The higher activity of CK in normal sera is due to the isoenzymes CK-MM and CK-MB present in muscle and cardiac tissues. CK-BB is usually present at very low concentrations. Both creatine kinase (CK) enzymes are dimers formed by the association of two muscle (M) and brain (B) subunits. Immunoinhibition with a specific antibody for both MM subunits and the individual unit of CK-MB allows the determination of the B subunit. The activity corresponding to half of CK-MB is measured through the increase in absorbance resulting from coupled reactions.
