Regulation of Glyceraldehyde 3-Phosphate Dehydrogenase by Phosphocreatine and Adenosine Triphosphate: IV. FACTORS AFFECTING IN VIVO CONTROL OF ENZYMATIC ACTIVITY

Abstract

Abstract Phosphocreatine inhibited glyceraldehyde 3-phosphate dehydrogenase crystallized from rabbit muscle or yeast. Creatine at the same concentration showed no inhibition. The inhibition was competitive with respect to glyceraldehyde 3-phosphate. At the approximate physiological concentrations of substrate (0.01 mm) and phosphocreatine (20 mm), the enzyme was 65 % inhibited. Phosphocreatine is not competitive with respect to NAD or orthophosphate. The inhibition by phosphocreatine was maximal at pH 7.4 and decreased about 17 % with a pH fall to 6.8 or a rise to 8.5. Incubation of the muscle enzyme with phosphocreatine produced a time-dependent inactivation which was prevented by the addition of glyceraldehyde 3-phosphate. This time-dependent inactivation and the instantaneous inhibition were additive. Magnesium partially relieved the inhibitory effects of phosphocreatine. The properties of the inhibitory effects of phosphocreatine and ATP are compared. Since the phosphocreatine level in muscle may decrease very rapidly during muscle contraction, the inhibition on triose phosphate dehydrogenase may be lessened substantially, and ATP resynthesis will be promoted. Further physiological consequences of phosphocreatine and ATP inhibition in the resting and contracting muscle are considered.