Molecular heterogeneity of creatine kinase isoenzymes

Cytoplasmic creatine kinase (ATP:creatine N-phosphotransferase, EC 2.7.3.2) is a dimeric enzyme exhibiting three isoenzymes (MM, MB and BB). The two subunits have been reported to have identical molecular weights (Mr) of 41 000. We have demonstrated that the M subunits from human, canine, rabbit, mouse and bovine tissue have similar apparent Mr values of 43 000 as determined by SDS-polyacrylamide gel electrophoresis. In contrast, the Mr of the B subunits was different from that of the M subunit and varied with each species (human Mr 44 500; canine Mr 46 000; rabbit Mr 44 000 and mouse Mr 49 000). Cyanogen bromide cleavage showed all M subunits to have identical fragments, while B subunits exhibited cleavage products with patterns unique for each species. Despite the differences in Mr and cyanogen bromide fragment patterns, all B subunits were capable of producing enzymatically active hybrid (MB) molecules in combination with M subunits from any species tested. Mitochondrial creatine kinase subunits exhibited identical molecular weights and were similar to the M subunits and failed to combine with either the cytosolic M or B subunits. Thus, B subunits appear less conserved during evolution compared to M subunits, but have retained the amino acid sequences essential for subunit interaction and enzymatic activity.