Creatine kinase (ATP: Creatine N-phosphotransferase, EC184.108.40.206) is a dimeric enzymecomposed of two types of monomer subunits, M (Muscular) and B (Brain). The subunits combine to form three distinct CK isoenzymes, CK-BB (CK-1), CK-MB (CK-2) and CK-MM (CK-3). CK-MM is the predominant form of CK in skeletal muscle. CK-BB is found in brainand smooth muscle. CK-MB is found in a high concentration in the myocardium (between 14 and 42%) and to a lesser extent skeletal muscle. In the absence of disease, most CKactivity in serum is due to the CK-MM isoform.
Damage to the myocardium, as will occur in acute myocardial infarction (AMI), will result in increased circulating levels of the CK-MB isoform. Typically CK-MB levels become elevated 4 to 6 hours after the onset of chest pain, peak between 12 to 24 hours and return to a baseline within 48 hours. Determination of CK-MB usually on admission and at 6 hours, 12 hours, and 24 hours later, is recommended when AMI is suspected. Myocardial damage is very likely when the total CK activity is above 190 U/l, the CK-MB activity is above 24 U/l (+37°C) and the CK-MB activity fraction exceeds 6% of the total.