Inhibition of rat liver nicotinamide adenine dinucleotide kinase by reduced nicotinamide adenine dinucleotide phosphate.

Rat liver NAD kinase (ATP : NAD 2’-phosphotransferase, EC 2.7.1.23) was purified about 70-fold. The MichaelisMenten constants (Km) for NAD and ATP were 8 x 10e4 M and 2 x low3 M, respectively. NAD kinase activity was markedly inhibited by NADH and also NADPH. The Ki of NADH was approximately 1 X 10q4 M, and that of NADPH was approximately 5 X 10M5 M. Both inhibitions were competitive with NAD, and the inhibition of NADH and NADPH was partially additive. NADP had little or no inhibitory effect on the enzyme activity. &Hydroxymercuribenzoate (2 X 10u5 M) inhibited NAD kinase activity. This inhibition was reversed by I-cysteine or 2-mercaptoethanol. The possible role of the reduced pyridine nucleotides in the control of NADP synthesis has been considered.