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MB-fraction of creatine kinase in serum

 
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Last reviewed: 05.07.2025
 
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Reference values (norm) of creatine kinase MB fraction activity in blood serum: 6% of total CC activity or 0-24 IU/l.

Creatine kinase in the cardiac muscle consists of two isoenzymes: CK-MM (60% of total activity) and CK-MB (40% of total activity). CK-MB is a dimer, consists of two subunits: M (muscle) and B (brain). The MB fraction cannot be considered strictly specific for the myocardium. 3% of skeletal muscle creatine kinase is represented by this fraction. Nevertheless, an increase in CK-MB activity is considered the most specific for myocardial infarction - it accounts for more than 6% of the total CK (up to 25%). An increase in CK-MB activity is observed already 4-8 hours after the onset of the disease, the maximum is reached after 12-24 hours, on the 3rd day the activity of the isoenzyme returns to normal values in uncomplicated myocardial infarction. With expansion of the myocardial infarction zone, the activity of CK-MB is elevated for a longer period, which allows diagnosing a prolonged and recurrent infarction. The maximum activity of CK-MB is often achieved earlier than the maximum activity of total creatine kinase. The degree of increase in the activity of creatine kinase and CK-MB corresponds to the size of the affected myocardial zone. If thrombolytic therapy is started in the patient in the first hours of myocardial infarction, the peak activity of creatine kinase and CK-MB may appear earlier than usual, which is explained by faster washout of the enzyme from the affected zone (result of reperfusion - restoration of patency of the thrombosed coronary artery).

In the blood, carboxypeptidase cleaves terminal lysines of the peptide dimer of KK-MB to form two main isoforms: KK-MB 1 and KK-MB 2. In the blood serum of a healthy person, the KK-MB 2 /KK-MB 1 ratio is less than or equal to 1.5. After myocardial infarction, the activity of KK-MB 2 rapidly increases and the KK-MB 2 /KK-MB 1 ratio becomes greater than 1.5. In clinical practice, this ratio is used for early diagnosis of myocardial infarction and the onset of reperfusion during thrombolytic therapy.

The conducted studies have shown that in humans, 2 types of macro-CK can be detected during electrophoretic separation of creatine kinase. Macro-CK type 1 is CK-MB associated with IgG, less often with IgA. During electrophoresis, macro-CK type 1 is located between CK-MM and CK-MB. It is detected in 3-4% of hospitalized elderly patients, more often in women than in men. This type of creatine kinase can be present in the blood of patients for years and is not associated with any disease. Macro-CK type 2 is mitochondrial creatine kinase (oligomer of mitochondrial creatine kinase). During electrophoresis, it migrates to the cathode as CK-MB. Macro-CK type 2 indicates serious cell damage, is observed in severe diseases (myocardial infarction, shock, malignant tumors, hepatitis, liver cirrhosis, severe heart failure) and is a prognostically unfavorable sign.

Various tumors can produce CK-MB or CK-MM, which account for 60% or more of the total creatine kinase activity. Therefore, if CK-MB accounts for more than 25% of the total creatine kinase, a malignancy should be suspected as the cause of the increased enzyme activity.

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