Creatine kinase MB - enzyme activity


CK–MB is one of three separate forms of the enzyme creatine kinase. CK–MB is found mostly in heart muscle. It rises when there is any disease or damage to heart muscle cells.
CK–MB levels, along with total CK, are tested in persons who have chest pain to diagnose whether they have had a heart attack. Since a high total CK could indicate damage to either the heart or other muscles, a high CK–MB suggests that the damage was to heart muscle.
If the relative index is more than 2.5–3, the heart is the likely muscle damaged.

Relative index (RI) = (CK-MB (μg/L) / Total activity CK (U/L)) x 100 = > 2.5-3

A high CK with a very low relative index suggests that other muscles were damaged.

Creatine kinase is a dimeric enzyme. Cytoplasmic CK consists of two subunits with a combined molecular mass of 86 000 daltons. Molecular weight of individual subunits is half that of intact molecule. Each subunit of CK is either of M and B type. Combinations of M or B subunits will produce three different major isoenzymes of CK-MM (skeletal muscle), CK-MB (heart) and BB (brain). The isoenzymes have the same molecular weight and catalyze the same reaction, but differ in molecular structure and in their source.
CK-MM (CK-3) is found predominantly in skeletal muscle, the richest source of CK-MB (CK-2) is myocardial muscle and CK-BB (CK-1) is found mainly in brain and intestine.

Plasma CK activity is a reflection of tissue turnover, hence the isoenzyme composition in plasma is a balance between the amount of enzyme in different tissues, the relative isoenzyme composition of the different tissues, the rate of turnover of the different tissues, and the half-life of the different isoenzymes in blood.
Besides these usual forms (CK-MM, CK-BB, CK-MM) of CK, there are other atypical forms occasionally found in blood.
Two forms of macro-CK have been described. Macro-CK type 1 is a complex of CK-BB with IgG and rarely with IgA. It is usually found in the blood of older women. The macro-CK type 2 is oligomeric CK of mitochondrial origin found predominantly in the blood of severely ill persons and those with malignancy.