Metabolism of NAD and N1-methylnicotinamide in growing and growth-arrested cells.

Nicotinamide is metabolized primarily into NAD and N1-methylnicotinamide in cultured cells of normal rat kidney. The metabolic pathways for the nicotinamide metabolites are independently regulated and are influenced by the growth stage of the cells. N1-Methylnicotinamide levels are 1.5--2-fold elevated in cells growth-arrested by treatment with histidinol, thymidine, or picolinic acid, or by serum starvation. This increase is due to a more rapid rate of synthesis rather than decrease in excretion. The rates of both synthesis and degradation of NAD are increased in serum-starved cells so that the NAD concentration is the same as it is in growing cells. NAD and N1- methylnicotinamide levels are not significantly increased when the intracellular nicotinamide concentration is increased 20-fold by addition of excess nicotinamide to the culture medium, demonstrating that the size of the nicotinamide pool does not limit synthesis of these compounds. In medium containing normal amounts of nicotinamide, the apparent first-order rate constant for the decay of NAD, radioactively labeled in the nicotinamide moiety, is about 4 h-1. Labeled N1-methylnicotinamide is not metabolized, but rather is excreted into the medium with a first-order rate constant of 3.9 h-1. The rate of loss of label from NAD, but not from N1-methylnicotinamide, is increased about twofold by addition of excess nicotinamide to the culture medium. This could be explained by a dilution of a labeled nicotinamide pool which is formed during NAD degradation and which is recycled into NAD but not into N1-methylnicotinamide. The results demonstrate a rapid turnover of NAD at the bond joining nicotinamide and ADP-ribose, in agreement with previous studies. In addition, the results show that nicotinamide is metabolized into N1-methylnicotinamide with what appears to be a carefully regulated synthetic mechanism. The existence of significant amounts of N1-methylnicotinamide in cultured cells raises the question of the physiological importance of this compound.