Kinetic properties and isozyme composition of myosin in the mdx mutant mouse

Skeletal muscle fibers from muscular dystrophic mice (C57BL/10-mdx) 1-4 months of age show elevated free Ca2+ concentrations both at resting and stimulated states, although contractility of adult (2-12 months old) mouse is similar to that of normal mouse. To evaluate the sensitivity of the contractile system of adult mdx mouse muscle to elevated free Ca2+ concentration, Mg2(+)-adenosine triphosphatase (ATPase) activity was examined using myosin, myosin B, and reconstituted actomyosin. Myosin Mg2(+)-ATPase activity of the mdx mouse was significantly higher than that of the normal mouse. Myosin B ATPase activity of the mdx mouse was also higher than that of normal mouse in free Ca2+ concentrations between 10(-9) and 10(-5) M, though there was no difference in the Ca2+ concentration required for half maximal activation of ATPase activity, 2 x 10(-7) M. Polymerized actin (FA) isolated from normal and mdx mice activated rabbit myosin Mg2(+)-ATPase identically, while activation of Mg2(+)-ATPase in mdx myosin by rabbit FA was significantly lower than that in normal mouse myosin. Rapid Pi liberation by Mg2(+)-ATPase in mdx mouse myosin was about half that of normal mouse myosin, being consistent with low activation of Mg2(+)-ATPase activity by rabbit FA. Polyacrylamide gel electrophoresis in the presence of pyrophosphate showed that myosin molecules of mdx and normal mice were both composed of three isozymes, although the fast migrating myosin isozyme (M1) was decreased while the slow migrating band (M3) was increased in mdx myosin. Subunit composition of myosin analyzed by polyacrylamide gel electrophoresis in the presence of SDS showed that the content of the smallest light chain (LC3) in mdx myosin was lower than that of normal mouse myosin, which agreed with findings that mdx myosin contained less M1 isozyme than normal myosin. These results indicated that the lowered response of mdx muscle fibers to elevated Ca2+ concentration can be attributed to the isozyme composition of myosin in mdx mouse.