Vitamin A is structurally related to carotene. Carotene
is converted into vitamin A in the liver. Two molecules of vitamin
A are formed from on molecule of beta carotene.
Oxidation: If you compare the two molecules, it is
clear that vitamin A (retinol) is very closely related to half
of the beta-carotene molecule. One way in which beta-carotene
can be converted to vitamin A is to break it apart at the center
and is thought to be most important biologically. The breakdown
of beta-carotene occurs in the walls of the small intestine
(intestinal mucosa) and is catalyzed by the enzyme beta-carotene
dioxygenase to form retinal.
Trans-Retinal - Chime
in new window
Reduction Reaction: The retinal reduced to retinol
by retinaldehyde reductase in the intestines. This is the reduction
of an aldehyde by the addition of hydrogen atoms to make the
alcohol, retinol. See the graphic
in new window of this reduction.
Esterification Reaction: The absorption of retinol
from the alimentary tract is favored by the simultaneous absorption
of fat or oil, especially if these are unsaturated. Retinol is
esterified to palmitic acid and delivered to the blood via chylomicrons.
Finally the retinol formed is stored in the liver as retinyl
esters. See the graphic
in new window of this ester reaction (alcohol, retinol plus
acid, palmitic acid yields ester, retinyl palmitate).
Retinyl Palmitate - Chime
in new window
This is why cod liver oil used to be taken as a vitamin A
supplement. It is also why you should never eat polar bear liver
if you run out of food in the Arctic; vitamin A is toxic in excess
and a modest portion of polar bear liver contains more than two
Beta-carotene, on the other hand, is a safe source of vitamin
A. The efficiency of conversion of beta-carotene to retinol depends
on the level in the diet. If you eat more beta-carotene, less
is converted, and the rest is stored in fat reserves in the body.
So too much beta-carotene can make you turn yellow, but will
not kill you with hypervitaminosis.