There are four lipid vitamins: vitamin A, vitamin D, vitamin E, and vitamin K. Each of them contain rings and long aliphatic (—CH2—) side chains. The lipid vitamins are highly hydrophobic, although each possesses at least one polar group. Ingested lipid vitamins are absorbed in the intestine by a process similar to the absorption of other lipid nutrients. After digestion of any proteins that may bind them, they are carried to the cellular interface of the intestine as micelles formed with bile salts. The study of these hydrophobic molecules has presented several technical difficulties, so research on their mechanisms has progressed more slowly than that on their water-soluble counterparts. Lipid vitamins differ widely in their functions.
Vitamin A, or retinol, is a 20-carbon lipid molecule obtained in the diet either directly or indirectly from β-carotene [Monday's Molecule #40]. Carrots and other yellow vegetables are rich in β-carotene, a 40-carbon plant lipid whose enzymatic oxidative cleavage yields vitamin A.
Vitamin A exists in three forms that differ in the oxidation state of the terminal functional group: the stable alcohol retinol, the aldehyde retinal, and retinoic acid. All three compounds have important biological functions. Retinoic acid is a signal compound that binds to receptor proteins inside cells; the ligand–receptor complexes then bind to chromosomes and can regulate gene expression during cell differentiation. The aldehyde retinal is a light sensitive compound with an important role in vision. Retinal is the prosthetic group of the protein rhodopsin; absorption of a photon of light by retinal triggers a neural impulse.
From Horton et al. Principles of Biochemistry, 4th ed. © 2007, Laurence A. Moran and Pearson/Prentice Hall
[Photo Credit: The picture of the carrots is from The Food Network.]