A contractile protein possessing Ca 2+ sensitivity (natural actomyosin) from leucocytes. Its extraction and some of its properties

Abstract

1. 1. Methods have been developed for the isolation of contractile protein from equine leucocytes resembling natural actomyosin of the smooth muscle. 2. 2. This protein exhibited ATPase (EC 3.6.1.3) activity and underwent super-precipitation at low ionic strength in the presence of ATP, Mg 2+ and Ca 2+, but revealed no superprecipitation in the absence of either Ca 2+ or Mg 2+ even in the presence of ATP. Superprecipitation of this protein was enhanced by an increase of Ca 2+ concentration in a medium containing Mg 2+ and its intensity was closely associated with enhancement of the Mg 2+-dependent ATPase (EC 3.6.1.3) activity of this protein by an increase of Ca 2+. The free Ca 2+ concentration at which the Mg 2+-dependent ATPase activity of the protein was half activated was about 1 · 10 −6 M. Superprecipitation of this protein was enhanced by an increase of Mg 2+ concentration in a medium containing Ca 2+. In this case, too, the superprecipitation was closely associated with enhancement of the ATPase activity of this protein by increasing Mg 2+ with a constant Ca 2+ concentration. The ATPase activity was half activated in the presence of about 3 mM Mg 2+. 1. 3. Electron micrographs of the protein showed thick and thin filaments at low ionic strength and a relatively high ATP level conditions in which muscular actomyosin was dissociated into myosin aggregates and fibrous actin. At a relatively low ATP level, both thin and thick filaments were aggregated with each other, as in actomyosin from the muscle, which seemed to be subject to superprecipitation.