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>The effects of calcium and magnesium ions on the adenosine triphosphatase and inosine triphosphatase activities of myosin A
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The effects of calcium and magnesium ions on the adenosine triphosphatase and inosine triphosphatase activities of myosin A
p1. The effects of Casup2+/sup and Mgsup2+/sup on the enzymic activity of myosin were studied with myosin preparations treated by the ion-exchange resin Chelex-100. A reaction mixture containing 0·05m-potassium chloride was chosen in which the effects of univalent ions such as Ksup+/sup, Nasup+/sup and Clsup?/sup do not change significantly with small variations in their concentrations. 2. The relationship between the rate of hydrolysis of ATP or ITP and the concentration of Casup2+/sup suggests that a relatively weak binding of Casup2+/sup either to myosin or to the substrate nucleotide is responsible for the activation of the enzymic activity. According to the experiments with an ultrafiltration technique, the binding of Casup2+/sup to myosin proceeds in at least two steps, the first occurring at one site on every 500000 atomic mass units of myosin with an apparent association constant, iK/isubapp./sub, 1·3×10sup6/supmsup?1/sup, and the second seeming to be so weak that its binding parameters cannot be determined by the method used. The first type of Casup2+/sup binding is not observable with iN/i-ethylmaleimide-modified myosin, yet this modified myosin shows activation by Casup2+/sup of its adenosine triphosphatase and inosine triphosphatase. 3. The inhibition by Mgsup2+/sup can be related to a binding reaction of Mgsup2+/sup with myosin having iK/isubapp./sub ～10sup6/supmsup?1/sup. Mgsup2+/sup replaces the Casup2+/sup bound tightly to myosin. The iK/isubapp./sub for Mgsup2+/sup–myosin binding calculated by assuming a competition between Casup2+/sup and Mgsup2+/sup for the same site is 2·1×10sup5/sup?3·0×10sup5/supmsup?1/sup. When myosin is modified with a thiol reagent (ip/i-mercuribenzoate) at a certain ratio to myosin, the inhibition by Mgsup2+/sup becomes unobservable. 4. The behaviour of the hydrolytic activity of myosin on ATP or ITP in the presence of both Casup2+/sup and Mgsup2+/sup is consistent with the explanation that the inhibition by Mgsup2+/sup is due to the tight binding of Mgsup2+/sup to myosin, whereas the activation by Casup2+/sup is caused either by a weak binding of Casup2+/sup to myosin or by CaATPsup2?/sup or by both./p
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