1980 Volume 26 Issue 6 Pages 571-584
The effect on the tryptophan-NAD pathway of feeding rats on a diet containing an amino acid mixture simulating rice protein plus the limiting amino acids, 0.4% of lysine ⋅ HCl, 0.2% of threonine, 0.100 of valine, 0.2% of isoleucine, 0.100 of methionine and 0.100 of histidine ⋅ HCl (basal diet + amino acids), was investigated using male weanling rats of the Sprague Dawley strain, and compared with the reference (basal diet), which did not contain the supplement of the limiting amino acids. These two diets were niacin-free. The weight gain in the “basal diet+amino acids” group was higher than that in the “basal diet” group in the early stage and the food intakes of rats in both groups were almost the same. Consequently, tryptophan intake per g rat body weight in the former group was lower than that in the latter group. Nevertheless, free tryptophan, total niacin and NAD content per g of liver and kidney of both groups were almost the same and remained constant during this experimental period. This was attributed mainly to the fact that tryptophan oxygenase [EC 1. 13. 11. 11] and quinolinate phosphoribosyl transferase [EC 2. 4. 2. 19] activities of organs in the “basal diet+amino acids” group were higher, and urinary excretion of tryptophan and its metabolites in the group was smaller than that in the “basal diet” group in the early stage. In the middle stage, tryptophan intake per g of rat body weight in both groups fell to half the level of the early stage. Nevertheless, NAD content was maintained at a constant level, urinary excretion of tryptophan and its metabolites was decreased, and picolinate carboxylase [EC 4. 1. 1. 45] activity of liver dropped. These results indicate that a strict regulation mechanism must exist for maintenance of NAD content of liver and kidney of rat for at least the 54-day feeding of these incomplete diets.