Regulation of the Catabolic Ornithine Carbamoyltransferase of Pseudomonas fluorescens

The catabolic ornithine Carbamoyltransferase of Pseudomonas is a high molecular weight enzyme which, in vivo , catalyzes the phosphorolysis of citrulline. The enzyme exhibits pronounced allosteric properties which have been studied in great detail for the reverse direction of the reaction, the carbamoylation of ornithine. The interpretation of the kinetic data according to the “concerted transition” model of Monod, Wyman and Changeux led to the suggestion that the enzyme is an octamer built up by the aggregation of identical subunits. The present work was aimed at verifying that point and provided the following observations.1 Sucrose density gradient sedimentation and polyacrylamide gel electrophoresis of the dissociated protein showed that the enzyme is made up of eight identical subunits with respect to both charge and size. The apparent molecular weight of the subunits is 39000. 2 The number of subunits is eight as determined by crosslinking of the enzyme in the presence of glutaraldehyde followed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl‐sulfate. 3 The enzyme undergoes a self‐associating reaction which is under the control of its substrates, products and allosteric effectors. 4 The study of the effect of those ligands on the kinetic behaviour and on the distribution of the aggregation states of ornithine Carbamoyltransferase suggests that the active form of this enzyme is an octamer but that several inactive or less active octameric, tetramcric and dimeric states exist.