Regulation of pyruvate kinase and phosphoenolpyruvate carboxykinase activity in adult Fasciola hepatica (Trematoda).

F. hepatica pyruvate kinase and phosphoenolpyruvate (PEP) carboxykinase were found to have properties of regulatory enzymes in the dissimilation of PEP and the control of metabolic flow. Mn²⁺ and K⁺ were required for pyruvate kinase activity. In the presence of fructose-1, 6-diphosphate (FDP), Mg²⁺ could substitute for Mn²⁺. FDP caused a 4-fold increase in the Mn²⁺ activated pyruvate kinase activity. This was accompanied by a 12-fold decrease in apparent K_m(PEP) and a 3-fold decrease in apparent K_{m (ADP)}. ATP markedly inhibited F. hepatica pyruvate kinase, but this inhibition was relieved by FDP. Estimates of metabolic levels indicated that the pyruvate kinase is saturated with PEP and ADP in vivo, but will be highly sensitive to fluctuations in the physiological concentrations of FDP and ATP. NADH doubled the activity of the PEP carboxykinase reaction and decreased the apparent K_{m (PEP)} for this enzyme 3-fold. While the maximal activity of the PEP carboxykinase reaction was substantially higher than the pyruvate kinase reaction, the steady state concentration of PEP suggests that the PEP carboxykinase will not be saturated with this substrate.