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

NAD kinase is inhibited by free Mg 2+ , free ATP and by a number of other adenine nucleotides. Inhibition by ATP, ADPMg − , ADP‐ribose 2− and AMP 2− is competitive with respect to both ATPMg 2− and NAD + . ATP appears to bind equally well to the free enzyme and to binary enzyme · substrate complexes. The other adenine nucleotides studied have markedly less affinity for the free enzyme than for binary complexes. Mg 2+ probably inhibits by forming a complex with NAD + , which binds to the enzyme but is inactive in the reaction. Low concentrations of Mg 2+ reduce K m NAD+ in an unexplained manner. ATPCa 2− is a substrate for NAD kinase, but the rate‐limiting step of the reaction may be different when other metals activate the enzyme.