Monovalent antibodies against MM-creatine kinase remove the M line from myofibrils.

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Abstract
Column-purified antibodies against creatine kinase (EC 2.7.3.2) from chicken skeletal muscle (the homodimeric isoenzyme designated MM-CK) bind specifically to the M lines of chicken pectoral muscle myofibrils. Incubation of myofibrils with monovalent Fab'' [antigen-binding] fragments of these antibodies solubilizes most of the myofibril-bound creatine kinase, concomitantly removing most of the electron-dense material from the M lines. MM-CK is possibly an integral part of the M-line structure and MM-CK molecules apparently form the M bridges that are responsible for the principle M-line substriations.