Control of Glycolysis in Skeletal Muscle from Fetal Rhesus Monkeys

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Abstract
Extract: In our studies of metabolic control mechanisms in skeletal muscle from rhesus fetus we have determined the tissue levels of the metabolic intermediates and cofactors of the glycolytic pathway and have calculated the mass-action ratios for each reaction. Skeletal muscle from rhesus fetuses (Macaco mulatto), 90–155 days of gestational age, and from adult rhesus monkeys was used in these experiments. The apparent equilibrium constants for hexokinase and phosphofructokinase (PFK) in these tissues were over 1,000 times larger than the massaction ratios at all ages studied; the corresponding values for pyruvate kinase were more than 800 times different. The data suggest that these three enzymes are rate-limiting for fetal skeletal muscle as early as 54% of gestation. The next step was to study some of the numerous factors that modify these nonequilibrium reactions. 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-PO4 (F6P) phosphorylated per mg of protein was 2–3 times greater in the two 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 inorganic phosphate (P1) or cyclic AMP returned PFK activity to the uninhibited levels (pH 7.0). Relief of ATP inhibition of F6P phosphorylation with P1 and cyclic AMP was also observed at pH 7.0 in extracts of 100-day fetal skeletal muscle. Speculation: Control of carbohydrate metabolism in the fetus and newborn (30, 31) has recently become a point of considerable interest. For example, it is now known that the mammalian fetus possesses an endocrino~ogic autonomy and many hormones have been identified in the fetal circulation early in gestation. The mere presence of a hormone, however, does not necessarily imply that it is physiologically or biochemically active since tissue responsiveness may not have developed. It is generally agreed that in the adult, one of the primary effects of many hormones is to change the intracellular concentration of cyclic AMP (26). However, for the hormone to have its effect the cell must be capable of responding to such a change. In an earlier paper (7), we suggested that insulin and epinephrine affect carbohydrate metabolism in rhesus fetal muscle by changing the level of cyclic AMP. But even the simplest hormonemediated action has its origin in a chain of events, every link of which must be intact if the system is to work. These considerations are relevant to the perinatal period, for the enzymes in the adult cell are not necessarily present in the corresponding fetal cell (30). Our data provide evidence that as early as midterm the enzyme, PFK, which is generally agreed to be rate-limiting for glycolysis in adult tissues, is also rate-limiting in fetal muscle and is sensitive to changes in the level of effector molecules such as cyclic AMP. These results provide support for the hypothesis that in the overall regulation of glycolysis and the action of such hormones as epinephrine there is no major difference between fetal and adult muscle.