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Articles

Mode of Action of the Toxin from Pseudomonas phaseolicola

II. Mechanism of Inhibition of Bean Ornithine Carbamoyltransferase ^1,2

^a Department of Plant Pathology, University of Hawaii, 3190 Maile Way, Honolulu, Hawaii 96822

A chlorosis-inducing toxin of Pseudomonas phaseolicola was examined for inhibition of ornithine carbamoyltransferease prepared from acetone powder of bean (Phaseolus vulgaris L.) plants. The enzyme has a pH optimum at 8.5, involves a ternary complex reaction mechanism, and shows Michaelis constants of 5.0 mM and 1.7 mM for ornithine and carbamoylphosphate, respectively. Assuming reversible catalysis, Michaelas constants of 11 mM and 3.3 mM are calculated for citrulline and arsenate. Toxin induces allosteric competitive inhibition in relation to carbamoylphosphate and a noncompetitive mode of inhibition in relation to ornithine, except at high toxin concentrations where uncompetitive inhibition is observed. In the backward assay, competitive inhibition is observed for both arsenate and citrulline. Inhibition is increased with preincubation time and shows saturation kinetics with regard to toxin concentration.

¹ This research was supported by National Institutes of Health Grant IR01 AE09477 to S.S.P.

² Hawaii Agricultural Experiment Station Journal Series No. 1358.