A mitochondrial NADP+-dependent reductase related to the 4-aminobutyrate shunt. Purification, characterization, and mechanism.

Succinic semialdehyde reductase, a NADP+-dependent enzyme, was purified from whole pig brain homogenates. The enzyme preparation migrates as a single protein and activity band on analytical gel electrophoresis. Succinic semialdehyde reductase (Mr 110,000) catalyzes the reduction of succinic semialdehyde to 4-hydroxybutyrate. The equilibrium constant of the reaction is Keq = 5.8 X 10(7) M-1 at pH 7 and 25 degrees C. The inhibition kinetic patterns obtained when 4-hydroxybutyrate or substrate analogs are used as inhibitors of the reaction catalyzed by the reductase are consistent with an ordered sequential mechanism, in which the coenzyme NADPH adds to the enzyme before the aldehyde substrate. A specific aldehyde reductase was also purified to homogeneity from brain mitochondria preparations. Its catalytic properties are identical to those of the enzyme isolated from whole brain homogenates. It is postulated that two enzymes, i.e. a NAD+-dependent dehydrogenase and a NADP+-dependent reductase, participate in the metabolism of succinic semialdehyde in the mitochondria matrix.