Download LECTURE NOTES: Vitamin A and Carotenoids

VITAMIN A AND CAROTENOIDS:
Complex biochemistry and metabolism
of a highly-regulated nutrient
Oct 8, 2014
BIOCHEMISTRY OF VITAMIN A
Chief functions of vitamin A
-The aldehyde, also know as retinal, is needed for the function
of the light-sensitive cells in the retina (called: photoreceptors)
-The acid form, also known as retinoic acid, is needed to
regulate the activity of MANY genes, perhaps thousands.
one famous retinoic-acid dependent gene is MUCIN,
a protein needed for form a healthy cornea in the eye
Curiously, vitamin A deficiency effects both main components
of the eye: the retina, and the cornea!
VITAMIN A IS A FAT-SOLUBLE VITAMIN:
-stored in fat droplets in cells
-body can accumulate > 1 year supply
-absorption from diet much better if there is
also fat in the diet
-in the United States, both deficiency and toxicity occur
-worldwide, there is a great deal of vitamin A deficiency:
one of the world’s major nutrition problems
HISTORY OF THE DISCOVERY OF VITAMIN A
McCollum and others found during the period
1910-1920 if animals were fed highly purified diets,
the addition of egg-yolk, or of butterfat, was necessary
to maintain health.
Osborne and Mendel (1916) later found that codliver oil also supplied this nutrient (cod liver oil
does not have any carotene).
This led to the conclusion that a trace nutrient
(vitamin) was present in some dietary fats.
The active principle was named in 1913 by
McCollum: FAT-SOLUBLE VITAMIN A
•1919 Steenbock found that the bright-yellow
carotene fraction of plants had vitamin A activity.
Steenbock therefore argued that carotene was the
source of dietary vitamin A.
1929 Moore demonstrated the conversion of
carotene to vitamin A, by showing with chemical
methods that vitamin A accumulated in the liver of
a chick fed only carotene mixed with vegetable oil.
This data, from 1919, was important in
showing that SOME component in the
carrot was a source of vitamin A.
Each of these, in some way, contributes
to activity of “vitamin A”.
WHERE DO WE GET VITAMIN A IN THE DIET:
•
Dairy products (as retinyl esters). Vitamin A
deficiency does not occur where people drink
milk, or consume cheese or butter
•
Green and yellow vegetables (spinach, sweet
potatoes, carrots). We convert the beta-carotene
to vitamin A, in the digestive tract
•
Vitamin pills, and vitamin A added to foods,
such as breakfast cereal
These carotenoids all have vitamin A activity.
ZEAXANTHIN
LUTEIN
Do these carotenoids have
vitamin A activity?
In small intestine
Much of our vitamin A comes from the cleavage of β-carotene.
The conversion of β-carotene to vitamin A only creates safe amounts –
excess dietary vitamin A is harmful, but excess
dietary β-carotene only turns your skin a bit orange!
This form of vitamin A is:
-added to the diet as a vit A supplement
-used to transport vitamin from intestine to liver
-the formed stored in liver and other tissues
What change must occur, for humans to use retinyl
palmitate as a form of vitamin A?
OVERVIEW OF VITAMIN A METABOLISM:
PART 1
DIETARY
CONSTITUENTS
B-carotene
Retinol
Partly cleaved
to Retinol
(5%-20%)
Retinol:
~80% absorbed
into cells that line
ihe intestine
Retinyl
palmitate
Enzyme hydrolysis
Converted to
Retinyl palmitate,
excreted to
lymphatics
Transported
to liver
>75% of total
body vitamin A
stored in liver,
mostly as
retinyl palmitate
LIVER storage
~ 100 mg
OVERVIEW OF VITAMIN A METABOLISM:
PART 2
Stored as
Retinyl Palmitate
LIVER storage
~ 100 mg
Excreted to plasma as Retinol,
carried in
Retinol Binding Protein (RBP)
Absorbed by the retina, and
converted to Retinal
Retinal forms part of rhodopsin,
a light-sensitive protein that
allows you to see!
Much plasma Retinol
recycled back
to liver
Absorbed by skin, mucosa,
other tissues, converted
to Retinoic Acid
Retinoic acid directs
gene expression needed
for healthy skin and
mucous membranes
This diagram from the text provides some sense
of the COMPLEXITY of vitamin A biochemistry.
