Substrate channeling of NADH and binding of dehydrogenases to complex I.

The binding of porcine heart mitochondrial malate dehydrogenase and beta-hydroxyacyl-CoA dehydrogenase to bovine heart NADH:ubiquinone oxidoreductase (complex I), but not that of bovine heart alpha-ketoglutarate dehydrogenase complex, is virtually abolished by 0.1 mM NADH. The malate dehydrogenase and beta-hydroxyacyl-CoA enzymes compete in part for the same binding site(s) on complex I as do the malate dehydrogenase and alpha-ketoglutarate dehydrogenase complex enzymes. Associations between mitochondrial malate dehydrogenase and bovine serum albumin were observed. Subtle convection artifacts in short-time centrifugation tests of enzyme association with the Beckman Airfuge are described. Substrate channeling of NADH from both the mitochondrial and cytoplasmic malate dehydrogenase isozymes to complex I and reduction of ubiquinone-1 were shown to occur in vitro by transient enzyme-enzyme complex formation. Excess apoenzyme causes little inhibition of the substrate channeling reaction with both malate dehydrogenase isozymes in spite of tighter equilibrium binding than the holoenzyme to complex I. This substrate channeling could, in principle, provide a dynamic microcompartmentation of mitochondrial NADH.