100 SUBCELLULAR PARTICLES 



this cytochrome from both yeast and heart muscle. The isolated cytochrome is a 

 water-soluble molecule of molecular weight 13,000. It is only very recently that 

 we have come to appreciate that the least complex water-soluble forms in which 

 the cytochromes and the flavoproteins can be isolated bear the same relation to 

 their physiological forms as does nicotinamide to diphosphopyridine nucleotide. 

 For that reason the properties of these simple forms are highly deceptive, and 

 two generations of biochemists have understandably fallen into a trap. 



To the best of our knowledge, cytochrome c in its classical form does not exist 

 as such in mitochondria or in the electron transport particle. Widmer and Crane 

 in our laboratory have discovered that cytochrome c exists in the form of one or 

 more lipid-soluble complexes in the electron transport particle ( 32). One such 

 complex can be isolated by treating ETP with deoxycholate; another can be pre- 

 pared by mixing cytochrome c and the Q lipoprotein. These complexes are 

 insoluble in water and soluble in various organic solvents including hydro- 

 carbons. The lipid-complexes of cytochrome c still retain enzymatic activity and 

 can substitute for the water-soluble form in restoring enzymatic activity. There 

 may well be a family of lipid-cytochrome c\ which function at different parts of 

 the chain and which may be peculiar to either the succinic or DPNH chain. One 

 locus of action is certainly between coenzyme Q and cytochrome a in the succinic 

 chain, but other loci are not excluded. 



The particular point I want to stress is that cytochrome c functionally resembles 

 coenzyme Q more closely than the fixed oxidation-reduction components. It is a 

 relatively small molecule. It is readily taken up by the Q lipoprotein. It is 

 readily extracted from the electron transfer particle by lipid-splitting reagents 

 such as isobutyl and butyl alcohols and deoxycholate, and it can be reinserted into 

 the particles from which it was extracted. The fact that the water-soluble form 

 has catalytic properties is not surprising in view of the ease with which the lipid- 

 complex is formed by interaction of cytochrome c with the phosphatides of various 

 lipoproteins. 



The exact chemical composition of the various lipid complexes of cytochrome 

 c has yet to be clarified and this problem is now under active investigation by 

 F. L. Crane and K. Ambe. 



My colleagues, J. Jarnefelt, and H. Tisdale, and I have isolated cytochrome c^ 

 for the first time in our laboratory as a water-soluble molecule in homogeneous 

 state with a molecular weight of about 380,000 (15). It can readily be shown that 

 this form is derived from a complex of cytochrome c^ and a specific lipoprotein 

 which contains 66 per cent by weight of lipid. When this complex is exposed to 

 the action of sodium lauryl sulfate or Duponol, the cytochrome is released from 

 the complex, and the lipoprotein and cytochrome c^ can then be isolated sepa- 

 rately. Again we find evidence of lipid, in the form of a lipoprotein, being closely 

 associated with the oxidation-reduction components of the electron transfer chain. 



