the vitamin, with the result that FAD carries a pair of electrons and two hydrogens. The
monoamine oxidases (see section 8.2.6) are also flavoenzymes.
Vitamin B 2 deficiency is seldom seen, and its symptoms of leg ulceration (cheilosis),
skin symptoms, a purplish tongue, eye disturbances, and photophobia are vague.
8.2.8.3 Nicotinamide and Nicotinic Acid (Niacin)
Nicotinamide (8.45) and nicotinic acid (8.46, niacin)—which have also been referred to
as vitamin B 3 or B 5 —are simple pyridine-3-carboxylic acid derivatives occurring in
liver, yeast, and meat. In the form of nicotinamide-adenine dinucleotide (NAD+) or its
phosphorylated form (NADP+), nicotinamide is the most important electron carrier in
intermediary metabolism. Unlike FAD, it adds a hydride ion (i.e., one pair of electrons
and one hydrogen) only.
Nicotinamide deficiency leads to pellagra[pelle agra(Italian)=rough skin], which
manifests itself in dermatitis (skin rash), pigmentation, a red and inflamed tongue, diar-
rhea, and weakness. People who consume large amounts of corn in an unbalanced diet
are prone to develop the disease.
8.2.8.4 Vitamin B 6 (Pyridoxine)
Vitamin B 6 (8.47, pyridoxine) is a pyridine-alcohol, but its biologically active forms are
pyridoxal 5-phosphate and the corresponding pyridoxamine. Like all the members of the
vitamin B complex, it occurs in yeast, bran, wheat germ, and liver. It is a coenzyme of
ENDOGENOUS MACROMOLECULES 505