Effect of bivalent cations on the adenosine triphosphatase of actomyosin and its modification by tropomyosin and troponin

Abstract

1. After removal of tropomyosin and troponin from the ‘natural’ actomyosin complex, the adenosine triphosphatase activity of the resulting ‘desensitized’ actomyosin is stimulated to the same extent by various bivalent cations with an ionic radius in the range 0·65–0·99å when tested at optimum concentration of the metal ion in the presence of 2·5mm-ATP at low ionic strength and pH7·6. Under identical conditions the adenosine triphosphatase activity of myosin alone is stimulated to an appreciable extent only by Ca2+ (ionic radius 0·99å). 2. Tropomyosin narrows the range of size of the stimulatory cations by inhibiting specifically the adenosine triphosphatase activity of ‘desensitized’ actomyosin when stimulated by Ca2+ or the slightly smaller Cd2+ (ionic radius 0·97å). Tropomyosin has no effect on the adenosine triphosphatase activity of ‘desensitized’ actomyosin when stimulated by the smaller cations, nor on the Ca2+-activated adenosine triphosphatase activity of myosin alone. 3. The adenosine triphosphatase activity of the ‘natural’ actomyosin system (containing tropomyosin and troponin) stimulated by the smallest cation, Mg2+ (ionic radius 0·65å), is low when the system is deprived of Ca2+ but high in the presence of small amounts of Ca2+. This sensitivity to Ca2+ seems to be a unique feature of the Mg2+-stimulated system. 4. The changes in specificity of the myosin adenosine triphosphatase activity in its requirement for bivalent cations caused by interaction with actin, tropomyosin and troponin primarily concern the size of the metal ions. The effects on enzymic properties of myofibrils due to tropomyosin and troponin can be demonstrated at low and at physiological ionic strength.