PRIMARY EV'ENT IN MUSCLE ACTION 



strate as a deforming agent. Such predictions have been borne 

 out by experiment. Thus in a series of homologous substrates* 

 the equihbrium constants inferred from kinetic analysis are in the 

 order, A'^tp > ^utp > -^itp > ^'tpp (6,26), and wherever a cor- 

 responding comparison has been carried out on ability to deform 

 a myosin system, just the same order has been observed (57). 

 Mg2+, which is found kinetically either to leave K unchanged 

 or (more frequently) to increase it, is par excellence an enhancer 

 of deformation; indeed recent experiments (1,14,69) indicate 

 that Mg + + (or a substitute such as Mn"*"+) is indispensable for 

 contraction (69). In considering this indispensability of Mg+"^ 

 it should be recalled that our model assigns to this cation 

 the role of charging positively the sites to which ATP^~ is 

 adsorbed. Finally, there are to be considered the clever but 

 somewhat more complicated experiments of Laidler and his 

 associates (46,47), in which they studied the dependence of 

 -^ATP on bulk dielectric constant, ionic strength, and hydrostatic 

 pressure. The dependence of A^tp ^^ these "electrical" param- 

 eters was in every case strong, confirming in a general way the 

 postulated condensation of charged ATP with charged protein, 

 the release of oriented water, and the consequent increase in 

 volume and entropy of the system. Moreover, the results ob- 

 served on varying ionic strength were quite consistent with the 

 idea that anionic ATP binds to cationic protein in dilute KCl, 

 and to anionic protein in more concentrated KCl (compare the 

 results on reversal of ATP deformation cited above). The re- 

 sults observed on varying bulk dielectric constant, however, 

 were unclear, for although the free-energy change of the binding- 

 deformation process proved to be proportional to inverse dielec- 

 tric constant, its magnitude increased with decreasing dielectric 

 constant under conditions in which both ATP and protein were 

 anionic. 



To this point we have been primarily concerned with at- 

 tempting to explain the mechanical deformation of Mg++-myosin 



* UTP = uridine triphosphate; ITP = inosine triphosphate: TPP = 

 tripolyphosphate. 



621 



