The contribution of tryptophan to the regulation of the NAD+ level in mouse liver.

It has been observed that the NAD level is lower in regenerating and growing mouse liver than in resting liver of adult mice. Also it has been found that in proliferating liver the NAD biosynthesis from nicotinamide and nicotinate is unchanged whereas that from tryptophan is reduced. Under these conditions the enzymatic activity of the kynureninase is decreased and the activity of the kynurenine aminotransferase as well as of the tryptophan pyrrolase is not altered. From these and other data of the literature it is concluded that the NAD biosynthesis from tryptophan is important for the regulation of the hepatic NAD level, through this pathway and the NAD degradation nicotinamide is formed which is transported to the non-hepatic tissues to act as a NAD precursor there. The possible interrelation between the NAD metabolism and the DNA synthesis is discussed.