COMPONENTS OF THE ENERGY-COUPLING MECHANISM 33 



concept of the structure proposed by Sjostrand is now under some modi- 

 fication and refinement by other workers, notably Robertson [3]. In any 

 case, alternation of oriented lipid and protein molecules in unimolecular 

 layers appears to be the basic structural plan. 



In the light of these considerations it is clear that the permeability 

 and physical state of the mitochondrial membranes could logically be 

 expected to be functions of the activity or state of the catalytically active 

 proteins which apparently make up such a large part of the structural 

 mass of the membranes. Thus the swelling-contraction cycle of the mem- 

 branes and their characteristic selective permeability may be attributed to 

 mechano-chemical changes of the respiratory and coupling enzymes, 

 analogous to the mechano-chemical activities of the actomyosin complex. 

 Furthermore work of Gamble in our laboratory [4] has demonstrated that 

 the membranes are also the site of perhaps the most prominent reaction 

 of mitochondrial active transport, namely the active binding of K+. 

 Isolated digitonin fragments of the membranes bind K + specifically 

 during coupled phosphorylation to an extent which can account nearly 

 completely for the entire activity of intact mitochondria. 



Finally it should be pointed out that the selective permeability of the 

 mitochondrial membranes may be an element in physiological control 

 mechanisms. For example, it has been assumed in some recent specula- 

 tions on the mechanism of the Pasteur reaction [5] that ATP generated 

 by mitochondria is segregated or compartmented in the mitochondria, so 

 that it does not "mix" with glycolytically generated ATP. 



"Partial reactions" and the mechanism of oxidative 

 phosphorylation 



No attempt will be made to review in any detail thedexelopment of ideas 

 and the experimentation which have led to current outlines of knowledge ; 

 recent reviews by Slater [6], Lehninger [i, 2, 7, 9, 10], and Chance [8] 

 may be referred to. However, some of the most valuable information has 

 come from study of the so-called "partial reactions" of oxidative phos- 

 phorylation which are reflections of the fact that at least some if not all 

 the intermediate reactions are reversible. The most fundamental dis- 

 covery was probably the finding that the uncoupling agent dinitrophenol 

 stimulates hydrolysis of ATP, indicating that a "leak" in the coupling 

 mechanism occurs in the presence of this reagent (cf. [11]). Since DNP can 

 release respiration from its dependence on ADP in the absence of in- 

 organic phosphate (cf. [6]), the site of action of DNP appears to be at a 

 point prior to the uptake of phosphate. 



A second "partial reaction" of great significance is the ATP-P,^- 

 exchange. In the absence of net electron transport the terminal phosphate 



