The inhibitory action of the light meromyosin component on the myofibrillar and actomyosin atp-ase.

The protein component of light meromyosin [LMM-1] was shown earlier to relax glycerinated muscle fibres and actomyosin. Presently its influence on ATP-ase activity of myofibrils, actomyosin, myosin and heavy meromyosin has been studied. LMM-1 decreases Mg-ATP-ase activity of myofibrils and of reconstructed actomyosin by 25-- 30% and does not change [or slightly increases] Ca-ATP-ase activity of this protein and of myosin; besides LMM-1 is able to increase Mg-ATP-ase of HMM substantially. LMM-1 markedly inhibits [preliminary data] the activation of ATP-ase activity of HMM by actin. It is suggested that LMM-1 protein interacts with myosin and decreases the actin-myosin affinity, displacing actin out of the complex. It reacts only with one of the heads of myosin. Probably this suggestion can account for a relatively slight inhibition of ATP-ase activity of complex by LMM-1. LMM-1 represents a natural and specific inhibitor of Mg-AM-ATP-ase activity, included in the structure of myosin protofibrils and interacting with the myosin active site region.