78 



INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



8,000 lb/in.-, the points begin to fall below the line. The shape of the 

 curve again points to an interplay between two effects of pressure (3). 

 At low pressures, renaturation of active sites dominates, while at high 

 pressures most sites have been renatured and the direct inhibition of 

 enzyme-substrate breakdown becomes more important. The curve in fig- 

 ure 4 was calculated for volume changes of 108 and 63 cc/mole, which fit 

 the data slightly better than the 97 and 56 cc/mole found from the pH 

 data. The fit could undoubtedly be improved by further juggling of num- 

 bers, but it is already within experimental error. 



32 uM ATP 

 1.47 yM CA 

 30- pH8.4 



PRESSURE (PSD 

 J I I I 



Fig. 4. Pressure increment in 

 myosin ATP-ase activity as a 

 function of pressure (4). For 

 discussion of data points see 

 text. The curve is the calculated 

 increment for a two-fold pres- 

 sure effect, on enzyme-substrate 

 breakdown and on reversible de- 

 naturation. 



4000 



8000 



12000 



DISCUSSION 



Now that the existence of reversible denaturation is established, the 

 hydrolysis of ATP by myosin involves at least four known single-step 

 reactions. First, the enzyme combines with substrate, which cannot be 

 limiting under the present experimental conditions. The enzyme combines 

 with calcium, which increases the rate but does so independently of tem- 

 perature and pressure. The I'eversible denaturation of the enzyme and the 

 breakdown of the enzyme-substrate complex are both sensitive to l)oth 

 temperature and pressure. Several reaction schemes have been suggested 

 (2, 16, 18, 19, 24), into one of which these four reactions may eventually 

 be fitted. The present reversible denaturation may correspond to a sug- 

 gested deformation or configurational change. 



Table 1 lists some of the temperature and pressure characteristics of 

 myosin reactions. The volume change for solation of actomyosin gels 

 (14) is nearly that for reversible denaturation. Pressure favors the sol 



