Electron paramagnetic resonance reveals age-related myosin structural changes in rat skeletal muscle fibers

We tested the hypothesis that low specific tension (force/cross-sectional a rea) in skeletal muscle from aged animals results from structural changes i n myosin that occur with aging. Permeabilized semimembranosus fibers from y oung adult and aged rats were spin labeled site specifically at myosin SH1 (Cys-707). Electron paramagnetic resonance (EPR) was then used to resolve a nd quantify the structural states of the myosin head to determine the fract ion of myosin heads in the strong-binding (force generating) structural sta te during maximal isometric contraction. Fibers from aged rats generated 27 +/- 0.8% less specific tension than fibers from younger rats (P < 0.001). EPR spectral analyses showed that, during contraction, 31.6 +/- 2.1% of myo sin heads were in the strong-binding structural state in fibers from young adult animals but only 22.1 +/- 1.3% of myosin heads in fibers from aged an imals were in that state (P = 0.004). Biochemical assays indicated that the age-related change in myosin structure could be due to protein oxidation, as indicated by a decrease in the number of free cysteine residues. We conc lude that myosin structural changes can provide a molecular explanation for age-related decline in skeletal muscle force generation.