Which CK isoenzyme is primarily located in myocardial tissue?

Which CK isoenzyme is primarily located in myocardial tissue?

• A. CK-MM
• B. CK-MB
• C. CK-BB
• D. CK-NB

Answer: (B)

CK isoenzymes reflect different tissue distributions, and that pattern is what makes CK-MB the one associated with heart muscle. The enzyme is a dimer made from M and B subunits, giving three main forms: CK-MM (two M subunits) mostly in skeletal muscle, CK-BB (two B subunits) mainly in brain, and CK-MB (one M and one B subunit) enriched in cardiac tissue. Because the heart contains the CK-MB form, it is the isoenzyme most closely tied to myocardial tissue. In clinical practice, CK-MB levels rise when heart muscle is damaged, serving as a marker of myocardial injury. Modern terminology uses CK-MB rather than CK-NB, which is an older term sometimes used for the MB fraction.

CK isoenzymes reflect different tissue distributions, and that pattern is what makes CK-MB the one associated with heart muscle. The enzyme is a dimer made from M and B subunits, giving three main forms: CK-MM (two M subunits) mostly in skeletal muscle, CK-BB (two B subunits) mainly in brain, and CK-MB (one M and one B subunit) enriched in cardiac tissue. Because the heart contains the CK-MB form, it is the isoenzyme most closely tied to myocardial tissue. In clinical practice, CK-MB levels rise when heart muscle is damaged, serving as a marker of myocardial injury. Modern terminology uses CK-MB rather than CK-NB, which is an older term sometimes used for the MB fraction.