Origin of variable extraction of myosin from myofibrils treated with salt and pyrophosphate

When isolated myofibrils are treated with solutions containing NaCl and pyrophosphate to imitate conditions in meat being cured, considerable dijfirences are observed between myofibrils in the concentration of NaCl required for extraction of the myosin‐containing A‐band. The diferences in extent of extraction at an intermediate concentration of NaCl are between bundles of myofibrils rather than within them, indicating that the variation derives from differences between muscle fibres. The extraction behaviour is determined by the myosin isoenzyme content of the muscle fibre. Thus, the proportion of myofibrils extracted at an intermediate concentration of NaCl in preparations from various bovine and rabbit muscles reflects the documented proportion of fast‐contracting fibres in each muscle. Rabbit M plantaris myofibrils from fast white fibres are extracted in a low concentration of NaCl, whereas those myofibrils containing slow myosin are extracted only in a high Concentration of NaCl. Myofibrils probably from fast red fibres require an intermediate concentration of NaCl for extraction.