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We have described the isolation from brain of a Mg++/Ca++-ATPase resembling muscle actomyosin (Sci. 12 1 1701 1968). Like myofibrils (Perry and Grey 1 Biochem. J. 64, 184, 1956), the ATPase activity isolated from rat and cat brain is dependent upon the cone. of ATP and cation. When the conc. of ATP exceeds that of Mg++ 1 enzyme activity is inhibited. With increasing conc. of Mg++ 1 the optimum conc. of ATP also increases. This does not occur with Ca++ . Polyethylenesulfonate( PS)(I0-6M)causes the splitting of actomyosin into actin and myosin; the Mg++-ATPose activity is correspondingly diminished (Barony and Joisle 1 BBA.,41,192,1960). PS, in conc. effective against muscle actomyosin, also decreases the Mg++-ATPase activity of brain preparations . We have isolated from bovine brain myosin-like and actin-like proteins which interact with each other, as well as with their counterparts isolated from muscle, with a resultant increase in viscosity. The addition of ATP(I0-4M) causes on abrupt decrease in the viscosity which increases again over a period of on hour. These studies further establish the presence in brain of a contractile actomyosin-like protein

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Federation proceedings. 28 no. 2

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