CONTROL OF GLYCOLYSIS IN SKELETAL-MUSCLE FROM FETAL RHESUS-MONKEYS

In studies of metabolic control mechanisms in skeletal muscle from the rhesus fetus the tissue levels of the metabolic intermediates and cofactors of the glycolytic pathway were determined and the mass-action ratios for each reaction were calculated. Skeletal muscle from rhesus fetuses (Macaca mulatta), 90-155 days of gestational age, and from adult rhesus monkeys was used. The apparent equilibrium constants for hexokinase and phosphofructokinase (PFK) in these tissues were over 1000 times larger than the mass-action ratios at all ages studied; the corresponding values for pyruvate kinase were more than 800 times different. These 3 enzymes are apparently rate-limiting for fetal skeletal muscle as early as 54% of gestation. Factors that modify these non-equilibrium reactions were studied. Increasing the ATP concentration had a marked effect on the PFK activity of both fetal and adult muscle, first increasing and then inhibiting enzyme activity. At maximum PFK activity, the amount of fructose-6-phosphate (F6P) phosphorylated per mg of protein was 2-3 times greater in the 2 fetal than in the adult series. At a concentration of 0.3 mM, citrate decreased PFK activity of the 100-day fetal muscle; a further decrease occurred at 1.2 mM citrate. At a citrate level of 0.3 mM, the addition of Pi or cyclic AMP returned PFK activity to the uninhibited levels (pH 7.0). Relief of ATP inhibition of F6P phosphorylation with Pi and cyclic AMP was also observed at pH 7.0 in extracts of 100-day fetal skeletal muscle.