142 Albert L. Lehninger, et al. 



Table IV 



Respiration-releasing action of R factor 



Test system contains digitonin fragments (100 \xg. N), 0-01 m p-hydroxy- 

 butyrate, 01 m phosphate labelled with ^^p^ pH 6-5, water, and additions 

 shown to volume of 3-0 ml. Incubated 30 minutes at 23°. ADP was added 

 at • 0024 M as shown. 



Rate of respiration 

 m[imoles acetoacetate 



* P : 2e = moles AT^sp formed per mole of acetoacetate. 



uncoupling and releasing activities. Some preparations 

 release respiration fully with negligible uncoupling, whereas 

 others uncouple substantially but not completely, a behaviour 

 which suggests occurrence of two factors in the preparation. 

 The R factor activity thus differs from that of DNP and all 

 known uncoupling agents we have studied, and the R factor 

 is thus more properly described as a "respiration-releasing" 

 factor. The extra respiration of the digitonin fragments which 

 is stimulated by the R factor is completely inhibited by 

 antimycin A and is also blocked completely by presence of 

 serum albumin. Tests of the site of action in the respiratory 

 chain indicate that the R factor stimulates electron transfer 

 in the span p-hydroxybutyrate to cytochrome c and also in the 

 span (ascorbate) cytochrome c to oxygen, indicating that its 

 action may be on all three uncoupling sites. 



There are as yet insufficient data at hand to define the mode 

 of action of the R factor precisely in terms of the reaction 

 mechanism postulated in Fig. 2. However, it appears neces- 

 sary to postulate that the factor acts at a point other than 