There are many BINDING proteins in the cell.
Mysteries of vitamin A regulation (there are MANY):
Plasma vitamin A is carried on a binding-protein,
called PLASMA RETINOL BINDING PROTEIN.
Levels in healthy adults are held at 50-60 micrograms/100ml.
What sets that plasma level? We don’t know that mechanism.
The same level in plasma is seen for intakes between 0.5 mg/day
and 5 mg/day.
In the liver, storage can vary from 50-250 mg total.
Your tissues use up about 500 micrograms/day or less.
For Americans the hepatic stores are usually enough
for 1 year or longer.
The Eye: Vitamin A is critical!
These light-sensitive cells require
presence of RETINAL.
RETINAL: Cis-Form
Signals to
brain
RETINAL: Trans-FORM
The Cis-Retinal is part of the protein RHODOPSIN
(the visual pigment). When light causes the conversion
to Trans-Retinal, the affects the protein, which then initiates
signal transduction to the optic nerve.
All-trans-RETINOL (vitamin A alcohol) circulates in the blood,
and is delivered to tissues carried on a binding protein.
In the tissues, it’s converted to RETINAL, which is used
for function of the photoreceptors.
The conversion requires: NAD+, and the enzyme
retinol dehydrogenase.
ZINC IN THE BIOCHEMISTRY OF VITAMIN A
MANY of the enzymatic transformations, such
as packaging vitamin A into binding proteins,
require zinc.
Zinc deficiency often leads to defects in the
functions of vitamin A-dependent mechanisms.
The specific details are being clarified. This
illustrates the interdependence of different
micronutrients for total function.
The whole cycle is very complex!
Fig. 10-7, p. 383
Active in the
retina
Active in control
of gene activity
LACK OF RETINAL FOR THE PHOTORECEPTORS
CAUSES AN EARLY SYMPTOM OF VITAMIN A
DEFICIENCY;
IMPAIRED DARK ADAPTATION
The rod photoreceptors, critical for night vision,
are very sensitive to deprivation of RETINAL.
BOTH FORMS OF VITAMIN A ARE NEEDED FOR VISION
COMPONENT OF THE
PHOTORECEPTOR
PROTEIN (RHODOPSIN)
IN THE RETINA
NEEDED TO ACTIVATE
EXPRESSION OF
MANY GENES, INCLUDING
GENES IN THE CORNIA
WITHOUT RETINOIC ACID, EPITHELIA TENDS TO
REMAIN IN THE SQUAMOUS FORM.
ON THE CORNEA, THIS LEADS TO DRY
SCALY TISSUE AND IRREVERSIBIBLE
SCARRING OF THE CORNEA.
THIS IS A MAJOR DISEASE THAT CAN
RESULT FROM VITAMIN A DEFICIENCY.
One of the proteins that requires retinoic acid
for production of its mRNA is mucin…
If mucin is not made, the cornea becomes
more and more like SKIN.
DIFFERENT TYPES OF EPITHELIA
TYPICAL OF SKIN CELLS:
DRY AND SCALY
TYPICAL OF CORNEA:
MUCUS-SECRETING
Retinoic acid is required
for this cell type to develop.
During 15-20 weeks
on vitamin A deficient,
expression of mRNA
for mucin5 and ASGP
declines in the cornea
of rats.
These proteins play an
essential role in maintaining
the normal mucous
epithelium of the cornea.
The paper on bypass surgert deals with some
aspectof vitamin A deficiency, following a
procedure to reduce the size of the stomach and
intestine.
Homework assgnment: Do an Internet search,
for the number of bypass operations done
in the US each year.
Are patients with bypass now the most important
group of nutritionally-deficient Americans?
Website, Britain: Cystic Fibrosis Medicine
http://www.cfmedicine.com/htmldocs/CFText/vitamins.htm
Malabsorption of fat soluble vitamins is likely in most patients with CF,
particularly those who are pancreatic insufficient. Biochemical evidence of fat
soluble vitamin deficiency has been found by two months of age in untreated
screened infants with CF (Sokol et al, 1989; Feranchak et al, 1999). Patients
should have plasma levels checked annually (Borowitz et al, 2002; Cystic
Fibrosis Trust, 2002; Sinaasappel et al, 2002) and receive supplementation
with the fat soluble vitamins A, D and E.
