[Creatine kinase and its isozymes].

Creatine kinase (CK; EC 2.7.3.2) is found in a variety of striated and smooth muscles, and the brain, and is an important enzyme regulator of high-energy phosphate production and utilization within contractile tissues. Serum CK activity is measured routinely as a sensitive indicator of injuries to the skeletal muscle and myocardium. CK has three isozymes (CK-MM, CK-MB and CK-BB) in cytoplasm and two isozymes (non-sarcomeric and sarcomeric) in mitochondria. CK isozymes provide more specific information about injured tissue because of their tissue distribution. CK-MM is useful in skeletal muscle diseases, such as muscle dystrophy, CK-MB in acute myocardial infarction (AMI), and CK-BB in brain damage and malignant tumor of the gastro-intestinal tract. CK-MB is measured either by enzyme activity or mass concentration and is measured as a marker not only in the diagnosis of AMI but also in suspected AMI and unstable angina. Mitochondrial CK, a useful indicator for pinpointing the severeness of muscle injuries, and CK-linked immunoglobulin are recognized as macro-CK because of their large molecular size.