Which enzyme has isoforms MM, MB, and BB?

• A. CK
• B. LDH
• C. AST
• D. ALT

Answer: (A)

Creatine kinase is the enzyme that exists as three main isoforms named by their subunit combinations: CK-MM, CK-MB, and CK-BB. These isoforms arise from different pairings of the M and B subunits and are distributed in tissues with high and variable energy needs: CK-MM is abundant in skeletal muscle, CK-MB in heart muscle, and CK-BB in brain. The enzyme’s primary role is to shuttle high-energy phosphate from phosphocreatine to ADP to rapidly regenerate ATP during muscle contraction or other energy-demanding processes. Other enzymes do not use these MB/BB/MM designations for their isoforms—the LDH family has different isoforms (such as LDH-1 to LDH-5) based on H and M subunits, while AST and ALT do not have isoforms named MB or BB. So the enzyme with isoforms MM, MB, and BB is creatine kinase.