Control of Ornithine Decarboxylase Activity in Physarum by Polyamines

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Abstract
The addition of putrescine, spermidine, or spermine, to cultures of P. polycephalum rapidly reduced the activity of ornithine decarboxylase [EC 4.1.1.17], with maximal inhibition, 80-90%, occurring after 90 min. This response was not due to a decrease in enzyme molecules, but rather to the rapid conversion of the active enzyme to a stable, catalytically less active form. This response to exogenous polyamines was not accompanied by the appearance of a macromolecular inhibitor (antizyme), either free or bound to the enzyme. Physiological levels of the polyamines also inhibited this enzyme in vitro both competitively and non-competitively, and promoted complete yet reversible inactivation of this enzyme in the absence of reducing agents. The control of this enzyme by endogenous polyamine levels may be distinct from its response to exogenous polyamines.