Creatine kinase (CK), also known as creatine phosphokinase (CPK) or phospho-creatine kinase, is an enzyme that catalyzes the reversible transfer of a high-energy phosphate group between phosphocreatine and ATP. There are five known CK isozymes, three located in the cytosol (CKMM, CKMB, CKBB) and two in the mitochondria (Mi_SCK and Mi_uCK). Structurally, cytosolic CK is a 86 kDa dimeric enzyme of two identical or non-identical chains, CKM and/or CKB whereas mitochondrial CK is an octameric enzyme associated with mitochondrial membranes. The compartmentation of CK isoenzymes allows for direct association with ATP-providing or consuming processes both linked via metabolite channeling through the creatine phosphate shuttle. Thus CK isoenzymes allow for the generation of a large pool of phosphocreatine, considered to be a temporal and spatial ATP buffering system in tissues with large, fluctuating energy demands, such as skeletal muscle, heart, brain and spermatozoa.

Expression of cytosolic isoenzyme patterns differ between tissues, with skeletal muscle expressing high levels of CK-MM (98%) and low levels of CK-MB (1%), the myocardium (heart muscle) expressing CK-MM at 70% and CK-MB at 25–30% and the brain and neural tissue expressing mainly CK-BB. CKM is also found in serum from normal animals and levels vary based on muscle mass, age, physical activity and underlying pathologies.

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