Regulation and Properties of an NADP+ Oxidoreductase Which Functions as a γ-Hydroxybutyrate Dehydrogenase

A number of naturally occurring biological intermediates have been found to inhibit competitively the activity of a highly purified NADP⁺-dependent oxidore-ductase which catalyzes the simultaneous oxidation of γ-hydroxybutyrate to succinic semialdehyde, and the reduction of D-glucuronate to L-gulonate. Of the inhibitors studied, those with the lowest K_i are the α-keto analogues of the branched chain or aromatic amino acids. The V_max and K_m 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 mercapto-ethanol, 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.