Kinetic studies with rat-brain succinic-semialdehyde dehydrogenase

A simple procedure is described that gives an approximately 100-fold purification of rat brain succinic-semialdehyde dehydrogenase with a high yield. The enzyme exhibits a relatively low Km value for succinic semialdehyde (2.5 microM) and is inhibited by high concentrations of that substrate in an uncompetitive manner with respect to NAD+ (Ki = 150 microM). p-Hydroxybenzaldehyde was shown to give competitive inhibition with respect to succinic semialdehyde and uncompetitive inhibition with respect to NAD+. Initial rate studies in the presence of a fixed concentration of this inhibitor allowed a more accurate estimation of the kinetic parameters for the uninhibited reaction. The results of these studies, together with analysis of the dead-end inhibition by AMP and the effects of NAD+ and 3-acetylpyridine--adenine dinucleotide as alternative acceptors in the reaction, were consistent with the enzyme-catalysed reaction obeying a compulsory-order mechanism in which NAD+ was the first substrate to bind to the enzyme and NADH was the last product to dissociate from it.