108 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



with its large volume decrease, activates three enzymatic sites which 

 then catalyze the phosphorylation reaction which proceeds with a de- 

 crease in volume. 



In contractions of the glycerated fiber produced by pressure at pH 

 5.6 the tension developed is controlled by reaction (A) with no indication 

 of an inhibitory reaction which would be expected if reaction (Dj were 

 implicated. At pH 7.0, however, the inhibition of tension via reaction 

 (D) becomes a significant factor, particularly at the high pressures. 

 Apparently the two reactions have a sufficiently different pK value to 

 eliminate the action of pressure on reaction (D) at the lower pH. In 

 the pressure contracture in muscle there is no indication of reaction (D), 

 and reaction (A) appears to be in control over the physiological range of 

 pressures. In contrast to the extensive action of pressure in contracture, 

 the maximum tetanus tension may be decreased, increased or unchanged 

 by pressure. In the auricular muscle of the turtle at 5°C tension at the 

 peak of the twitch is inhibited by pressure with a volume decrease of 

 120 cc/mole. Such a variable effect of pressure on maximum tension 

 raises the possibility that where the pressure effect is small tension may 

 have escaped from the controlled reactions (A) and (D) and be governed 

 in respect to pressure by factors inherent in reaction (B), such as the 

 fibrous state of the protein. 



In regard to temperature, the major simplification which appears is 

 that the tension-temperature relation is governed by the })K of the active 

 sites. Since a major factor in determining the pK is the phosphorylation 

 reaction (D), the heat constant represents an energy of activation, AH* 

 and has a value of 12,000 cal. This is an important consideration since 

 the value of 36,000 cal., resulting from the three active sites being involved 

 per unit tension, would be quite incompatible with results of thermal 

 studies on the muscle if it represented a heat of reaction. In the schema a 

 aH of 36,000 is assigned to reaction (D). As regards reaction (A), it ap- 

 pears to be temperature-independent. In reaction (B), however, there is 

 the possibility, as indicated by the linear temperature dependence of the 

 glycerated fiber at pH 6.15 that this reaction has a AH of about 6,000 cal. 



Another possibility which has not been dealt with in this paper is that 

 a decrease in tension may involve the return of the contracted unit to 

 the inactive form by another pathway (G). In the light of Polissar's 

 and Goodall's consideration this requires examination, particularly in 

 relation to certain irreversible effects of pressure on tension in the glyc- 

 erated fiber. 



REFERENCES 



1. Brown, D. E. S., K. F. Guthe, H. C. L.\wler and M. P. Carpenter. The 

 pressure, temperature and ion relations of myosin ATPase. In press. 



