pH and temperature dependence of the double inhibition of D-glyceraldehyde-3-phosphate dehydrogenase by ATP and quinaldate.

ATP and quinaldate, two inhibitors of D-glyceraldehyde-3-phosphate dehydrogenase, act antagonistically if [Pi] greater than 1 mM, at pH 7.5 and 25 degrees C. However, the type of interaction of the two inhibitors changes with pH and temperature. Antagonism is strongest at pH values near neutrality and decreases at higher pH. Above pH 9.5 the two inhibitors act nearly additively. At pH 8.5 and [Pi] = 5 mM there is antagonism above 20 degrees C between the two inhibitors whereas there is synergy below 20 degrees C. A lag period in the time course of the enzyme reaction was detected when both inhibitors were present. The lag period is a function of pH. Below pH 8.5 the pH-dependence of the lag period resembles a titration curve with a pKapp greater than 8.