356 R. Lemberg, P. Clezy and J. Barrett 



intermediate, different from the magnesium porphyrins isolated by Granick 

 and Bogorad (Granick, 1950; Bogorad and Granick, 1953) from Chlorella 

 mutants. 



Chlorophyll in the chloroplasts, haem a in the mitochondria, and haem a^ 

 in much smaller bacterial particles perhaps derived from the protoplasmic 

 membrane (Moss, 1954; Tissieres, 1954) are all closely associated with 

 phospholipids. While the lipid character of chlorophyll is due to its phytol 

 ester group, neither haem a nor haem a^, are esters, but have lipophilic character 

 owing to their large alkyl side chains. 



Nothing is as yet known about the oxidation-reduction potential of cyto- 

 chrome fiTa- Nor is it as yet ascertained that cytochrome flg acts as oxidase in 

 the cytochrome system of Aerobacter, although the balance of the evidence 

 appears to be in favour of this hypothesis (Tissieres, 1952; Moss, 1952; 

 Chance, 1953). The concentration of haemin a^, in Aerobacter is variable, 

 but can be almost half as great as the concentration of haem a in ox heart 

 muscle (14-43 mg/kg of dry weight). It is an induced enzyme, requiring 

 oxygen for its formation, but in contrast to cytochrome oxidase is sensitive 

 to iron-deficiency. While the recent studies of Layne and Nason (1958) and 

 Horio (1958) do not yet exclude the possibihty that their oxidase preparations 

 from Pseudomonas contain mixtures of cytochrome a.^ with cytochrome of 

 type c, a double-headed enzyme containing haem Og as well as cytochrome c 

 type prosthetic groups is another possibility. 



SUMMARY 



1 . Porphyrin a, the iron-free prosthetic group of cytochrome oxidase, is a 

 dicarboxylic porphyrin substituted with formyl, with alkylvinyl (on the 

 pyrrole ring opposite to that bearing the formyl) and with an a-hydroxyalkyl 

 on the pyrrole ring in between (pyrrole I). The alkylvinyl group and probably 

 the a-hydroxyalkyl group contain large alkyl groups. Two partial structural 

 formulae are suggested which are in harmony with the available evidence. 



2. Cryptoporphyrin a is another formyl-porphyrin which occurs in heart 

 muscle in the form of a haematin compound, though only in small amounts. 

 It greatly resembles chlorocruoroporphyrin from which it appears to differ 

 only by the presence of a large alkyl group on the vinyl side chain. 



3. Chlorin a^ is the iron-free prosthetic group of cytochrome a^. It is a 

 dihydroporphyrin without formyl groups and with one vinyl (or alkylvinyl) 

 and one a-hydroxyalkyl side chain instead of the two vinyl side chains of 

 protoporphyrin. 



Acknowledgement 



This research has been carried out with grants from the Australian National 

 Health and Medical Research Council. 



