Investigation of the heterogeneity of the MM isozyme of creatine kinase.

MM creatine kinase (MM CK) from human skeletal muscle was resolved into 21 subspecies by isoelectric focusing. Rabbit and bovine MM CK's were also shown to have patterns of 21 bands. These results are compatible with the presence of six different M subunit forms in skeletal muscle, which by random association into dimers, could produce 21 different subspecies of creatine kinase. Two-dimensional isoelectric focusing of human MM CK yielded a diagonal pattern, indicating that the various enzyme subspecies were not induced by ampholyte-protein interaction. Heat inactivated serum appeared to stabilize various CK subspecies and also produced one further anodally migrating MM CK form. Fresh serum had combined inhibitory and stabilizing effects on the MM CK subforms, and allowed for the identification of four anodally migrating CK subspecies not present in tissue extract. These findings are of critical importance because of the widespread use of CK isoenzymes in serum for diagnostic purposes.