, Mevissen G, Fischer M, Olsen W, Goodspeed D, Genini M, Boll W, Semenza G and
Mantei N (1992) Regulation of intestinal lactase in adult hypolactasia. J Clin Invest 89:524 –
529.
Madzarovova-Nohejlova J (1973) Trehalase deficiency in a family. Gastroenterology 65:130 –133.
McNair A, Gudman Hoyer E, Jarnum S and Orrild L (1972) Sucrose malabsorption in Greenland.
Br Med J 2:19 –21.
Naim HY, Jacob R, Naim H, Sambrook JF, and Gething M-J H (1994) The pro-region of human
intestinal lactase-phlorizin hydrolase. J Biol Chem 269:26933–26943.
Nichols BL, Eldering J, Avery S, Hahn D, Quaroni A and Sterchi E (1998) Human small
intestinal maltase glucoamylase cDNA cloning homology to sucrase isomaltase. J Biol Chem
273:3079 –3081.
Oberholzer T, Mantei N and Semenza G (1993) The pro sequence of lactase-phlorizin hydrolase
is required for the enzyme to reach the plasma membrane: An intramolecular chaperone?
FEBS Lett 333:127–131.
Ouwendijk J, Moolenaar CEC, M, Peters WJ, Hollenberg CP, Ginsel LA and Fransen JAM
(1996) Congenital sucrase-isomaltase deficiency. Identification of a glutamine to proline
substitution that leads to a transport block of sucrase-isomaltase. J Clin Invest 97:6331– 6341.
Ringrose RE, Preiser H, and Welsh JD (1979) Sucrase-isomaltase (Palatinase) deficiency
diagnosed during adulthood. Dig Dis Sci 25:384 –387.
Ruf J, Wacker H, James P, Maffia M, Seiler P, Galand G, von Kieckebusc A, Semenza G and
Mantei N (1990) Rabbit small intestinal trehalase. Purification, cDNA cloning, expression, and
verification of glycosylphosphatidylinositol anchoring. J Biol Chem 265:15034 –15039.
Semenza G, Auricchio S, and Mantei N (2000) Small intestinal disaccharidases, ch. 75, in The
Metabolic and Molecular Basis of Inherited Disease (Scriver CR, Beaudet AL, Sly WS and
Valle D eds) vol 1, pp 1623–1650, McGraw-Hill, New York.
Swallow DM and Hollox EJ (2000) The genetic polymorphism of intestinal lactase activity in
adult humans, ch. 76, in The Metabolic and Molecular Basis of Inherited Disease, 8th ed
(Scriver CR, Beaudet AL, Sly WS and Valle D eds) vol 1, pp 1651–1663, McGraw-Hill, New
York.
Veitch AM, Kelly P, Segal I, Soies SK and Farthing MJ (1998) Does sucrase deficiency in black
South Africans protect against colonic disease? Lancet 351:183.
Wacker H, Keller P, Falchetto R, Legler G and Semenza G (1992) Location of the two catalytic
sites in intestinal lactase phlorizin hydrolase: Comparison with sucrase-isomaltase and other
glycosidases, the membrane anchor of lactase phlorizin hydrolase. J Biol Chem 267:18744 –
18752.
Wang Y, Harvey CB, Hollox EJ, Phillips AD, Poulter M, Clay P, Walker-Smith JA and Swallow
DM (1998) The genetically programmed down-regulation of lactase in children. Gastroenterology 114:1230 –1236.
Wang Y, Harvey CB, Pratt WS, Sams VR, Sarner M, Rossi M, Auricchio S and Swallow DM
(1995) The lactase persistence/non-persistence polymorphism is controlled by a cis-acting
element. Hum Mol Genet 4:657– 662.
Wasserman D, Hoekstra JH, Tolia V, Taylor CJ, Kirschner BS, Takeda J, Bell GI, Taub R, and
Rand EB (1996) Molecular analysis of the fructose transporter gene (GLUT5) in isolated
fructose malabsorption. J Clin Invest 10:2398 –2402.
Downloaded from dmd.aspetjournals.org at ASPET Journals on June 17, 2017
FIG. 3. Map of the world to show the distribution in the Old World of the lactase persistence allele (from published lactose tolerance studies) and the LCT haplotypes
A, B, C, and U, from our own unpublished studies.