Equilibrium binding of coenzymes and substrates to nicotinamide-adenine dinucleotide phosphate-linked isocitrate dehydrogenase from bovine heart mitochondria.

1. The stoicheiometries and affinities of ligand binding to isocitrate dehydrogenase were studied at pH 7.0, mainly by measuring changes in NADPH and protein fluorescence. 2. The affinity of the enzyme for NADPH is about 100-fold greater than it is for NADP+ in various buffer/salt solutions, and the affinities for both coenzymes are decreased by Mg2+, phosphate and increase in ionic strength. 3. The maximum binding capacity of the dimeric enzyme for NADPH, from coenzyme fluorescence and protein-fluorescence measurements, and also for NADP+, by ultrafiltration, is 2 mol/mol of enzyme. Protein-fluorescence titrations of the enzyme with NADP+ are apparently inconsistent with this conclusion, indicating that the increase in protein fluorescence caused by NADP+ binding is not proportional to fractional saturation of the binding sites. 4. Changes in protein fluorescence caused by changes in ionic strength and by the binding of substrates, Mg2+ or NADP+ (but not NADPH) are relatively slow, suggesting conformation changes. 5. In the presence of Mg2+, the enzyme binds isocitrate very strongly, and 2-oxoglutarate rather weakly. 6. Evidence is presented for the formation of an abortive complex of enzyme-Mg2+-isocitrate-NADPH in which isocitrate and NADPH are bound much more weakly than in their complexes with enzyme and Mg2+ alone. 7. The results are discussed in relation to the interpretation of the kinetic properties of the enzyme and its behaviour in the mitochondrion.