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Isozymic Differentiation of Cardiac and Muscle Pyruvate Kinase in the Ringed Seal Phoca hispida

To determine whether the unusual capacity of muscle of diving mammals to undergo extended periods of anaerobiosis is due to the expression of specialized forms of the rate-limiting enzymes of glycolysis, we examined the isozymic nature of the pyruvate kinase expressed in various tissues of the ringed seal Phoca hispida. Our electrophoretic results indicate that heart and skeletal muscle contain different forms of pyruvate kinase. Both of these forms are distinct from the cathodally migrating type K isozyme in the liver. Kinetic results show that both these forms are distinct from other mammalian type M pyruvate kinases in having an acid pH optimum and a sensitivity to alanine inhibition. The affinity of the seal muscle pyruvate kinase for phosphoenolpyruvate (PEP) is lower than that of the heart form or of other mammalian type M isozymes. While the seal heart isozyme shows a decrease in affinity for PEP with a decrease in temperature, the PEP affinity of the muscle form is independent of temperature. This permits a more constant enzyme activity in the more heterothermic muscle. Muscle pyruvate kinase, with its acid pH optimum, lowered affinity for PEP, and increased sensitivity to alanine inhibition, is well suited for a tissue which undergoes prolonged anaerobic periods and then switches to aerobic utilization of lipids. We conclude that the differentiation of this distinct muscle pyruvate kinase is a special adaptation which may partially account for the ability of seal muscle to withstand considerable periods of anaerobiosis.