COMPONENTS OF THE ENERGY-COUPLING MECHANISM 47 



rat, in Ehrlich ascites tumour cells, and also in erythrocytes and extracts 

 of Escherichia coli. Of great significance is the finding that C-factor 

 activity is especially rich in contractile tissues like skeletal muscle and car- 

 diac muscle. We have earlier called attention to the possibility that C- 

 factor may bear a relation to the mitochondrial membrane analogous to 

 that born by actin to myosin. While this analogy is only suggestive at this 

 stage, it is of interest to point out that erythrocyte membranes can be 

 induced to change shape in the presence of ATP [51] and it is now well 

 known from studies of Abrams and others that bacterial protoplast mem- 

 branes also undergo swelling-contraction cycles which are metabolism- 

 dependent [52]. 



It is also significant that the partlv purified specimens of C-factor 

 contain some ATP-ase activity, which suggests that they may be related 

 to the factor described by Pullman et al. [2^^ which is capable of restoring 

 oxidative phosphorylation in heart preparations. 



Other factors in mitochondrial contraction 



There is evidence that mitochondrial substances other than C-factor 

 are necessary in contraction. It has been found bv measuring the light- 

 scattering envelope of intact mitochondria [^^t^] that the ratio of light 

 scattered at 135 to that scattered at 45' to the incident beam measures a 

 change in mitochondrial configuration induced bv ATP which is not 

 measureable by simple light absorption or bv light scattered at 90 . This 

 change is promoted by substance(s) "leaking" from mitochondria stored 

 simply at o in sucrose which are apparently not identical with C-factor. 



Lastly, the rather puzzling effect of L-thyroxine in stimulating mito- 

 chondrial contraction by ATP [54] must be mentioned. L-thvroxine is 

 thus not only a swelling agent, but can also stimulate contraction. 



Concluding remarks 



A number of soluble mitochondrial factors having significant action of 

 an apparently enzymic nature on oxidative phosphorylation and mito- 

 chondrial swelling and contraction have now been recognized. These 

 include (i) the soluble ATP-ADP exchange enzyme, (2) M-factor, (3) 

 C-factor, (4) sucrose-extracted contraction factor, as well as earlier des- 

 cribed entities such as (5) U-factor [^^^ (presumablv an uncoupling fatty 

 acid) and (6) R-factor, a protein fraction which releases respiration from 

 its dependence on ADP but which does not uncouple phosphorvlation 

 [56]. With the protein factors from beef heart mitochondria separated by 

 Titchener and Linnane [24] and by Pullman et al. [25], as well as the in- 

 creasing successes in dissociation and recognition of the respiratorv carriers, 

 a significantly large number of elements of the mitochondrial membrane 



