Regulation and properties of an NADP+ oxidoreductase which functions as a gamma-hydroxybutyrate dehydrogenase

A number of naturally occurring biological intermediates have been found to inhibit competitively the activity of a highly purified NADP+-dependent oxidoreductase which catalyzes the simultaneous oxidation of gamma-hydroxybutyrate to succinic semialdehyde, and the reduction of D-glucuronate to L-gulonate. Of the inhibitors studied, those with the lowest Ki are the alpha-keto analogues of the branched chain or aromatic amino acids. The Vmax and Km for this enzyme are affected by pH; consequently, changes in substrate concentration can markedly alter the pH optimum. The enzyme has been found to be inhibited by reducing agents such as dithiothreitol and mercaptoethanol, protected against this inhibition by oxidizing agents such as oxidized glutathione or H2O2, and finally, protected against heat inactivation by the presence of either NADP+ or NADPH.