Hysteretic interaction of NADH and Mg2+ with mammalian NADH:CoQ reductase from beef heart.

Preincubation of submitochondrial particles (SMP) from beef heart in a reaction mixture containing low concentrations of Mg2+ induces a time lag in the NADH:oxidase activity. Preconditioning of the SMP by NADH, but not by NAD+, prevents the Mg2+-related time lag. The data obtained show that there exists a tight binding site for Mg2+ regulating the rate of electron transfer from NADH to the natural acceptor. The ability of Mg2+ to form a catalytically inactive complex with the enzyme is regulated by NADH.