Re: How does the body interconvert enantiomers of Vitamins A and E?

Hi Penni. A very god question. Actually the body handles isomers of these vitamins differently requiring a seperate explanation for each one. Interconvergence doesn't count in one while only exceptionally in the other.

Vitamin E is a mixture of several similar substances comprised of two groups known as tocopherols and tocotrienols in alpha, beta, delta and gamma forms. Each of these occur in mirror image forms, righthanded (the D form) and left handed (the L form). They all have biological activity but in varying amounts. The alpha form occurs naturally as the d-isomer. It has the greatest biological activity. The left handed isomer is not as biologically active as the d-form and is not absorbed as well either. The synthetic isomer of d-a-tocopherol (used in food supplements) is called dl- alpha tocopherol which contains equal parts of right and left handed stereo isomers. Differences in activity among the various isomers appear to be due primarily to differences in retention times in the body. (Absorption depends on the presence of bile and plasma concentrations are correlated to plasma lipoprotein and lipid concentration.) The biological activity of the d-isomer of alpha tocopherol is twice as potent as the synthetic version, dl-alpha tocopherol. However, the biological activity of all forms of Vitamin E is calculated and defined as International Units (I.U.'s). So for example, a supplement of Vitamin E containing 100 I.U.'s of dl-alpha tocopherol will equal 100 I.U.'s of d-alpa tocopherol.

With regards to Vitamin A, many mono-and di-cis isomers of retinol are found, but are present in fish liver, whereas in humans with the exception for a specific stereoisomerization in the retina, isomers are absent. The term Vitamin A is appled to a nuber of substances with very similar structure and activity. We are only concerned with those structures that are biologically active. The most dominant form of Vitamin A is retinol, called Vitamin A1 alcohol having an all trans configuration and also Vitamin A2 called 3-dehydroretinol which also has the all trans structure. (A2 is usually mixed with A1 but has only 30 to 40% of the biological activity of A1.) Retinal is the aldehyde form of Vitamin A also called retinene 1. Oxidation of retinal is changed to retinoic acid.

The stereoisomerization mentioned in the retina is simplified as follows: All-trans-retinol from the blood is oxidized by alcohol dehydrogenase (with NADP) to retinol which is isomerized in the retina to 11-cis- retinal. This combines with opsin to form rhodopsin (needed for night vision). (The all-trans isomer will not combine with opsin). On exposure to light rhodopsin undergoes a sequence of changes eventually splitting off retinal back into the all-trans configuration. It can undergo isomerizations again for reutilization or be reduced to retinol by alcohol dehydrogenase and returned to the bloodstream.

Beta carotene (known as Provitamin A) has 20 cis-trans isomers but it occurs in nature mainly as the all-trans isomer. B-carotene is converted to retinal which is reduced to retinol. One molecule of B-carotene produces 2 molecules of retinol, but alpha and gamma carotene yields only 1 molecule of retinol because those compounds are not as symmetrical as is B-carotene. Excess is joined with glucuronic acid and is excreted from the body.

Let me leave you with this anecdote. As Alice puzzled over why the little girl in the mirror held her apple in the opposite hand she began to wonder what would happen to this image if she stepped through the looking glass on the opposite side?

Hope that helps. For more go to www.cognis.com/cognis/testveris/vitamin E June02.pdf.

My references were: Van Nostrand's Scientific Encyclopedia

How to Live Longer and Fell Better-Linus Pauling

Chemistry for the Health Sciences-Sackheim and Lehman