Creatine kinase
Creatine kinase is most abundant in cells of cardiac and skeletal muscle and in brain, but also occurs in other tissues such as smooth muscle. Isoenzymes of creatine kinase Creatine kinase consists of two protein subunits, M and B, which combine to form three isoenzymes, BB (CK-1), MB (CK-2) and MM (CK-3).
CK-MM is the predominant isoenzyme in skeletal and cardiac muscle and is detectable in the plasma of normal subjects.
CK-MB accounts for about 35 per cent of the total CK activity in cardiac muscle and less than 5 per cent in skeletal muscle; its plasma activity is always high after myocardial infarction. The use and limitations of CK-MB estimation are considered in Chapter 22. It may be detectable in the plasma of patients with a variety of other disorders in whom the total CK activity is raised, but this accounts for less than 6 per cent of the total CK activity.
CK-BB is present in high concentrations in the brain and in the smooth muscle of the gastrointestinal and genital tracts. Increased plasma activities may occur during parturition. Although they have also been reported after brain damage, for example trauma or cerebrovascular accident, and in association with malignant tumours of the bronchus, prostate and breast, measurement is not of proven value for diagnosing these conditions. In malignant disease, plasma total CK activity is usually normal.
There are also other forms of CK. One is a mitochondrial form seen in hepatic disease, certain tumours and critically ill patients. There are also type 1 and type 2 macroenzyme forms of CK. Type 1 macroenzyme is associated with autoimmune disease such as rheumatoid arthritis and is thought to be CK complexed with IgG; type 2 macroenzyme is a oligomer of mitochondrial CK.
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