ON NEGATIVE COOPERATIVITY OF RAT-LIVER 6-PHOSPHOGLUCONATE DEHYDROGENASE

The effect of NADPH concentration on the activity of rat liver 6-phosp hogluconate dehydrogenase (EC 1.1.1.44) has been studied. The influenc e of NADPH concentration on the cooperativity of the enzyme was observ ed. NADPH used at concentrations up to 200 mu M appeared to be a compe titive inhibitor with respect to NADP(+), without any cooperative effe ct towards the coenzyme NADP(+)) binding. At higher concentrations of NADPH (above 300 mu M) the negative cooperativity displayed by the enz yme is confirmed by a significant decrease in the value of the Hill co efficient for NADP, from 1.1+/-0.2 down to 0.6+/-0.1 (p<0.05).