Control of Rate of Intracellular Respiration 71 



This is interesting, because Polis and Shmukler (1957) found 

 that mitochrome (the haemprotein) was bound so firmly to 

 albumin that the two were not separated by electrophoresis. 

 It seems likely that this binding occurred through the lipid 

 uncoupling agent. 



Pressman and Lardy (1952) isolated from microsomes a 

 heat-stable, acetone-soluble fraction which stimulated the 

 respiration of liver mitochondria in a medium deficient in 

 phosphate acceptor, and stimulated the ATPase. The active 

 substances were later identified as a mixture of lonsf-chain 

 fatty acids (Pressman and Lardy, 1955, 1956). Unsaturated 

 fatty acids such as oleic acid were found to be especially 

 active as uncoupling agents (Pressman and Lardy, 1956). 



Chemical studies of the uncoupling lipid extractable by 

 *50octane from mitochrome preparations have shown that it 

 also is an unsaturated fatty acid. 



It appears likely that the high ATPase and low phos- 

 phorylative activity of aged liver mitochondria are at least 

 partly due to the liberation of the lipid uncoupling material. 

 The pH-activity curve of the ATPase of liver mitochondria 

 aged for 2 hours at 30° has two peaks, at 6 -3 and 9 (Myers and 

 Slater, 1957a). The same peaks of activity were obtained with 

 liver mitochondria to which was added either oleic acid or an 

 acid fraction isolated from the isooctane extract of the mito- 

 chrome preparation. Lower concentrations of the lipids 

 stimulated only in the region of 6-3. The further activation 

 of the ATPase of liver mitochondria, particularly in the 7 • 4 

 and 8-5 regions, which occurs when the suspension aged at 

 30° for 2 hours is subjected to freezing and thawing (Myers and 

 Slater, 1957a), is probably not due to the liberation of the 

 uncoupling lipid. 



Possible control of metabolism by uncoupling agents 



The isolation from mitochondria and from microsomes of 

 substances which uncouple oxidative phosphorylation in 

 freshly prepared liver mitochondria raises the question of the 



