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Creatine kinase (CK), also known as phosphocreatine kinase or creatine phosphokinase (CPK) is an enzyme (EC 184.108.40.206) expressed by various tissue types. Its function is the catalysis of the conversion of creatine to phosphocreatine, consuming adenosine triphosphate (ATP) and generating adenosine diphosphate (ADP).
Every CK enzyme consists of two subunits, which can be either B (brain type) or M (muscle type). There are, therefore, three different isoenzymes: CK-MM, CK-BB and CK-MB. The genes for these subunits are located on different chromosomes: B on 14q32 and M on 19q13. In addition to those, there are two mitochondrial creatine kinases, the ubiquitous and sarcomeric form.
Isoenzyme patterns differ in tissues. CK-BB occurs in all tissues, and levels do rarely have any significance. CK-MB, however, is present at low levels (1%) in muscle and high levels (30%) in myocardium (heart muscle).
CK is often determined routinely in emergency patients. In addition, it is determined specifically in patients with chest pain and acute renal failure. Normal values are usually between 25 and 200 U/L. This test is not specific for the type of CK that is elevated.
Elevation of CK is an indication of damage to muscle. It is therefore indicative of injury, rhabdomyolysis, myocardial infarction, myositis , myocarditis, malignant hyperthermia and neuroleptic malignant syndrome.
Isoenzyme determination has been used extensively as an indication for myocardial damage in heart attacks. Troponin measurement has largely replaced this in many hospitals, although some centres still rely on CK-MB.
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