a remarkable number of enzymes, most of which are involved in amino acid metabolism.
By forming a Schiff base, pyridoxal phosphate acts as an electron sink and weakens all
three bonds about the α-carbon of amino acids. It therefore functions in transamination,
decarboxylation, deamination, racemization, and aldol cleavage, and can also influence
theβandγcarbons of an amino acid. Glutamic acid decarboxylase (chapter 4) is also
a pyridoxal enzyme and is involved in GABA synthesis. If it is inhibited by hydrazine-
type compounds through Schiff-base formation, the resulting lack of GABA may lead
to a seizure.
The typical B 6 avitaminosis is very similar to riboflavine and niacin deficiency and
is manifested by eye, mouth, and nose lesions as well as neurological symptoms.
8.2.8.5 Pantothenic Acid
Pantothenic acid (8.48), a hydroxyamide, occurs mainly in liver, yeast, vegetables, and
milk, but also in just about every other food source, as its name implies [pantos(Greek)=
everywhere]. It is part of coenzyme A, the acyl-transporting enzyme of the Krebs cycle
and lipid syntheses, as well as a constituent of the acyl carrier protein in the fatty-acid
synthase enzyme complex.
There is no record of pantothenic acid deficiency in humans, since all food
contains sufficient quantities of this vitamin. Experimentally, however, neurological,
gastrointestinal, and cardiovascular symptoms result from a diet lacking in pantothenic
acid.
8.2.8.6 Biotin
Biotin (8.49), a thiophene-lactam, occurs in yeast, liver, kidney, eggs, vegetables, and
nuts. It functions as a cocarboxylase in a number of biochemical reactions. It binds CO 2
in the form of an unstable carbamic acid on one of the lactam nitrogens. The carbamate
carboxyl is then donated easily.
506 MEDICINAL CHEMISTRY