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Metabolism and Enzymology

Pyrroline-5-Carboxylate Reductase in Soybean Nodules ¹

Comparison of the Enzymes in Host Cytosol, Bradyrhizobium japonicum Bacteroids, and Cultures

Department of Biology, Washington University, St. Louis, Missouri 63130, Division of Biological and Biomedical Science, Washington University, St. Louis, Missouri 63130

Characteristics of pyrroline-5-carboxylate reductase (P5CR) from Bradyrhizobium japonicum bacteroids and cultured rhizobia were compared with those of the enzyme in soybean nodule host cytosol. Reductase from host cytosol differed from that in bacteroids in: (a) the effect of pH on enzymic activity, (b) the capacity to catalyze both reduction of pyrroline-5-carboxylic acid and NAD⁺-dependent proline oxidation, (c) apparent affinities for pyrroline-5-carboxylic acid, and (d) sensitivities to inhibition by NADP⁺ and proline. The K₁ for proline inhibition of P5CR in bacteroid cytosol was 1.8 millimolar. The properties of P5CR in B. japonicum and bacteroid cytosol were similar. The specific activities of P5CR in the cytosolic fractions of the nodule host and the bacteroid compartment were also comparable.