362 THE VIEWPOINT OF A PHYSIOLOGIST 



and, in some instances, the temperature characteristic of the total 

 metabolic system. The explanation of such acclimation effects is 

 by no means clear. It is unlikely that qualitative changes in en- 

 zyme proteins occur, since protein structure appears to be genetic- 

 ally fixed. There may, however, be quantitative changes in en- 

 zymes, a sort of enzyme induction. 



Among microorganisms enzyme synthesis can be induced by 

 forcing the use of unfamiliar substrates, for example sugars which 

 the organism had not been able to metabolize. Enzyme induction 

 has been demonstrated in animals — the synthesis of tryptophane 

 peroxidase in rat liver induced by injection of L-tryptophane 

 (Knox, 1951; Lee and Williams, 1953). There is no evidence, 

 however, that physical factors such as salinity, temperature, and 

 oxygen partial pressure can similarly induce enzyme synthesis. 

 The following hypothesis might explain the effects of physical 

 agents in terms of enzyme induction: It is recognized but not 

 sufficiently emphasized that there are numerous alternate paths 

 in digestion and in intermediary metabolism. Some of these paths 

 are followed by certain tissues and animals and at certain stages 

 in a life cycle more than are other paths. Parasitic helminths in 

 hypoxia switch to glycolytic systems and excrete the acids 

 formed. Cecropia pupae use a cyanide-insensitive oxidative path- 

 way (presumably a flavoprotein ) until cytochrome oxidase is 

 activated by hormones of metamorphosis. If one path is affected 

 more than another by temperature, by oxygen pressure, or by 

 salinity, some of its substrates might pile up, and these might 

 serve as intermediate substrates of an alternate path. These sub- 

 stances might then induce synthesis of some limiting enzyme of 

 the alternate path. The limiting temperatures and the tempera- 

 ture characteristics of the overall metabolism might be changed 

 according to relative importance of given pathways. In fact, a 

 change in slope of the rate-temperature curve (change in tem- 

 perature characteristic) could effectively increase or decrease the 

 absolute rate of oxygen consumption at a given temperature. 



What is needed is a quantitative assay of a variety of known 

 alternate paths under different acclimation states, a very difficult 

 task. An enzymatic analysis of adaptive phenotypic variation is 



