Metabolism of NAD after whole-body irradiation: IV. Incorporation of nicotinamide by isolated perfused liver and by intact rats

The metabolism of nicotinamide was investigated in organs of normal and x-irradiated intact rats (24 h after 1000 R whole-body irradiation) and in isolated perfused rat liver. Rates of reactions were calculated from the data on liver perfusion. At low substrate levels, incorporation of nicotinamide seems to involve pathways via nicotinamide mononucleotide, whereas at high substrate levels the Preiss-Handler pathway via nicotinic acid may play a role. The changes after irradiation are explained as the result of an increased catabolism of NAD which cannot be entirely compensated for by re-synthesis.