Niacin Antagonists
A number of compounds structurally related to niacin act as antagonists in animals and/or in microorganisms. Pyridine-3-sulfonic acid, and 3-acetylpyridine are examples. It is probable that such antagonists are used in the synthesis of DPN or TPN analogues and that these combinations are not physiologically active. Thus the organism is deprived of its normal complement of these coenzymes. In most cases additional niacin overcomes the antagonism. The compound 6-aminonicotinamide produces niacin deficiency symptoms in rats at a level of 15 to 30 mg per kg of diet. The symptoms are prevented by the further addition of 150 to 300 mg of niacinamide per kg of ration. The 6-amino analogue is quite active against some tumors in rats, and this tumor inhibition is reversed by niacin-amide.
Occurrence of Niacin
In natural sources the vitamin occurs as free nicotinic acid, bound nicotinic acid, or nicotinamide in coenzyme form. Ghosh and others determined the content of these forms in a variety of natural food products. About 40 per cent of the nicotinic acid of oil seeds and most of it in cereals is present in bound form. Yeast, crustacea, fish, animal tissues, and milk contain principally free and amide forms of nicotinic acid. The vitamin is rather widely distributed in both plant and animal tissues. Certain animal organs, including liver, kidney, and heart, as well as lean meat and some fish flesh are outstanding sources. Yeasts, peanuts, wheat germ, and dried legumes are in the same category. Fresh legumes and a few green vegetables are good sources, whereas milk, eggs, and most fruits are generally classed as poor sources.
The niacin content of 160 Indian foodstuffs, many of which are common constituents of the American diet, has been reported. According to the data of this report, dried yeast may contain from 20 to 62, rice polishings around 28, wheat germ 7, sheep liver 15, and pork muscle 2.8, all in milligrams per cent. Milk and eggs contain only 0.1 mg per cent. Elvehjem and co-workers also gave an extensive list of foods and their niacin content. Niacin, like other B vitamins, resides in wheat primarily in the bran and middling fraction. These workers list the niacin-content of patent flour 0.80, fancy first clear flour 1.68, second clear flour 5.55, clean shorts 5.44, and wheat feed and screenings 19.2 mg per cent.
