Myosin isoelectric point

All the known fibrous proteins of muscle are globulins which are insoluble at the isoelectric point (I.P.) in absence of salt (Table VI). This is true also of L-myosin which is sometimes described as water-soluble. In salt free solutions, Donnan effects are very marked as the pH is moved away from the I.P., and the protein swells and finally dissolves. 

Under these conditions, F-actomyosin is certainly in solution, so that the optimum at pH 9.0 can be attributed to L-myosin. The shift of the optimum to pH 9 in presence of Mg and Ca ions corresponds to the influence of these two ions upon the isoelectric point (c/. Section III, 3). Whether the optimum at pH 6.3 is that of L-myosin in absence of Ca++... 

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