Relative Roles of Niacin and Tryptophan in Maintaining Blood Pyridine Nucleotides, Nitrogen Balance and Growth in Adult Rats

Abstract

Young adult rats, depleted of blood pyridine nucleotides (PN) by a niacin-tryptophan-deficient ration, were fed supplements of L-tryptophan from 0 to 0.30% of the ration. Nitrogen equilibrium was evident on the lower levels of tryptophan supplementation, growth on the intermediate levels and synthesis of blood PN on the higher levels. This sequence suggests a preferential use of tryptophan in the order listed by niacin-tryptophan-deficient rats. When similarly depleted rats were fed no supplement or constant physiological levels of tryptophan, niacin, or tryptophan plus niacin, those receiving niacin showed small weight losses comparable to the unsupplemented group, while those receiving tryptophan or tryptophan plus niacin showed comparable weight gains. “Total circulating” PN values of the niacin-supplemented group decreased slightly or were just maintained at a constant level only slightly above the unsupplemented group. “Total circulating” PN values of the tryptophan plus niacin-supplemented groups increased gradually, and in a two-week period were a little higher than the tryptophan-supplemented groups. At the end of a 30-day period this difference had disappeared. This supports the suggestion that at physiological levels tryptophan contributes to PN synthesis in niacin-tryptophan-depleted rats to a greater extent than does niacin.