Elevated circulating CK-MB concentration 2019-01-26T17:05:00Z Increased circulating CK MB isoform The concentration of CK-MB (= creatine kinase MB) in the blood circulation is above the upper limit of normal. All CK isoforms are encoded by separate nuclear genes and, in most tissues, a single cytosolic CK isoform is co-expressed together with a single mitochondrial CK isoform (mtCK). Cytosolic muscle-type CK (M-CK) and brain-type CK (B-CK) form homodimers or heterodimers, e.g. MM-CK in skeletal muscle, MM, MB and BB-CK in heart, or BB-CK in brain, kidney, spermatozoa, skin and many other tissues. MtCK is situated in the outer mitochondrial compartment and occurs as sarcomeric mtCK (smtCK) expressed mainly in muscle tissue and as ubiquitous mtCK (umtCK) expressed in a large number of other cells and tissues. CK-MB levels can rise upon myocardial injury, CK-MM rises upon skeletal muscle damage, and CK-BB can rise with brain injury. Abnormal circulating creatine kinase activity