The Structure of Porphyrin a, Cryptoporphyrin a and Chlorin a, 361 



forms will be large; (2) the ratio — will be larger for cytochrome a.^^ than for 



pyridine haemochrome a^. 



In some bacteria where cytochrome a^ occurs, cytochromes a^^ and b^ are also found. 

 In others, cytochrome 02 occurs with c type cytochromes (as well as cytochromes aj 

 and 61). Castor and Chance {J. biol. Chem. 234, 1587, 1959) have demonstrated that 

 in some bacteria cytochrome a^ can function as an oxidase and that it also combines 

 with carbon monoxide. Nevertheless it appears to us that in other organisms it may 

 function similarly to cytochrome c in other electron transporting sequences. Like 

 cytochrome c, cytochrome a^ appears to be a di-imidazole complex from consideration 

 of its spectroscopic properties, which resemble those of pyridine haemochrome a. 

 The redox potential of cytochrome Oj we would expect to have a value around 0-20 V. 

 We then speculate that in bacteria there exist the transport systems: 



O2 02 ^1 ^i substrate 



O2 02 (^ b substrate 



E° = 0-30 0-20 005 



These systems are very like the graded redox systems of animal cells. 



Terminal oxidase systems with a redox potential around 0-30 V can utilize oxygen 

 with an almost optimal efficiency. There are now two ways of raising the redox 

 potential and destabilizing the iron-protein complexes sufficiently for such efficient 

 reactions to occur with oxygen 



(1) by introducing — CHO groups into the porphyrin ring, e.g. porphyrin a; 



(2) by addition of hydrogen to one of the pyrrole rings, e.g. chlorin 02- 



In these proceedings it has been indicated why a chain of catalysts is required. In 

 Pseudomonas oxidase, Horio (this volume, p. 302) has found that the oxygen-combining 

 haem a^ is accompanied by a type c haem. Okunuki and his school in their studies 

 on "cytochrome a" have brought forward evidence that the initial oxidase combines 

 with oxygen reversibly, and that it is only the presence of a second cytochrome which 

 enables it to act as a cytochrome oxidase and not as an oxygen transporting haem- 

 protein. Such may be the case with cytochrome a^. Evidently more model experiments 

 are required to establish this argument. 



Extractability of Ferro- and Ferricytochrome c 



Slater: In Morrison's experiments, is the degree of extractability of cytochrome c from 

 mitochondria a function of the degree of swelling of the mitochondria? 



Morrison: I do not believe that the degree of swelling of the mitochondria has much 

 effect on the extractability of the cytochrome c since the structure of the mitochondria 

 is destroyed by the acetone and salt treatment. 



Margoliash: Your treatment with boiling acetone would be expected to denature cyto- 

 chrome c, and the degree of this denaturation might depend on its oxidation state. 



Legge: Theorell's earliest preparation of cytochrome c made use of the differential 

 adsorption of oxidized and reduced cytochrome c on cellophane. 



If there is doubt as to whether cytochrome c is denatured by hot acetone, there 

 would be no doubt that the other proteins would be, and therefore perhaps be able 

 to adsorb the oxidized and reduced cytochrome differentially. 



Henderson : In connexion with the extraction of cytochrome c from mitochondria treated 

 with boiling acetone and then at 60'C, your experiments indicate that the well-known 

 difference between the adsorbability of Fe+++- and Fe++-cytochrome c is not involved. 

 Have you considered, however, the difference in the degree of modification brought 

 about by this treatment on Fe+++ as compared with Fe++ cytochrome c? Okunuki's 

 group (see, e.g. Hagihara et al. Nature, Land. 181, 1588, 1958) have reported that 

 Fe+++-cytochrome c is considerably more susceptible to modification than Fe++- 

 cytochrome c and that this is reflected largely in the increased adsorbability of the 

 former. It would seem that this could explain the observed results without implicating 

 the extraction of a 'cytochrome coupler compound'. 



