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 componentsof 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 necessaryto maintain health.

Osborne and Mendel (1916) later found that cod-liver oil also supplied this nutrient (cod liver oildoes 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, contributesto 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.

LUTEIN

ZEAXANTHIN

Do these carotenoids havevitamin A activity?

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!

In small intestine

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 retinylpalmitate as a form of vitamin A?

DIETARYCONSTITUENTS

B-carotene

Partly cleavedto Retinol(5%-20%)

Retinol

Retinol:~80% absorbed

into cells that lineihe intestine

Converted toRetinyl palmitate,

excreted to lymphatics

OVERVIEW OF VITAMIN A METABOLISM: PART 1

Transported to liver

LIVER storage~ 100 mg

>75% of total body vitamin Astored in liver,

mostly asretinyl palmitate

Retinyl palmitate

Enzyme hydrolysis

OVERVIEW OF VITAMIN A METABOLISM: PART 2

LIVER storage~ 100 mg

Excreted to plasma as Retinol, carried in

Retinol Binding Protein (RBP)

Absorbed by the retina, andconverted to Retinal

Retinal forms part of rhodopsin,a light-sensitive protein that

allows you to see!

Absorbed by skin, mucosa,other tissues, converted

to Retinoic Acid

Retinoic acid directs gene expression needed

for healthy skin andmucous membranes

Much plasma Retinolrecycled back

to liver

Stored asRetinyl Palmitate

This diagram from the text provides some senseof 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/dayand 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 enoughfor 1 year or longer.

The Eye: Vitamin A is critical!The Eye: Vitamin A is critical!

These light-sensitive cells requirepresence of RETINAL.

RETINAL: Cis-Form

RETINAL: Trans-FORM

Signals tobrain

The Cis-Retinal is part of the protein RHODOPSIN(the visual pigment). When light causes the conversionto Trans-Retinal, the affects the protein, which then initiatessignal 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 usedfor function of the photoreceptors.

The conversion requires: NAD+, and the enzymeretinol dehydrogenase.

ZINC IN THE BIOCHEMISTRY OF VITAMIN A

MANY of the enzymatic transformations, suchas packaging vitamin A into binding proteins,require zinc.

Zinc deficiency often leads to defects in thefunctions of vitamin A-dependent mechanisms.

The specific details are being clarified. Thisillustrates the interdependence of differentmicronutrients for total function.

Fig. 10-7, p. 383

The whole cycle is very complex!

Active in theretina

Active in controlof gene activity

LACK OF RETINAL FOR THE PHOTORECEPTORSCAUSES AN EARLY SYMPTOM OF VITAMIN ADEFICIENCY;

IMPAIRED DARK ADAPTATION

The rod photoreceptors, critical for night vision,are very sensitive to deprivation of RETINAL.

COMPONENT OF THEPHOTORECEPTOR

PROTEIN (RHODOPSIN)IN THE RETINA

NEEDED TO ACTIVATEEXPRESSION OF

MANY GENES, INCLUDINGGENES IN THE CORNIA

BOTH FORMS OF VITAMIN A ARE NEEDED FOR VISION

WITHOUT RETINOIC ACID, EPITHELIA TENDS TOREMAIN 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 CANRESULT 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.

TYPICAL OF SKIN CELLS:DRY AND SCALY

TYPICAL OF CORNEA:MUCUS-SECRETING

Retinoic acid is requiredfor this cell type to develop.

DIFFERENT TYPES OF EPITHELIA

During 15-20 weekson vitamin A deficient,expression of mRNAfor mucin5 and ASGPdeclines in the corneaof rats.

These proteins play anessential role in maintainingthe normal mucousepithelium 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 importantgroup of nutritionally-deficient Americans?

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!

Website, Britain: Cystic Fibrosis Medicine

http://www.cfmedicine.com/htmldocs/CFText/vitamins.htm

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 GENE #2(FOR MUCIN)REGULATORY

DOMAIN #1REGULATORY

DOMAIN #2

+ Retinoic acid

Dormant binding protein

(abundant in cell)

RA

ACTIVATED BINDING PROTEIN(called: Transcription factor)

RA

GENE #1 GENE #2

REGULATORYDOMAIN #1

REGULATORYDOMAIN #2 mRNA for

proteinmRNA IS MADE AFTER BINDINGOF ACTIVATED BINDING PROTEIN

Mucin expression dependent on these pathways.

To Mucinsynthesis

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 ToxicityVitamin 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 reportunits. Be able to convert, as needed!

BECAUSE OF OUR POLICY OF ADDING VITAMIN A(USUALLY, RETINYL PALMITATE) TO MOSTDAIRY PRODUCTS, VITAMIN A DEFICIENCYDOES NOT EXIST IN THE US IN PEOPLE WITHNORMAL GI-TRACT FUNCTION

IN FACT..WE ARE CONCEIVABLY MORE IN A SITUATION OF BEING OVERLOADED WITH VITAMIN A!

THIS MAY ACTUALLY BE RESPONSIBLE FOREXCESS SKELETAL FRACTURES. THERE HAS BEENA SHIFT AWAY FROM SUPPLEMENTING WITHRETINOL COMPOUNDS, AND TOWARD SUPPLEMENTINGWITH 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

Week 1 Week 16

Therapy with 9-cis-retinoic acid (Isotretinoin).

High-dose 13-cis-retinoic acid is EFFECTIVE becauseit sends cells into a differentiated phenotype.

The cells in the skin are dividing very rapidly, andthis produces “acne pustules”.

The retinoic acid effect is to make the cells more stableand 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 consumedat high doses, with no harmful side effects.