Myonase is localized in skeletal muscle myofibrils

A novel chymotrypsin-like proteinase termed myonase was previously purified from MDX-mouse skeletal muscle [Hori et al. (1998) J. Biochem. 123, 650-658]. Western blots and immunohistochemical analyses showed that myonase was present within myocytes of both MDX-mouse and control mouse, and subcellular fractionation showed that it was associated with myofibrils. No significant difference was observed on Western blots between the amounts of myonase in myofibrils of MDX-mouse and control mouse, but the amount of myonase recoverable as a pure protein was 5-10-fold more when MDX-mouse was the source of the skeletal muscle. Myofibrils also possessed an endogenous inhibitor of myonase, whose inhibitory activity at physiological pH (pH 7.4) depended on salt concentration, stronger inhibition being observed at a low salt concentration. Inhibition at alkaline pH (pH 9) was weak and independent of salt concentration. Myonase in myofibrils was partially released at neutral pH by a high salt concentration (>0.6 M NaCl). However, even at 4 M NaCl, more than 80% of myonase remained within the myofibrils. Under alkaline conditions, release of myonase from myofibril was more extensive. At pH 12, myonase was almost completely present in the soluble fraction. Release of myonase under these conditions coincided with the solubilization of other myofibrillar proteins.