Creatine kinase

Creatine kinase (CK) is an enzyme that uses ATP to convert creatine into phosphocreatine and adenosine diphosphate. It is an enzyme found in the inner mitochondrial membrane. Creatine kinase is a marker of tissue damage. There are three isoenzymes of creatine kinase: skeletal muscle CK-MM, myocardium CK-MB, & brain and smooth muscle CK-BB. Elevated levels are found in patients suffering from heart attack, severe muscle breakdown, and muscular dystrophy.

A study found increased levels of creatine kinase in the muscle biopses of patients with postviral fatigue syndrome. A study measured plasma creatine kinase as a surrogate measure of a lowered oxidative phosphorylation in skeletal muscle of ME/CFS patients (n=15) and healthy controls. They found low plasma creatine kinase levels before and 24 hours after an exercise challenge in ME/CFS patients and healthy controls. This suggested muscle mitochondria were normal, since 24 hours after strenuous exercise CK did not leak to the blood, as is the case in patients with defective oxidative phosphorylation. A 2019 study found markedly reduced serum CK concentrations in severe ME/CFS (n=56), but not in less severe cases.