Control of the steady-state concentrations of the nicotinamide nucleotides in rat liver.

1. The effects of injecting nicotinamide, 5-methylnicotinamide, ethionine, nicotinamide+5-methylnicotinamide and nicotinamide+ethionine on concentrations in rat liver of NAD, NADP and ATP were investigated up to 5hr. after injection. 2. Nicotinamide induced three- to four-fold increases in hepatic NAD concentration even in the presence of 5-methylnicotinamide or ethionine, whereas 5-methylnicotinamide or ethionine alone did not cause marked changes in hepatic NAD concentration. 3. Nicotinamide alone also induced a twofold increase in hepatic NADP concentration. However, in the presence of 5-methylnicotinamide+nicotinamide, the NADP concentration decreased by 25% after 5hr., and in the presence of nicotinamide+ethionine by 30% in the same time. In the presence of 5-methylnicotinamide or ethionine alone hepatic NADP concentrations fell by 50% after 5hr. 4. 5-Methylnicotinamide inhibited the microsomal NAD(+) glycohydrolase (EC 3.2.2.6) by 60% at a concentration of 1mm and the NADP(+) glycohydrolase by 40% at the same concentration. 5. The rat liver NAD(+) kinase (EC 2.7.1.23) was found to have V(max.) 4.83mumoles/g. wet wt./hr. and K(m) (NAD(+)) 5.8mm. This enzyme was also inhibited by 5-methylnicotinamide in a ;mixed' fashion. 6. The results are discussed with respect to the control of NAD synthesis. It is suggested that in vivo the NAD(P)(+) glycohydrolases are effectively inactive and that the increased NAD concentrations induced by nicotinamide are due to increased substrate concentration available to both the nicotinamide and nicotinic acid pathways of NAD formation.