Rat testis mitochondrial aldehyde dehydrogenase: kinetic evidence for a direct reaction between capronaldehyde and enzyme-bound NAD+ in the presence of Mg2+ ions.

In the presence of Mg2+ saturation curves of aldehyde dehydrogenase show a sharp maximum at capronaldehyde concentrations lower than 1 microM. Since the native enzyme is a dimer, kinetic data have been analyzed with a general rate equation (given as a ratio of two polynomials) that takes into account the presence of two binding sites for both substrates and two for Mg2+. Simulation of the saturation curves was only successful after allowing the formation of the stable complexes ES, ES2, ES2M, ES2M2, EM and EM2. Since ESM and ESM2 are highly reactive but very unstable, activity at low aldehyde concentration can be explained by assuming a direct reaction mediated by Mg2+. At concentrations higher than 1 microM, capronaldehyde effectively binds to the enzyme in a highly cooperative process, but the formation of ES2M and ES2M2 results in slower reaction rates. Since ES2M2 is inactive, increase of the Mg2+ concentration eventually leads to strong inhibition. Experiments at different NAD+ concentrations show that the enzyme binds two NAD+, but reaction takes place at one binding site.