Studies on Human Erythrocyte IMP: Pyrophosphate Phosphoribosyltransferase

Some kinetic properties of the enzyme, IMP: pyrophosphate phosphoribosyltransferase have been investigated. The effect of varying the concentration [S] of the purine substrate (hypoxanthine or guanine) on the initial velocity v of the reaction was studied at constant concentrations of P ‐Rib‐ PP and Mg 2+ . The velocity curves (plots of v against [S]) were hyperbolic. The velocity curves were sigmoid when the P ‐Rib‐ PP concentration was varied at constant concentrations of purine and Mg 2+ . Sigmoid velocity curves also resulted when the Mg 2+ concentration was varied in the presence of constant concentrations of P ‐Rib‐ PP and purine. Mn 2+ could not replace Mg 2+ in the system, but low concentrations of Mn 2+ were stimulatory and high concentrations were inhibitory in the presence of low concentrations of Mg 2+ and P ‐Rib‐ PP . The shape of the curves obtained by plotting the P ‐Rib‐ PP concentration against the initial velocity at constant concentrations of purine and Mg 2+ was pH dependent, being hyperbolic at pH 8.5 and sigmoid at pH 7.5 or less. Inorganic orthophosphate had a slight activating effect when the P ‐Rib‐ PP concentration was varied at constant concentrations of purine and high concentrations of Mg 2+ (6 mM). However, if the Mg 2+ concentration was low (1 mM), high concentrations of inorganic phosphate (12 mM) were inhibitory. The activity of the enzyme was reduced disproportionately with respect to the amount of protein present when it was highly diluted. The possibility that these findings are due to the enzyme having allosteric properties, and their bearing on the possible function of IMP: pyrophosphate phosphoribosyltransferase in the regulation of de novo purine biosynthesis are discussed.