Succinic Semialdehyde Dehydrogenases of Escherichia coli

Abstract

Two physically and genetically distinct forms of succinic‐semialdehyde dehydrogenase have been identified in Escherichia coli B. The two enzymes could be separated by filtration on Sephadex G‐150 and their apparent molecular weights were 200000 and 97000. The larger enzyme, which is specific for NADP, is induced by growth on γ‐aminobutyrate. Its induction is highly coordinated with that of γ‐aminobutyrate:2‐oxoglutarate transaminase, the enzyme which initiates degradation of γ‐aminobutyrate. The smaller enzyme, which is induced by growth on p ‐hydroxyphenylacetate, has been purified to 98% homogeneity by affinity chromatography in conjunction with conventional methods. Under standard assay conditions this enzyme acts preferentially with NAD but reduces NADP at 15% of the rate observed for NAD, primarily because of a difference in K m . Apparent K m values for succinic semialdehyde and NAD are 13.3 ± 1.3 μM and 33.7 ± 1.4 μM, respectively. The subunit molecular weight was estimated to be 55000, indicating that the native enzyme is dimeric. The NAD‐dependent succinic‐semialdehyde dehydrogenase is aiso induced by exposure of cells to exogenous succinic semialdehyde, a treatment which has no effect on the amount of other enzymes of p ‐hydroxyphenylacetate or γ‐aminobutyrate metabolism. Apparently the gene for this enzyme functions independently from the genes encoding the other enzymes of p ‐hydroxyphenylacetate degradation. As a consequence of its induction mechanism. this NAD‐dependent dehydrogenase is also present in extracts of E. coli B grown with γ‐aminobutyrate as sole nitrogen source, in addition to the NADP‐specific enzyme involved in γ‐aminobutyrate metabolism. Presumably the NAD‐dependent enzyme is gratuitously induced by succinic semialdehyde formed by transamination of γ‐aminobutyrate.