Kinetic studies on the two common inherited forms of human erythrocyte adenylate kinase

Abstract

1. The kinetic properties of two genetic variants of human erythrocyte adenylate kinase were studied at limiting concentrations of both ADP and MgADP- in the forward direction and at limiting concentrations of both AMP and MgATP2- in the reverse direction. 2. Primary reciprocal plots rule out the possibility of a Ping Pong mechanism for both forms of the enzyme. 3. Analysis of the kinetic data by an appropriate computer program gave the following Km values for the type 1 enzyme: AMP, 0.33mm±0.1; MgATP2-, 0.95mm±0.13; ADP, 0.12mm±0.03; MgADP-, 0.22mm±0.04. Values for the type 2 enzyme were: AMP, 0.27mm±0.03; MgATP2-, 0.40mm±0.05; ADP, 0.08mm±0.07; MgADP-, 0.20mm±0.04. 4. Product inhibition studies were done by studying the reverse reaction. With ADP as product inhibitor competitive inhibition patterns were obtained with AMP and/or MgATP2- as variable substrate. Similar results were obtained for product inhibition by MgADP- with AMP as variable substrate. The results are consistent with a Rapid Equilibrium Random mechanism. 5. Secondary plots of slope versus product concentration were linear. The data were fitted to the appropriate equation and analysed by computer to give values for the product inhibition constants. 6. Differences between the values of certain kinetic constants for the two forms of the enzyme were observed.