TETRAPYRROLES 41 



There are stillmajor gaps in the pathway to be filled in, particularly 

 in the area between porphobilinogen and protoporphyrin. Uroporphyrin- 

 ogen and coproporphyrinogen seem firmly established as intermediates 

 and the accumulation of the corresponding oxidized porphyrins in 

 cultures is probably due to spontaneous oxidation of the porphyrinogens. 

 Conversion of porphobilinogen to the uroporphyrinogen in occurs in at 

 least two steps catalysed by different enzyme fractions, one being a 

 porphobilinogen deaminase and the other having an isomerase function 

 (22). An enzyme fraction has been prepared from Rps. spheroides 

 which decarboxylates uroporphyrinogen to coproporphyrinogen (23). 

 This conversion is likely to proceed in several stages and the detection 

 by chromatography of traces of porphyrins with five to seven carboxyl 

 groups in enzymic reaction mixtures, as well as in the porphyrin mix- 

 tures excreted by whole cells, support this. The conversion of copro- 

 porphyrinogen to protoporphyrin has been studied with an enzyme 

 fraction purified twentyfold from beef liver mitochondria (24). The 

 reaction requires oxygen but is not inhibited by cyanide. The mechanism 

 is largely unknown but a tricarboxylic porphyrinogen and protoporphy- 

 rinogen are probable intermediates. There is also evidence that an 

 intermediate is, or can become, covalently bound to protein (25). 



Under conditions of iron deficiency coproporphyrin HI is always the 

 major porphyrin accumulated by photosynthetic bacteria; protopor- 

 phyrin and chlorophyll derivatives are not found. With adequate iron, 

 the porphyrin output is considerably less (1 to 10% of that with low 

 iron) but under these conditions protoporphyrin (Fig. 2), magnesium 

 protoporphyrin monomethyl ester, and chlorophyll derivatives pre- 

 dominate. This suggests that iron participates at a stage in the con- 

 version of coproporphyrinogen to protoporphyrin. Such a function for 

 iron is supported by whole cell experiments with Rps. spheroides 

 (26,27). Conversion of 5-aminolaevulate (ALA) to coproporphyrin III 

 occurs when iron-deficient cells are incubated anaerobically in the 

 light in the presence of phosphate and Mg2+only; no protoporphyrin is 

 formed. Addition of iron to such suspensions promotes synthesis of 

 protoporphyrin and free heme. Additional evidence for the involvement 

 of iron in protoporphyrin synthesis is provided by the inhibition by 

 o-phenanthroline of the mitochondrial enzyme system that converts 

 coproporphyrinogen to protoporphyrin (24). To establish the function 

 of iron in the conversion of coproporphyrinogen to protoporphyrin, 

 further enzymic studies are clearly needed, but so far there has not 

 been much success in preparing extracts of Rps. spheroides active in 

 this respect. 



Another interesting aspect of the conversion of coproporphyrinogen 

 to protoporphyrin in the photosynthetic bacteria concerns the nature 

 of the oxidant needed for the oxidative decarboxylation. The enzyme 

 systems from animal tissues show an obligatory requirement for 

 oxygen and alternative electron acceptors have not been demonstrated. 



