Control of Rate of Intracellular Respiration 65 



A comparison of the stimulation of the ATPase by DNP 

 (pK = 4-1) and p-nitrophenol (PNP; pK = 7-2) at different 

 pH's suggests that the phenolate ion is active rather than the 

 undissociated phenol (de Ronden, 1958, unpubhshed). 



All three phosphorylation steps of the respiratory chain 

 are uncoupled by DNP. A study of the pH-activity curves 

 of the ATPase of liver and heart mitochondria under various 

 conditions led to the conclusion that there are three different 

 DNP-stimulated ATPases, characterized by pH optima at 6 -3, 

 7-4 and 8-5 (Myers and Slater, 1957a). These different 

 enzymes have different sensitivities to DNP, the 6 • 3 enzyme 

 being the most sensitive, and the 8 • 5 enzyme the least. A 

 study of the pH-activity curves of oxidative phosphorylation 

 (Hiilsmann and Slater, 1957) led to the conclusion that the 

 8 • 5 enzyme was associated with X '^ I^ (the intermediate in 

 the phosphorylation between DPNH and fp), the 7-4 enzyme 

 was associated with X ^-^ I2 (the intermediate in the phos- 

 phorylation between cytochromes b and c), and the 6-3 

 enzyme was associated with X '^ I3 (the intermediate in the 

 phosphorylation at the oxygen end of the respiratory chain). 

 These results are summarized in Table I. 



Table I 



Properties of I's 



I Phosphorylaiive x^H optimum of reaction [DNP] required for 



step of X r^ I -{■ ADP 4- Pi 50 per cent maximum 



respiratory chain ^ X -\- I ■{■ ATP stimulation of ATPase 



M 



Ii DPNH + H+ + fp-^ 8-5 6x10-5 



DPN+ + fpHa 



12 2 6"+ 2 c- ^2 6-+ 7-4 1-8 XlO-^ 



2 c- 



13 2 a- + lOa + 2 H+-> 6^-3 8 X 10-^ 



2 a- + HgO 



The conclusion, drawn from the pH-activity curves of the 

 ATPase reaction (Myers and Slater, 1957a) and of oxidative 

 phosphorylation (Hiilsmann and Slater, 1957), that the DNP- 

 stimulated ATPase with pH optimum at 6-3 is associated 



CELL METAB. — 3 



