Double inhibition of D-glyceraldehyde-3-phosphate dehydrogenase and lactate dehydrogenase

The simultaneous action of ATP (partially uncompetitive inhibitor with respect to Pi) and quinaldate (purely non-competitive inhibitor with respect to Pi) on D-glyceraldehyde-3-phosphate dehydrogenase was analyzed kinetically. The interaction constant [as defined by Keleti and Fajszi (1971) Math. Biosci. 12 197] of the two inhibitors for the D-glyceraldehyde-3-phosphate dehydrogenase-Pi complex is greater than 1, which means that the two inhibitors act antagonistically. The kinetic analysis of the double inhibition shows that there is no ATP-enzyme-quinaldate ternary complex, but a quaternary complex with Pi is formed. The interaction of the two inhibitors on the enzyme-Pi complex depends on substrate (Pi) concentration. The antagonistic effect of the two inhibitors becomes additive at low Pi concentrations (about 1 mM). The simultaneous action of oxalate (purely uncompetitive inhibitor with respect to NAD) and quinaldate (partially mixed type inhibitor with respect to NAD) on lactate dehydrogenase was also analyzed. Oxalate and quinaldate act antagonistically on lactate dehydrogenase. However, at low NAD concentrations (about 0.06 mM) or at high quinaldate and low oxalate concentrations (around 7 and 1.7 mM, respectively) the antagonism turns into the simple summation of the effects of the two inhibitors.