Creatine kinase (CK), also known as creatine phosphokinase (CPK), is an enzyme found in various tissues of the body, with particularly high concentrations in muscles, heart, and brain tissues. Its primary function is to catalyze the reversible phosphorylation of creatine by adenosine triphosphate (ATP) to form phosphocreatine and adenosine diphosphate (ADP). This reaction helps to maintain cellular energy homeostasis by buffering ATP levels, particularly during periods of high energy demand.
Creatine kinase exists in multiple isoforms, which are differentiated based on their tissue distribution. The three main isoforms are CK-MM (found primarily in skeletal muscle), CK-MB (found predominantly in cardiac muscle), and CK-BB (found mainly in brain tissue).