milk, and eggs contain vitamin A itself, a cleavage product of β-carotene. It is necessary
for the growth of young animals since it regulates bone-cell formation and the shape of
bones. It also plays a vital role in vision, where cis-retinal (8.53), an isomer of retinol,
is formed upon irradiation with light and becomes attached to the protein rhodopsin as
a Schiff base. Light energy is thus transformed into atomic motion, resulting in hyper-
polarization of the plasma membrane of the rods in the retina. A single photon can
block the Na+permeability of rod membranes, amplifying the response a million fold.
In general metabolism, vitamin A also influences glycogen synthesis.
Avitaminosis Aresults in the loss of night vision (nyctalopia). Furthermore, the
removal of vitamin A from the diet causes the cornea of the eye to “dry out” (xeroph-
thalmia). However, excessive intake of vitamin A can result in severe and even fatal
toxicity.
The past twenty years have witnessed considerable progress in the synthesis and use
of other retinoid-like molecules related to vitamin A. The aromatic retinoid etretin
(8.54) and its ester etretinate (8.55) had some effectiveness in the treatment of psoriasis,
a disorder of skin. 13-cis-Retinoic acid (isotretinoin) produces sebaceous gland atrophy
and could prove useful in the treatment of severe acne vulgaris. Although these com-
pounds have toxic side effects and are not in regular use, they have opened up new ther-
apeutic possibilities. Retinoic acid (tretinoin,8.56) has been employed in the treatment
of acne.
ENDOGENOUS MACROMOLECULES 509