Pyruvate kinase isoenzymes in human serum

Pyruvate kinase activity in serum from presumably healthy individuals is labile, but can be maintained for a week when samples are stored frozen. Contrary to published data our electrophoretic and isoelectrofocusing studies with fresh sera show the presence of both K and M isoenzymes; the latter predominates. In freezer-stored serum the putative M isoenzyme showed a progressive decrease in pl value as well as a decreased electrophoretic mobility.