Creatine Kinase exists as dimeric molecules composed of M and B subunits that form the isoenzymes MM, MB, and BB. The subunits M and B are immunologically distinct. CK-MM and CK-MB are distributed primarily in the skeletel muscle and heart muscle, respectively, while CK-BB is present mainly in the brain and in tissues composed of smooth muscle.
Following acute myocardial infarction, CK-MB activity increases significantly and this elevation is highly specific for the laboratory diagnosis of myocardial infarction. Although total CK activity usually increases following myocardial infarction, in some patients only the CK-MB activity increases, while the total CK remains in the normal range. In this procedure CK activity is measured in the presence of an antibody to CK-M monomer. This antibody completely inhibits the activity of CK-MM and half of the activity of CK-MB, while not affecting the B subunit activity of CK-MB and CK-BB. Due to negligible concentrations of CK-BB in the circulation, the remaining activity, multiplied by a factor of 2, represents the activity of the CK-MB isoenzyme.