Nicotinic acid: Niacin

Nicotinic acid and pyridoxine are closely related substances. Both are derivatives of pyridine. This substance is a weak base, the molecule of which contains five CH-groups and a nitrogen atom arranged in a six-membered ring. As is customary with ring compounds, the various positions on the ring have numbers. In pyridine, nitrogen occupies the number 1 position. Nicotinic acid is 3-pyridine carboxylic acid. The structural formulas of the acid and its amide are given below:

The amide is more important than the acid itself, for some organisms require it and cannot manufacture it from the acid. Both acid and amide are white crystalline powders, without odor.

Nicotinic acid is one of the commonest growth factors needed by bacteria; in other words the ability to synthesize it is one most frequently lost. This may be due to its very wide distribution in soil and natural media. It is an essential requirement for those ciliate protozoa that have been studied (Colpoda and Tetrahymena) and also for most of the insects that have been investigated. Nicotinic acid is essential for the dog, the pig, and for man. However, if enough tryptophan is added to the diet, neither dog nor pig requites it. As noted previously, tryptophan is a precursor of nicotinic acid, and the tissues can synthesize the vitamin from tryptophan.

From a lack of nicotinic acid, dogs become afflicted with what is known as blacktongue disease. In man, nicotinic acid deficiency is the primary cause of pellagra. This serious disease, common enough in the United States, involves the lining of the digestive tract, the skin, and the nervous system.

Nicotinic acid amide enters into the composition of two substances of great importance for the oxidative metabolism of the cell. These are coenzyme I and coenzyme II. Presumably nicotinic acid deficiency is associated with a lack in the tissues of coenzymes I and II. But what connection there is between such a lack and the manifold disturbances present in a disease like pellagra is not at all clear.