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Phosphofructokinase activity in skeletal muscle extracts following administration of epinephrine.

T. Mansour

Published 1972 in Journal of Biological Chemistry

ABSTRACT

Abstract Phosphofructokinase in rabbit skeletal muscle was assayed before and after intravenous epinephrine administration. Assays at optimal pH (8.2) and in the presence of optimal substrate concentrations showed no significant difference in enzyme activity. Muscle extracts prepared in the presence of caffeine were assayed under conditions optimal for allosteric kinetics (pH 6.9 and low substrate concentrations) and in the presence of cyclic 3',5'-AMP. These assays showed that enzyme activity in epinephrine extracts was much higher than control activity. Under the following conditions control activity increased and activity in epinephrine extracts was not significantly changed: (a) if muscle extracts were prepared in the presence of fructose-1,6-P2; (b) when the substrate concentrations in the assay mixture were high; (c) when there was not enough coupling enzymes present to remove rapidly fructose-1,6-P2 formed; (d) when muscle extracts were prepared in the absence of caffeine. Studies on the kinetics of the enzyme at pH 6.9 showed that following epinephrine administration phosphofructokinase became less sensitive to ATP inhibition and had a higher affinity to its second substrate, fructose-6-P. Studies on purified phosphofructokinase revealed that enzyme incubated with fructose-6-P, fructose-1,6-P2, cyclic 3',5'-AMP or 5'-AMP could be modified to a form that was active independent of the presence of cyclic 3',5'-AMP, and less sensitive to inhibition by ATP and caffeine. We conclude that phosphofructokinase, following epinephrine administration, is converted to a form that is more active under assay conditions that favor enzyme inhibition. Such activation appears to be mediated through a combination of hexose phosphates and adenylate nucleotides that are known to be increased by epinephrine.

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