The effect of actin on the magnesium-activated adenosine triphosphatase of heavy meromyosin.

The evidence is now strong that magnesium plays an important role in the interactions between myosin, actin and ATP which are associated with the physiological activity of muscle. Although calcium activates the adenosine triphosphatase ofboth L-myosin and isolated myofibrils, in the presence of this cation alone neither contraction (Ashley, Arasimavicius & Hass, 1956) nor relaxation (Bendall, 1953; Bozler, 1952) of myofibrillar systems can occur. On the other hand magnesium, which is essential for the contraction-relaxation cycle, either inhibits or has little effect on L-myosin adenosine triphosphatase, depending on the ionic strength. Nevertheless, this cation strongly activates the hydrolysis of ATP by actomyosin systems at low ionic strength. Hence the mechanism by which actin modifies the role of magnesium in the myosin-adenosine-triphosphatase system is of great interest both on general enzymological grounds and for the understanding of the mechanicochemical process associated with contraction in muscle. An important feature of the magnesium-activation of actomyosin adenosine triphosphatase is that it does not occur at ionic strengths greater than 0-15-0-20. To explain this effect it has been suggested that at low ionic strength the enzyme is in the form of the actomyosin complex which is magnesium-activated, whereas as the ionic strength increases the complex is dissociated and the system assumes the enzymic characteristics of free Lmyosin, i.e. it is magnesium-inhibited (SzentGyorgyi, 1951; Hasselbach, 1952). A direct test of this hypothesis is difficult because actomyosin is insoluble under those ionic conditions at which magnesium-activation occurs, and is therefore not very amenable to physical studies that enable the physical state of the complex to be determined. The heavy meromyosins, however, offer advantages for studies of this kind, as they retain the biological activity of the original myosin and form a complex with actin which is soluble at low ionic strength (Szent-Gyorgyi, 1953; Gergely, Gouvea & Karibian, 1955). The present investigation is a study of the effects of magnesium on the enzymic activity of the heavy * Present address: Chemical Defence Experimental Establishment, Porton Down, nr Salisbury, Wilts. meromyosins and their complexes with actin. It provides evidence that actin can influence the enzymic activity of the heavy meromyosins even when viscometric studies suggest dissociation of the complexes formed by actin with these sub-units of the myosin molecule. Some of these findings have been briefly reported (Perry & Leadbeater, 1963).