Niacin Excretion in the Rat in Relation to Tryptophane, Pyridoxine, and Protein Content of the Diet

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Abstract
Addition of pyridoxine to a ration lacking this vitamin does not increase the excretion of niacin or N1-methylnicotinamide by the rat. Rats fed rations deficient in pyridoxine do not excrete less niacin or N1-methylnicotinamide than rats fed rations adequately supplemented with this vitamin. Addition of L-tryptophane to an adequate ration results in a marked increase in the excretion of both niacin and N1-methylnicotinamide. Addition of L-tryptophane to a diet deficient in pyridoxine results only in a very small increase in excretion of N1-methylnicotinamide; niacin excretion is unchanged. When rats are maintained on a diet deficient in pyridoxine for 22 days, and the diet is subsequently supplemented with pyridoxine, the increase in excretion of niacin after tryptophane is small. The excretion of N1-methylnicotinamide, however, increases greatly. The increase in excretion of niacin and its methylated amide is greater when tryptophane is fed in a diet containing casein than in a diet containing zein as the protein. Little or no increase in niacin or N1-methylnicotinamide is observed when the tryptophane intake is increased by feeding a high-protein diet. The significance of these results is discussed. It is concluded that pyridoxine is concerned in the action of tryptophane in increasing niacin excretion, and that other amino acids are also involved, perhaps as antagonists to tryptophane.