The effects of pyrophosphate on myosin heads and on fiber stiffness.
We have investigated the effect of magnesium pyrophosphate (MgPPi) on the orientation and stiffness of myosin heads in glycerinated muscle fibers. MgPPi is known to bind to the nucleotide site of myosin and to weaken the bond between myosin and actin. The spectra of paramagnetic probes attached to the reactive sulfhydryl on the myosin head were used to monitor the orientation of the head. Addition of 3 mM MgPPi to rigor fibers resulted in a shift of approximately 50% of the myosin heads from a state in which the probes are highly oriented to a state in which the probes are disordered. Experiments in which MgPPi dissociated labeled subfragment-1 from unlabeled fibers support the conclusion that the disordered probes are on myosin heads that are not attached to actin. Addition of MgPPi causes only a small decrease in fiber stiffness, measured by step changes in fiber length. We conclude that up to 50% of the myosin heads can be dissociated from actin with little change in muscle stiffness. Thus stiffness is not a linear measure of the fraction of myosin heads attached to actin. We present a model in which one head of the pair of a myosin molecule binds more weakly to actin and can be dissociated preferentially by the binding of MgPPi.
Cooke
R
R
Department of Biochemistry and Biophysics, University of California, San Francisco 94143-0524.
Pate
E
E
D000818:Animals; D011756:Diphosphates / Q000494:pharmacology*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D066298:In Vitro Techniques; D008274:Magnesium / Q000494:pharmacology*; D017616:Magnesium Compounds; D009119:Muscle Contraction / Q000187:drug effects*; D009132:Muscles / Q000187:drug effects / Q000502:physiology*; D009218:Myosins / Q000502:physiology*; D011817:Rabbits; D013113:Spin Labels
D011756:Diphosphates; D017616:Magnesium Compounds; D013113:Spin Labels; C034732:magnesium pyrophosphate; D009218:Myosins; D008274:Magnesium