totally unnecessary and without benefit. Some patients with inflammatory bowel
diseases, such as Crohn’s disease, may experience pernicious anemia if the inflammatory
disease affects their ileum (a portion of the small intestine). In these patients, the uptake
of the vitamin is impaired because they lack the so-called “intrinsic factor”—a gastric
glycoprotein necessary for the absorption of the vitamin. The administration of very
large amounts (milligrams) of vitamin B 12 forces the uptake of sufficient cobalamin to
cover the need, but this is only a treatment and not a cure for pernicious anemia.
8.2.8.8 Vitamin C (Ascorbic Acid)
Vitamin C (8.51, ascorbic acid) is a carbohydrate derivative. It occurs in citrus fruits, green
pepper, and fresh vegetables, including onions, but is rapidly hydrolyzed by cooking. Only
primates and the guinea pig are unable to synthesize ascorbic acid. Vitamin C is necessary
for the hydroxylation of proline to hydroxyproline, reducing the Fe atom in collagen
hydroxylase. Hydroxyproline is a major amino acid in the collagen that is present in all
fibrous tissues, the intracellular matrix, and capillary walls. Vitamin C is also used in
steroid hydroxylation in the adrenal gland, and finds a role in tyrosine metabolism.
In vitamin C avitaminosis, or scurvy,the joints become painful and the gums bleed
and deteriorate, resulting in tooth loss. Gangrene and infections may also occur, and wounds
do not heal properly. These symptoms all result from impaired collagen synthesis and
require 3–4 months to develop.
Increased doses of vitamin C are sometimes given after surgery or burns to promote
healing by increasing collagen synthesis. There is controversy about its purported activ-
ity of alleviating the common cold and other viral diseases when given in extremely
large doses. Experiments with radioisotope-labeled ascorbic acid have shown that the
normal body pool is about 20 mg/kg, which can be maintained by an intake of approx-
imately 100 mg/day (140 mg for smokers). The consumption of several grams per
day—as advocated by believers in “megavitamin” hypotheses—results in the excretion
of excess vitamin C. As opponents of megavitamin administration sometimes cynically
observe, the only obvious effect of megavitamin therapy is the production of brightly
coloured urine!
8.2.8.9 Vitamin A (Retinol)
Vitamin A (8.52, retinol) occurs in plants as the provitamin carotene; it is a highly
unsaturated terpenoid, with all of its double bonds trans(E) to one another. Fish liver oils,
508 MEDICINAL CHEMISTRY