Creatin kinase
Creatine kinase (CK , EC 2.7.3.2) is a predominantly cytoplasmic enzyme that catalyzes the phosphorylation of creatine to creatine phosphate by ATP. In the absence of ATP, the reaction proceeds in the opposite direction. CK is mainly localised in skeletal muscle, myocardium and brain tissue. It consists of two subunits , which could be two types – M ( muscle ) and B ( brain ), each with a relative molecular weight of around 40,000.
There are three creatine kinase isoenzymes and they are distinguished by the different representation of the subunits:
- CK-BB (CK-1, brain isoenzyme);
- CK-MB (CK-2, myocardial isoenzyme)
- CK-MM (CK-3, muscle isoenzyme)
In skeletal muscle, CK-MM predominates, but the CK-MB isoenzyme is also present. In the brain, we find the CK-BB isoenzyme, which we do not detect in the blood if the blood-brain barrier is intact. CK-MB is typical for the myocardium, but cardiac muscle also contains CK-MM.
The catalytic concentration of total CK increases within 3–6 hours from the onset of myocardial ischemia. Due to insufficient cardiospecificity, its determination in acute myocardial infarction is of limited importance. The total CK value is influenced by various factors (age, gender, muscle mass and physical activity).
The investigation of the CK-MB isoenzyme has greater diagnostic value even though CK-MB is not fully cardiospecific. The increase can also be caused by skeletal muscle damage (trauma, muscular dystrophy, intramuscular injection, resuscitation, defibrillation), extreme exercise and chronic renal insufficiency.
CK-MB can be determined as enzyme activity , which captures only active enzyme molecules, or immunochemically as protein in the form of mass concentration . In this case, we are talking about CK-MB mass , which is clearly preferred today. Determination of CK-MB mass is more specific and sensitive , as it also detects partially degraded molecules that have already lost their enzyme function.
According to the current recommendations, determination of CK-MB mass is acceptable only in case of unavailability of determination of cardiospecific troponins.
Furthermore, CK-MB mass is used to detect reinfarction at a time when a high cTn concentration still persists.
