EFFECTS OF TEMPERATURE: CELLULAR SYSTEMS 



797 



Table 15-1 



Effects of Quinine ox the Temperature Characteristics and Thermodynamic 

 Properties of Horse Serum Cholinesterase '' 



" From Robert et al. (1951; 



fects at the highest quinine concentration. The presence of 2,4-dinitrophenol 

 causes the appearance of a discontinuity in the Arrhenius plot for myosin 

 ATPase when ATP is the substrate (Koshknd, 1959). The curve, in fact, 

 becomes quite like that for the hydrolysis of ITP. This was interpreted as 

 resulting from a competition of the phenoxide ion of 2,4-dinitrophenol 

 with the S-NHo group of ATP for a group in a protein chain, releasing the 

 constriction normally imposed by the interaction of the 6-NH2 group 

 with the enzyme group. The possible effects of inhibitors on enzyme and 

 enzyme-substrate flexibility must therefore be considered in interpreta- 

 tions of changes in the temperature characteristics. 



Urease was claimed by Sizer (1939) to show // values of either 8700 or 

 11,700. In sulfite solution a critical temperature of 23° was observed, // 

 being 11,700 below and 8700 above this temperature. In various mixtures 

 of ferricyanide and ferrocyanide the // value was either of these figures and 

 it was suggested that in reducing solutions the configuration of the active 

 center was such that // = 8700 but as certain sulfhydryl groups became oxi- 

 dized the enzyme activity decreased and // = 11,700, these changes being 

 reversible. Kistiakowsky and Lumry (1949) reported that such a sudden 

 inflection at the critical temperature did not occur in the case of urease and 

 that in sulfite solution // rises progressively from 8830 to 15,000 as the tem- 

 perature is lowered. This change in the temperature characteristic was ex- 

 plained on the basis of an inhibition by sulfite, this inhibition exhibiting 

 an enthalpy change of — 29 kcal/mole. In other words, as the temperature 

 is decreased, the sidfite inhibition becomes important over a rather narrow^ 

 temperature range and alters //. They believe that many inflections in bio- 

 logical Arrhenius curves may be explained by the presence of reversible 

 inhibitors whose combinations with enzymes become particularly impor- 

 tant at lower temperatures, rather than on the basis of a shift from one lim- 



