ATPase activity and filament formation of partially purified myosin from leucocytes.

Myosin was isolated from leucocytes in horse arterial blood by the same procedures used for the isolation of myosin from skeletal muscle. The Ca2+-, EDTA-, and Mg2+-ATPase [EC 3.6.1.3] activities of the protein was 0.148, 0.147, and 0.001 mumoles/min/mg, respectively, in 0.5 M KCl at pH 7.0 and 25 degrees. The Ca2+-ATPase activity decreased with decrease in the ionic strength. No difference was found between leucocyte myosin and skeletal myosin in the pH profiles of Ca2+- and EDTA-ATPases. The rate and amount of the initial burst of Pi liberation of leucocyte myosin were 0.002 mumoles/min/mg and 0.83 moles/4.8 X 10(5)g, respectively. Leucocyte myosin aggregated into filaments of 0.3 mum length and 150 A diameter, which had a bare shaft and irregular projections. At high ionic strength, the protein bound to skeletal muscle F-actin to form a complex with the characteristic arrowhead structure.