THE INTERACTION BETWEEN ACTOMYOSIN AND ADENOSINE TRIPHOSPHATE. LIGHT SCATTERING STUDIES

High affinity of various smooth muscle myosins for skeletal F-actin demonstrated by enzyme kinetics and electron microscopy

Myosins isolated from mixed rabbit skeletal muscle and several smooth muscles differ with respect to the rate of ATP hydrolysis and the degree of inhibition of the potassium-activated ATPase activity at saturating actin concentrations. Kinetic studies indicate that smooth muscle myosins are tightly bound to skeletal F-actin in the presence of ATp. This observation led to the hypothesis that ATP is a poor dissociating agent for smooth muscle myosin and skeletal F-actin. The kinetic interpretation was verified by electron microscopic observations of myosin binding to F-actin filaments in the presence of ATP.

The mechanism for the promotion of tenderness in meat during the post-mortem process: a review

The post-mortem changes in the chemical composition and structure of striated muscle have been reviewed on the basis of various concepts that emerged from the studies of different investigators to explain the course of tenderization of meat during aging. These concepts include the changes in the sarcoplasmic proteins, myofibrillar proteins (such as complete dissociation of actomyosin, partial dissociation of actomyosin, cleavage of disulfide linkages, depolymerization of F-actin filaments, cleavage of myosin filaments, disorganization of Z-bands and the troponin-tropomyosin complex), sarcolemma, connective tissue elements (collagen fibrils, ground substance), and the protein-ion relationship of the muscle cells (more strictly, syncytia). The experimental evidence for and against each of the views is discussed critically in the light of certain fundamentals of biophysical chemistry and biochemistry. Finally, an alternative hypothesis has been presented based on the differential effect of the post-mortem formation of lactic acid (H+ ion concentration) on the intra- and extracellular components of muscle and the possible role of lysosomal cathepsins. Consequently, a series of biophysical, biochemical, and ultrastructural changes seem to account for the mechanism by which meat becomes tender during the aging process.

The binding of Mn2+ and ADP to myosin

The metal ion requirement of myosin-ADP binding was investigated by use of Mn2+. Mn2+ binds to two sets of noninteracting sites on myosin which are characterized by affinity constants of 10(6) and 10(3), M(-1) at 0.016 M KCl concentration. The maximum number of sites is 2 for the high affinity and 20-25 for the low affinity set. Binding of Mn2+ to the high affinity sites increases the affinity of ADP binding to myosin. F-actin inhibits ADP binding (Kiely, B., and Martonosi, A., Biochim. Biophys. Acta 172: 158-170 [1969]), but even at F-actin concentrations much higher than that required to saturate the actin binding sites of myosin or its proteolytic fragments, significant ADP binding remained. The actin insensitive portion of ADP binding was inhibited by 10(-4) M inorganic pyrophosphate or ATP. The results are discussed on the basis of a model in which actin and ADP bind to myosin at distinct but interacting sites.

Binding characteristics of myosin and F-actin from dog hearts

We used light scattering and ultracentrifugation to study the binding ratio of myosin and actin from normal and failing dog hearts and to determine the effect of temperature and adenosine triphosphate (ATP) on the binding. For comparison, similar studies were done on myosin and actin from rabbit skeletal muscle.

We found a higher combining ratio, by weight, for cardiac myosin and actin (4.8 : 1) than for skeletal myosin and actin (4.4 : 1). The dissociation constants were similar at 24°C for cardiac and skeletal proteins. But at 10°C, the dissociation constant was greater for cardiac proteins, and the difference appeared more pronounced when phosphate was used to buffer the solutions.

The dissociation constant at 10°C was greater for myosin and actin from failing dog hearts.

ATP caused dissociation of cardiac and skeletal F-acto-heavy meromyosin and F-actomyosin. There was evidence that the binding of cardiac proteins may not be as tight as that of skeletal muscle proteins.

Effect of ouabain on the ATPase of cardiac myosin B at high ionic strength

The effect of ouabain (10 -8 to 10 -3 m ) on the hydrolysis of ATP by beef cardiac myosin B was measured at various KC1 concentrations and at various interaction times at 25°C. Using an ionic strength of 0.24 m KC1 and a 10-minute contact time, 10 -7 and 10 -5 m ouabain approximately doubled the ATPase activity of freshly prepared natural actomyosin. After storage of the myosin B sample in 0.6 m KC1 at 2°C, ouabain had a small inhibitory effect in 0.24 m KC1 on the ATPase activity. At an ionic strength of 0.36 m KC1, only the inhibitory effect was observed. This effect was dependent on contact time and on calcium ion concentration. Previous treatment of the myosin B with EDTA removed the activating effect observed in 0.24 m KC1 at short contact times. Addition of 0.1 mM MgCl 2 restored the activating effect of ouabain. In the EDTA-treated myosin B in 0.6 m KC1 at high ouabain concentrations in the presence of 1 m m CaCl 2 , a small activation was observed. Addition of 0.1 m m MgCl 2 under the same experimental conditions produced a small deactivation. The effect of ouabain on the ATPase of myosin B in solution appears to be dependent on protein preparation, age of the protein preparation (which may affect aggregation), ionic strength, ouabain concentration, contact time, and divalent ions present. All effects induced by ouabain on the ATPase activity of myosin B may be related to the conformation of the myosin B in solution. However, no simple relationship was observed with any individual variable such as preparation, age, contact time, or ouabain concentration.