Evidence for the presence of a novel biosynthetic pathway for norspermidine in Vibrio.

Enzymatic studies of the cell extracts of Vibrio alginolyticus and V. parahaemolyticus provided evidence that there exists a novel biosynthetic pathway for norspermidine (NH2(CH2)3NH(CH2)3NH2), a major polyamine species. In this pathway, the Schiff base formed between aspartic beta-semialdehyde and 1,3-diaminopropane is first reduced by a NADPH-dependent enzyme to yield "carboxynorspermidine" (NH2(CH2)3NH(CH2)2CH(NH2)COOH), which is in turn decarboxylated by a pyridoxal phosphate dependent enzyme to form norspermidine. The end product and its intermediate were identified by gas chromatography - mass spectrometry. Experiments with L-[U-14C]aspartic acid resulted in appreciable incorporation of the label into norspermidine. Putrescine could replace 1,3-diaminopropane as a substrate to produce spermidine, but at a reduced rate. The enzyme activity was greatly enhanced by dithiothreitol. Since the activity of an aminopropyltransferase that utilizes decarboxylated S-adenosylmethionine as an aminopropyl group donor could not be detected in any of the cell extracts by our assay method, it was concluded that this novel pathway is primarily responsible for producing norspermidine and spermidine in these species.