COMPONENTS OF THE ENERGY-COUPLING MECHANISM 41 



case and it is therefore possible that the two laboratories are studying 

 reconstruction of different phosphorylation sites in the chain. 



Of greatest importance, however, is the fact that it now seems possible 

 to obtain from mitochondria in soluble and fairly stable condition, specific 

 protein factors which appear to be concerned in the mechanism of res- 

 piratory energy coupling. Identification of the specific enzymatic capabili- 

 ties of these reactions may represent a "breakthrough" to real under- 

 standing of the mechanism of oxidative phosphorylation. It is of course 

 quite possible that current hypotheses on the mechanism of phosphoryla- 

 tion and the postulated role of these factors are wrong. However, the 

 important thing is that these soluble factors are now at hand and that they 

 can be examined more carefully at the molecular level in reconstituting 

 oxidative phosphorylation. 



The swelling-contraction cycle of mitochondria 



An independent approach to the mechanism of oxidative phosphorvla- 

 tion comes from work on the contraction of mitochondria. Abundant 

 evidence now exists that both the swelling of mitochondria and their 

 active contraction, leading to uptake and extrusion of water respectivelv, 

 are phenomena which are geared to the activity or state of the respiratory 

 carriers and or the energy coupling mechanism bv which ATP is formed 

 in mitochondria (cf. [26]). The enzymes of respiration and phosphorylation 

 are located in the membranes and are thus in a strategic position to provide 

 mechano-chemical control over membrane properties, such as their mole- 

 cular geometry and their permeability. In the following discussion, 

 swelling and contraction will refer to those changes in membrane properties 

 specifically associated with the respiratory chain and the coupling 

 mechanisms which can lead to changes in the mitochondrial volume. The 

 transitory and purely osmotic changes which can be effected in mito- 

 chondrial volume for some seconds on altering merely the osmotic pressure 

 of the medium with solutes of varying degrees of penetrabilitv [27] will 

 not be discussed here. Such properties are of course common to all 

 structures bounded by semipermeable membranes. 



That mitochondrial swelling is a function of the activitv of the res- 

 piratory chain is shown most strikingly by the finding that swelling is 

 inhibited by respiratory inhibitors such as amytal, antimvcin A and cvanide 

 or by simple anaerobiosis [2S-30]. These factors inhibit swelling induced 

 by a variety of agents (cf. [26, 31]) such as phosphate, thyroxine, calcium, 

 phlorizin, and many others. At first it was concluded that this inhibition 

 was due to the maintenance of the carriers in the reduced state, particularly 

 DPNH [28, 29]. However, more extensive work by Chappell and Greville 

 [32] has shown that it is more likely that mitochondrial swelling requires 