The recommended daily supplements of the fat soluble vitamins for
pancreatic insufficient patients are:
• Infants: Vitamin A 4,000 IU (1,200 mcg), vitamin D 400 IU (10 mcg) and
vitamin E 37-75 IU (25-50 mg)
• Children over 1 year of age: Vitamin A 4,000 -10,000 IU (1,200 -3,000 mcg),
vitamin D 400 -800 IU (10-20mcg) and vitamin E 150-300 IU (100-200 mg)
• Adolescents and adults: Vitamin A 4,000 -10,000 IU (1200-3,000 mcg), vitamin
D 800-2,000 IU (20-50mcg) and vitamin E 150-300 IU (100-200 mg)
WHAT’S GOING ON? The pancreas often does not work well!
RETINOIC ACID INTERACTS WITH A DORMANT BINDING PROTEIN TO
ACTIVATE SOME GENES (Such as Mucin)
On this graph, Gene #2 is activated by retinoic acid.
(However, Gene#1 remains dormant.)
GENE #1
REGULATORY
DOMAIN #1
REGULATORY
DOMAIN #2
GENE #2
(FOR MUCIN)
RA
+
Retinoic acid
ACTIVATED BINDING PROTEIN
(called: Transcription factor)
Dormant binding
protein
(abundant in cell)
RA
GENE #2
GENE #1
REGULATORY
DOMAIN #1
REGULATORY
DOMAIN #2
mRNA IS MADE AFTER BINDING
OF ACTIVATED BINDING PROTEIN
Mucin expression dependent on these pathways.
mRNA for
protein
To Mucin
synthesis
How does vitamin A deficiency affect vision?
-What is the early symptom (mild deficiency)?
-What is the late symptom (severe deficiency)?
Vitamin A Deficiency and Toxicity
VITAMIN A UNITS: The tough reality
One IU of vitamin A is 0.3 micrograms
Some papers report IU, other papers report
units. Be able to convert, as needed!
BECAUSE OF OUR POLICY OF ADDING VITAMIN A
(USUALLY, RETINYL PALMITATE) TO MOST
DAIRY PRODUCTS, VITAMIN A DEFICIENCY
DOES NOT EXIST IN THE US IN PEOPLE WITH
NORMAL GI-TRACT FUNCTION
IN FACT..WE ARE CONCEIVABLY MORE IN A SITUATION
OF BEING OVERLOADED WITH VITAMIN A!
THIS MAY ACTUALLY BE RESPONSIBLE FOR
EXCESS SKELETAL FRACTURES. THERE HAS BEEN
A SHIFT AWAY FROM SUPPLEMENTING WITH
RETINOL COMPOUNDS, AND TOWARD SUPPLEMENTING
WITH β-CAROTENE, WHICH IS NON-TOXIC.
Risk ratios, ANY fracture, and serum vitamin A
Michaellson et al, New Eng J. Med, 2003
DIETARY VITAMIN A INTAKE (2 mg/day = 6,500 units/day)
SEEMED TO BE ASSOCIATED WITH FRACTURE RISK.
This dose would be considered harmless, by earlier guidelines.
There was no risk with higher intakes of β-carotene.
Serum vitamin A and hip fracture risk:
Opotowsky, Amer J. Med, 2004
Lowest
Highest
http://www.youtube.com/watch?v=xUgfuWmNnu0
Therapy with 9-cis-retinoic acid (Isotretinoin).
Week 1
Week 16
High-dose 13-cis-retinoic acid is EFFECTIVE because
it sends cells into a differentiated phenotype.
The cells in the skin are dividing very rapidly, and
this produces “acne pustules”.
The retinoic acid effect is to make the cells more stable
and they stop their excessive rate of division.
The Wikipedia article is very useful.
http://en.wikipedia.org/wiki/Isotretinoin
Only one form of vitamin A, shown here, can be consumed
at high doses, with no harmful side effects.