88 



INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



dependence of tension. When the relaxed fiber is immersed in 0.15 m 

 succinate buffer, 5 niM MgClo , and 10 niM ATP, tension develops as a 

 function of pH along a sigmoid curve beginning about pH 5.6 and reaching 

 a plateau at pH 6.8 (fig. L4). The relation is described by 



Y 



lof 



&u 



. [y/(l - y)] = (AH + pAV)/2.3RT + 3(pH - pK) {2) 



where y is the fractional tension, pK is the pH for half tension, and the 

 factor in) of 3 indicates that three active sites are involved per unit 

 tension. 



TENSION ly) 



< 7.0- 



o ATP 



• 10 mM ATP ♦ CP 



R I mM ATP.-CP 



pK= 6.45+2 log f^P/iq) 



pK 



4 6 .10 ,.20 



concentration mM 



Fig. 1. A: Tension in the glycerated rabbit psoas fiber in relation to pH. B: Vari- 

 ation in pK in relation to the concentration of adenosine triphosphate and creatine 

 phosphate. 



The addition of 16 mM creatine phosphate, a relaxing factor, to the 

 system shifts the curve to the right along the pH axis, increases the pK 

 to 6.5, but leaves the value of n unchanged. Conversely, the addition of 5 

 mM AMP decreases the pK with a resulting shift to the left. In all 

 instances, however, n retains the value of 3. 



In respect to creatine phosphate, the pK is defined as 



pK = 6.45 + 2 logio (CP)/10 [3] 



where 6.45 is the pK for 10 mM CP (fig. IB). It may be noted that here 

 n = 2, indicating a two-fold increase in pK per mole of i:)hosphate. If ATP 

 alone is used n also equals 2. 



The unique property of this type of fiber is the pronounced sensitivity 



