Studies on the special properties of actomyosin in the gel form. II. An analysis of the turbidity changes seen in gel suspensions.

The turbidity changes induced by MgATP in suspensions of actomyosin gel particles have been studied systematically over a wide range of MgATP concentrations at different temperatures with and without the regulatory proteins. An analysis of these changes distinguishes three separate protein-protein interactions in the gel: (1) The transient cyclic interactions between actin and myosin involved in the hydrolysis of MgATP and contraction. (2) Cross-links that cause turbidity in the original suspension. (3) Cross-links that cause the high turbidity usually associated with superprecipitation. In general, there was a good correlation between the half-time for reaching maximum turbidity during superprecipitation and the rate of hydrolysis. On the other hand, the actual magnitude of the turbidity increase was progressively diminished as the concentration of substrate was raised in the millimolar range. It appears that some essential phase of the superprecipitation process is limited by the same enzymatic step that limits the rate of hydrolysis. However, whether or not this phase leads to an increase in turbidity depends on the concentration of MgATP. Apparently, high physiological levels of MgATP (1-5 mM) inhibit the formation of the specific cross-links that cause high turbidity in the superprecipitate. It is proposed that MgATP interferes with these links when it can bind to a low-affinity site on myosin that is separate from the high-affinity active sites for hydrolysis. Thus, in contracting muscle, interfilament interactions analogous to those that increase turbidity and cause isodimensional shrinkage in the gel would be prevented by the high level of MgATP in the sarcoplasm. Observations relating the shortening of isolated myofibrils to their turbidity in suspension lend support to this interpretation.