Evidence for a defective thiol protease inhibitor in skeletal muscle of mice with hereditary muscular dystrophy.

The thiol protease inhibitor (TPI-d) from hind-limb skeletal muscle of dystrophic 60-day-old male mice (strain 129/ReJ/dy) has been purified to apparent homogeneity and compared with the thiol protease inhibitor (TPI-n) from hind-limb skeletal muscle of normal 60-day-old male littermates. While both TPI-d and TPI-n displayed identical properties on sodium dodecyl sulfate-polyacrylamide gels (14,800 relative mass), analytical isoelectric focusing gels (pI 4.5), and high performance liquid chromatography columns, TPI-d was unable to inhibit papain and cathepsin B after purification by isoelectric focusing. However, a component in the purified TPI-d preparation with an isoelectric point of 4.9 initially masked the functional state of TPI-d, using papain when assayed with the test proteases papain and cathepsins H and L. This inhibitory component was absent from TPI-n preparations. Pure TPI-d was also unable to inhibit in vitro myosin hydrolysis by cathepsin B, whereas TPI-n completely blocked cathepsin B catalyzed myosin hydrolysis. Given the central role of the thiol proteases, especially cathepsin B, in intracellular protein metabolism and the possibility that uncontrolled thiol protease activity in muscle leads to muscle protein breakdown and dystrophy, our data suggest that a modified (defective) thiol protease inhibitor (TPI-d) may be (one of) the end product(s) of the dystrophy gene in mice with the hereditary form of the disease.