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Multiple Pathways of Putrescine Biosynthesis in Escherichia coli

From the ¹ From the Program in Biochemical Sciences, Princeton University, Princeton, New Jersey 08540

Two routes of putrescine biosynthesis exist in Escherichia coli both emanating from the latter portion of the arginine-biosynthetic pathway. Isotope competition experiments indicate that both arginine and ornithine can give rise to putrescine. The formation of putrescine from ornithine is independent of the conversion of the latter to arginine.

The decarboxylation of ornithine to yield putrescine is catalyzed by cell-free preparations. This enzyme has been termed "biosynthetic" ornithine decarboxylase to distinguish it from the inducible degradative ornithine decarboxylase, from which it differs in many properties.

Cell-free extracts also carry out the conversion of arginine to putrescine. In this pathway, the arginine carboxyl group is first released as CO₂, and the amidine group is then removed as urea. The arginine decarboxylase has optimal activity at pH 8 and an absolute magnesium ion requirement. Both of these characteristics distinguish it from the catabolic arginine decarboxylase. Agmatine ureohydrolase, the enzyme catalyzing the second step of this pathway, is also present in cell-free extracts.

Arginine biosynthesis in E. coli involves a branched pathway, the branch point to putrescine and arginine occurring at ornithine. The existence of two pathways of putrescine biosynthesis is interpreted in relation to control mechanisms of the arginine biosynthetic pathway.

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