MANUEL F. MORALES AND JEAN BOTTS 



A theory based on this principle has been proposed collabora- 

 tively by A. F. Huxley and Niedergerke (39), and H. E. Huxley 

 and Hanson (40), and has been adopted by Bailey (3). Along 

 a different line, it has been shown that actin undergoes a self- 

 reaction in the course of which ATP is hydrolyzed. Spatially, 

 this self-reaction is at least a partial condensation, and although 

 the nature of the interaction between actin condensation and 

 ATP hydrolysis is quite unknown, one can reasonably conceive 

 of muscle actin so disposed that upon introduction of ATP the 

 material condenses axially. Such a theory has been proposed 

 by Straub (67) and has been occasionally advocated by Mom- 

 maerts (52). Either of these theories would be greatly strength- 

 ened if it could be shown, at least in vitro, that it is possible to 

 construct a contractile, work-performing system operating on 

 either principle. Actually this has never been accomplished, 

 and an examination (57) of present evidence bearing on these 

 theories reveals inconsistencies that, in our opinion, render the 

 theories highly improbable. By contrast, the theory to be 

 elaborated here stems from the brilliant Engelhard t experi- 

 ments (especially as improved by Weber and Szent-Gyorgyi), 

 showing that a thread spun from myosin will contract and do 

 work in the presence of Mg++ and ATP (20, 41). These experi- 

 ments not only make it probable that myosin, Mg^"*", and ATP 

 are parts of the muscle "machine" but also suggest that macro- 

 scopic contractility results from intrinsic contractility of molecu- 

 'ar chain elements, not from reactions in which mechanically 

 discontinuous elements clump together. 



Much physicochemical thinking and experimentation of 

 the past fifteen years has sought to explain the basic Engelhard t- 

 Weber-Szent-Gyorgyi experiments ; it is only recently that new 

 experiments have placed additional requirements on a theory of 

 the primary event. In the present essay we shall, correspond- 

 ingly, commence by outlining what seems a plausible theory of 

 why myosin deforms on ATP addition, and then we shall at- 

 tempt to assimilate complications by elaborating the basic 

 theory. 



610 



