In the immunoinhibition phase of the CK-MB assay, which subunit is inactivated?

• A. M subunit is inactivated
• B. B subunit is inactivated
• C. MB is inactivated
• D. BB is inactivated

Answer: (A)

Immunoinhibition in this CK-MB assay relies on targeting the subunit composition of the enzyme. Creatine kinase exists as isoenzymes that differ by their subunits: CK-MM (M/M), CK-MB (M/B), and CK-BB (B/B). An antibody that binds the M subunit is used to inactivate any CK isoenzyme containing M. Because CK-MB includes the M subunit (as part of its structure), it is inactivated in this phase, as are CK-MM. The CK-BB isoenzyme, which has no M subunit, remains active. This selective inactivation allows the assay to distinguish the M-containing forms from CK-BB and to derive the CK-MB fraction by comparing total CK activity with the residual activity after inhibition.