EFFECTS OF PRESSURE 851 



on one or more enzymes, and not necessarily on the one involved in the final 

 emission of light. A study of the effects of sudden pressure changes in such 

 isolated systems has led Strehler (1955) to postulate that high pressure in- 

 hibits the DPNH-FMN reaction (indicating a volume increase on activation) 

 and accelerates the oxidation of FMNHg via the luminescent pathway (indi- 

 cating an over-all volume decrease). It was pointed out that other inter- 

 pretations are possible but for our present purpose in discussing the effects 

 of pressure on the inhibition of luminescence it emphasizes that some re- 

 evaluation of the earlier conclusions must be made, since in the earlier 

 work the effect of pressure on only one enzyme was assumed. The sites of 

 actions of the various inhibitors in the sequence are not known and this 

 further complicates an analysis of the pressure effects on inhibition. The 

 actions of the inhibitors on the isolated systems with respect to pressure 

 have not been investigated as yet. 



The first demonstration that the inhibition of a metabolic system could 

 be reduced or eliminated by an increase in the pressure was in a study of 

 bacterial luminescence by Johnson et al. (1942 a). The inhibitions produced 

 by chloroform, ethyl ether, ethanol, procaine, and certain carbamates are 

 diminished by high pressure, whereas inhibitions by p-aminobenzoat^, 

 sulfanilamide, barbital, and chloral hydrate are scarcely affected or slightly 

 increased (Fig. 15-25). The explanation for these effects was formulated in 

 terms of shifts between native and denatured forms of some critical enzyme. 

 For example, inhibitors such as ethanol or the carbamates were believed to 

 lower the activation" energy for the reversible denaturation of the enzyme; 

 since denaturation involves an increase in the volume, high pressures would 

 tend to counteract the inhibition. Actually, if the interaction of the inhi- 

 bitor with the enzyme in any way involves an increase in the volume, an 

 antagonism by pressure would be observed. Also, since the hmiinescence 

 has now been shown to depend upon a multienzyme system, further pos- 

 sibilities for pressure effects are conceivable. 



A more detailed study of the inhibition by ethanol (Johnson et al., 

 1945) gave the results shown in Fig. 15-26 for the effects of pressure. A 

 consistent depression of the inhibition was seen at all concentrations of 

 the ethanol but the volume changes, obtained by logarithmic plots, were 

 154 ml/mole at 0.2 M and 60 ml/mole between 1 and 1.5 M, possibly in- 

 dicating that somewhat different mechanisms are responsible for the inhi- 

 bition at different concentrations, or that the actions on two or more steps 

 of the multienzyme system vary relatively. A disturbing factor, which 

 has never been corrected for in studies of this type, is seen at 24.4° and 

 0.5 M ethanol: increasing pressure accelerates the luminescent reaction 

 equally in both the presence and absence of ethanol, which leads to an ap- 

 parent decrease in the inhibition. If the stimulation were due to some 

 step unrelated directly to the ethanol inhibition, these results would not 

 actually imply a reduction of this inhibition. It may also be mentioned 



