Subcellular compartmentation of creatine kinase isoenzymes in guinea pig heart

In an attempt to determine whether a subcellular compartmentation of creatine kinase exists and if so, whether there is a different distribution of the 3 isoenzymes of CK in the cell, studies were carried out with the guinea pig heart which had been subfractionated by either isopycnic density gradient centrifugation or differential pelleting. Isoenzyme analysis of CK in the isolated subcellular fractions by electrophoresis on agarose gels, revealed that the MM isoenzyme occurred in the cytosol, myofibrils, and the microsomes while the MB isoenzyme (which is cardio-specific) was only found in the cytosol. Trace amounts of the BB isoenzyme were detected in the cytosol. Considerable CK activity was associated with the mitochondria, this did not represent the MM, the MB, or the BB isoenzymes but was a distinct and additional mitochondrial-specific form of CK. PH optima and kinetic studies were carried out to characterise and distinguish the mitochondrial isoenzyme from other CK isoenzyme activity. The evidence for a differential compartmentation of MM, MB, BB, and mitochondrial CK is discussed in relation to their possible cellular roles.