EFFECTS OF PH ON KINETICS OF HUMAN LIVER ORNITHINE - CARBAMYL PHOSPHATE TRANSFERASE

The effects of pH on the kinetics of human liver ornithine-carbamyl phosphate transferase have been examined. As the pH of the assay increases, the concentration of ornithine which gives maximum activity decreases, and inhibition becomes apparent at each pH if an optimal ornithine concentration is exceeded. The Km for ornithine decreases as the pH is raised from 6 to 8, but the Km for the zwitterion form (pK′2 = 8.69) remains constant over this range, indicating that the zwitterion is the actual substrate of human ornithine-carbamyl phosphate transferase. Further, pH does not affect the Km for carbamyl phosphate. Hence, when either carbamyl phosphate or the zwitterion of ornithine bind, apparently no groups at the active center of the enzyme or of the enzyme-substrate complex ionize. However, the effect of pH on Vmax implies that an ionizing group in the enzyme-substrate complex with an apparent pK of 6.6 affects the rate of the breakdown of the complex into free enzyme and products. © 1968, American Chemical Society. All rights reserved.