144 Albert L. Lehninger, et al, 



respire at high rates in the complete absence of a phosphate 

 acceptor system, whereas normal mitochondria require the 

 acceptor system for maximum rates of respiration. 



Summary 



Studies of the properties and requirements of the ATP- 

 32p. and ATP-ADP exchanges occurring in phosphorylating 

 mitochondrial fragments indicate that phosphate is incorpor- 

 ated in a labile reaction step which precedes the reaction in 

 which ADP is taken up. This conclusion is strongly supported 

 by the finding that ^^Pj incorporation may be inhibited by 

 azide, ageing, ionic strength and sucrose, without affecting 

 incorporation of ADP. Similarly these agents also abolish the 

 DNP-sensitivity of the ATP-ADP exchange. Location of the 

 relative sites of action of a wide variety of uncoupling agents 

 is thus made possible by such approaches, for which the 

 behaviour of the ATP-ADP exchange enzyme in soluble 

 purified form toward uncoupling agents provides an un- 

 equivocal basis. 



A soluble protein fraction (R factor) obtained from rat 

 liver mitochondria which may contain mitochrome but which 

 is probably a mixture, uncouples phosphorylation and inhibits 

 the ATP-^^Pj exchange at high concentrations. Such R 

 factor preparations release respiration in the absence of ADP. 

 Unlike DNP, however, the R factor releases respiration at 

 concentrations which cause little uncoupling of phosphoryla- 

 tion. Addition of R factor to respiratory fragments thus 

 produces "loose coupled" systems characterized by high 

 P : 2e ratios but no respiratory stimulation by ADP. 



REFERENCES 



Beyer, R, E., and Glomset, J. (1956). Ada chem. scand., 10, 1041. 

 Chance, B., and Hollunger, G. (1957). J. Amer. chem. Soc, 79, 2970. 

 Chance, B., and Williams, G. R. (1956). Advanc. EnzymoL, 17, 65. 

 Cooper, C, and Lehninger, A. L. (1956a). J. biol. Chem., 219, 489. 

 Cooper, C, and Lehninger, A. L. (19566). J. biol. Chem., 219, 519. 



