Kinetic Studies of the Interaction of Pigeon‐Liver NAD Kinase with Adenine Nucleotides and Divalent Cations

1 NAD kinase is inhibited by free Mg2+, free ATP and by a number of other adenine nucleotides. 2 Inhibition by ATP, ADPMg−, ADP-ribose2− and AMP2− is competitive with respect to both ATPMg2− and NAD+. 3 ATP appears to bind equally well to the free enzyme and to binary enzyme · substrate complexes. 4 The other adenine nucleotides studied have markedly less affinity for the free enzyme than for binary complexes. 5 Mg2+ probably inhibits by forming a complex with NAD+, which binds to the enzyme but is inactive in the reaction. 6 Low concentrations of Mg2+ reduce KmNAD+ in an unexplained manner. 7 ATPCa2− is a substrate for NAD kinase, but the rate-limiting step of the reaction may be different when other metals activate the enzyme.