MYOSIN ATP-ASE ACTIVITY IN RELATION TO 

 TEMPERATURE AND PRESSURE 



Karl F. Guthe, Department of Zoology, University of 

 Michigan, Ann Arbor, Michigan 



A HE MOLECULAR EVENTS in iiiuscular Contraction are still poorly under- 

 stood, although the protein myosin has long been regarded as the actively 

 contracting substance. For more than twenty years, the important role of 

 adenosine triphosphate (ATPj in muscle energetics has been recognized. 

 In 1939, the discovery that myosin catalyzes the hydrolysis of ATP (6) 

 was received with great enthusiasm. Although Mommaerts (16j has re- 

 cently presented evidence against the view that ATP splitting is the im- 

 mediate source of contractile energy, it is nevertheless clear that myosin's 

 ability to contract is closely linked to its ATP-ase activity (25) . Whether 

 contractility is linked to ATP-binding, to ATP-splitting or to a steady- 

 state transference by myosin of the terminal phosphate of ATP is still 

 debated. In any case, ATP-splitting is a useful tool for studying the 

 physicochemical properties of myosin. 



Myosin ATP-ase is not a very simple system. Its complexity has been 

 shown in many investigations. Only one enzyme is present, and it removes 

 only the terminal phosphate from ATP, but its activity depends on the 

 relative concentrations of magnesium, calcium, sodium, potassium and hy- 

 drogen ions, as well as temperature and pressure. Besides these factors, 

 myosin's ATP-ase activity is also influenced by the presence of the other 

 muscle protein actin. No attempt will be made to summarize all the influ- 

 ences, because there are many recent reviews of this subject (1, 15, 19, 21, 

 23, 25) . Instead, data will be presented to show that myosin undergoes re- 

 versible denaturation. Preliminary notes (3, 10) will be followed by a 

 more complete report elsewhere (4, 9). 



Reversible denaturation is now a quite familiar concept because it oc- 

 curs in bacterial luminescence, as clearly shown by the analysis of lumi- 

 nescence in terms of the theory of absolute reaction rates (5, 7). Ac- 

 cording to this theory (11), the rate of a simple reaction depends on the 

 energy needed to form some unstable intermediate complex. The effect 

 of pressure on the rate depends on the volume change between the re- 

 actant and the activated complex, while the effect of temperature depends 

 on the corresponding heat change. These changes may depend on the 

 folding or unfolding of the molecule (11, 22). Since pressure tends to fold 



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