Metastable creatine kinase MM and complexes of creatine kinase BB and immunoglobulin G, "atypical" isoenzymes with similar electrophoretic mobility.

Metastable creatine kinase MM isoenzyme was isolated and partially purified from homogenates of myocardium and skeletal muscle by gradient elution on carboxymethyl cellulose. This variant isoenzyme migrated between the MM and MB isoenzymes on agarose electrophoresis, accounted for 3.5% of the total creatine kinase activity in each tissue, was not a macromolecule, and had stable electrophoretic mobility only in borate buffer (0.02 mol/L). By comparison, the creatine kinase isoenzymes with similar "atypical" electrophoretic mobility in serum specimens were complexes of the BB isoenzyme and immunoglobulin G. These complexes were measured by a radioimmunoassay specific for the creatine kinase B-subunit and eluted predominantly with the MB isoenzyme in a commercial anion-exchange reagent system.