Influence of dietary change from a niacin- and protein-free diet to a niacin-free 30% casein diet on niacin formation from tryptophan in rats.

Abstract

Niacin formation from tryptophan was compared in two groups of rats, one fed a niacin- and protein-free diet (group A) and the other, niacin-free 30% casein diet (group B) for one day after preliminary feeding of a niacin- and protein-free diet for three days (experiment I). In the other experiment, L-tryptophan was added to the last day's diets (experiment II). NAD and the total niacin contents in liver as well as urinary excretion of pyridine compounds were significantly higher in group I-B than in group I-A. L-Tryptophan supplementation reversed the total niacin content in liver, while total tryptophan intake was about two times higher in group II-B. Since the activities of kynurenine metabolizing enzymes had not changed, we considered that it was not due to a difference in kynurenine metabolism. Urinary excretion of quinolinic acid tended to be higher in group II-A than in group II-B. However, quinolinate phosphoribosyltransferase activity did not differ significantly. On the other hand, the activities of α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase (ACMSDase) in both liver and kidneys were found to be significantly higher in group II-B than in group II-A. This might be responsible for the lower content of liver total niacin in group II-B.