EFFECTS OF TEMPERATURE: CELLULAR SYSTEMS 773 



Certainly most metabolic processes involve a variety of multienzyme sys- 

 tems of different types. Respiration normally is the over-all result of en- 

 zyme activity in monolinear chains, polylinear chains, branched chains, 

 cycles, and regenerative systems all operating simultaneously. It would 

 appear extremely unlikely that the variation of respiration with the tem- 

 perature could be interpreted in specific terms, or that temperature char- 

 acteristics could be assigned to certain enzymes functioning in the total 

 system. The presence of an inhibitor always introduces an additional fac- 

 tor into an already very complex situation and so the effects of temperature 

 on inhibition must also in the general case be considered to be difficult to 

 analyze. However, in some instances the use of a selective inhibitor may 

 simplify the problem to some extent. If certain pathways can be blocked, 

 or their importance in the over-all process minimized, the metabolic system 

 may become more simple and the effects of temperature more readily 

 interpretable. 



One more temperature-dependent characteristic of multienzyme systems 

 must be mentioned before proceeding to a consideration of cellular meta- 

 bolism. When an inhibitor acts on a multienzyme system in the steady state, 

 it requires a definite time for the system to reach a new steady state, if 

 such occurs. This transition time has been discussed in Chapter 7 and it 

 was pointed out there that in a monolinear chain the rate of change of the 

 concentration of the intermediate depends on the difference between the 

 rates of formation and utilization, i.3., d{B)ldt = v^ — v^. When i\ = v^ 

 in the steady state, d(B)jdt = 0. When reaction 2 is inhibited, d{B)ldt = 

 ^1 — (1 — i)v2. A change in the temperature will alter v^ to g^v^ and Vg 

 to g2V2, so that now d{B)jdt = g-i^v^ — (1 — '?)g2^2- The rate at which (B) 

 changes and, hence, the transition time will depend on the relative effects 

 of the temperature change on i^ and v^. If reaction 1 is speeded relative to 

 reaction 2, the transition time will be less because the rate of change of 

 (B) will be greater; conversely, if reaction 2 is relatively speeded, the tran- 

 sition time will be lengthened. These effects on transition time may well 

 be of importance in the effects of inhibitors on cells and tissues at different 

 temperatures. For example, it is quite possible that more damage will be 

 done to a tissue by an inhibitor if the metabolism cannot reach a new steady 

 state as rapidly. 



EFFECTS OF TEMPERATURE: CELLULAR SYSTEMS 



The effects of temperature variations on the activities of cells and their 

 sensitivity to enzyme inhibition are very complex, since not only is the 

 cell a summation of hundreds of enzymes and multienzyme systems, each 

 responsive in a characteristic way to temperature, Ijut the metabolic and 

 functional activities are dependent on a structural organization that is 

 also influenced by temperature changes. A complex system is usually more 



