KM Brindle, FF Brown, ID Campbell, DL Foxall, RJ Simpson
The exchange of hydrogen and deuterium atoms between the C-2 position of lactate and solvent was monitored in suspensions of human erythrocytes by using a non-invasive spin-echo p.m.r. method that permits continuous assessment of the rate and the extent of exchange. Exchange rates were measured in cells suspended in buffers made in 2H2O and 1H2O after the addition of L-[2-1H]lactate and L-[2-2H]lactate respectively. The rate of exchange is dependent on the activities of four glycolytic enzymes (fructose bisphosphate aldolase, triose phosphate isomerase, glyceraldehyde phosphate dehydrogenase and lactate dehydrogenase) and on the concentrations of their substrates. The dependence of the exchange on the following substrates was studied: (1) lactate, (2) the triose phosphates and fructose 1,6-bisphosphate and (3) pyruvate. Observation of the exchange in vitro, in a system produced by mixing the isolated enzymes, permits determination of the individual isotope-exchange equilibrium velocities of the enzymes. The dependence of the equilibrium velocity of human erythrocyte lactate dehydrogenase on NAD+ + NADH concentration was measured. Possible applications of these methods are discussed.