The major CK activity in normal serum is due to CK-MM and CKMB isoenzymes, found mainly in skeletal and heart muscle. CK-BB is ussualy present in serum at low concentration.
Both, creatine kinase (CK) enzymes are dimers formed by the association of two sub-units from muscle (M) and nerve cells (B). The immunoinhibition from an specific antibody of both, MM subunits and the single M sub-unit of CK-MB, allows the determination of the B sub-unit. The CK-B activity corresponding to half of CK-MB is measured by the increasing rate of absorbance resulting from the following coupled reactions.