MANUEL F. MORALES AND JEAN BOTTS 



indeed, the Churney experiments on myosin fibers approach the 

 Katchalsky (42) experiments (on polymethylacrylate threads) 

 in the extent to which they suggest polyelectrolytic behavior. 

 There are other mechanical effects of ATP which seem best ex- 

 plainable in terms of an electrostatic model. Perhaps the most 

 striking of these is that while ATP causes contraction of myosin 

 structures immersed in 0.05 M KCl and 0.001 M MgCl2, it 

 causes extension of myosin particles (7) dissolved in 0.60 M KCL* 

 The possible significance of this result becomes apparent on not- 

 ing that the conditions for contraction are thought (65) to favor 

 positive charging of the myosin, whereas under the conditions 

 for extension the myosin is certainly negatively charged; thus 

 again the opposite effects of ATP addition are very simply ex- 

 plainable in elementary terms. Finally it may be added that 

 certain relaxing effects of relatively massive ATP concentrations 

 could conceivably be attributed to "overcharging" initially 

 positive myosin with more than enough ATP anions (15,63); 

 in this case it is possible, however, that interactions of ATP 

 with divalent cations also play a role (57). 



Combination of chemical kinetic inferences with mechanical 

 observations provides further support for the model in question. 

 As already mentioned, when certain conditions are met, it is 

 possible to infer from a kinetic analysis the equilibrium constant 

 of the binding-deformation reaction between myosin and an 

 enzymatic substrate (6,54,59). From the present theory it 

 follows that in comparing various substrates the bigger is this 

 equilibrium constant the more effective should be the substrate 

 in deforming myosin. Also, for any one substrate, any "modi- 

 fier" (11) or other agency which increases the apparent equilib- 

 rium constant should also increase the effectiveness of the sub- 



* An analogous reversal of mechanical effect can also be shown on one 

 and the same myosin film (32,58). Although the question of whether ATP 

 distends or dissociates myosin particles remains controversial, we feel that the 

 result under well prescribed circumstances is distension. Either process, 

 however, suggests the operation of forces directed in the sense opposite 

 to contraction. 



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