216 Discussion 



state which seems to be required for haem formation, and that the porphyrin be kept 

 in solution. I feel that Neve is using the right approach, that is, to purify and fraction- 

 ate until one does or does not finally isolate an enzyme system which appears to be 

 specific. 



LocKWOOD : The entry of iron into a porphyrin structure with four ^NH groups seems 



surprising and it was for this reason that Heikel in Rimington's laboratory compared 

 the non-enzymic incorporation of iron into coproporphyrin, coproporphyrinogen and 

 protoporphyrin using, however, concentrations far in excess of physiological con- 

 ditions. With Teepol to keep the reactants in solution (or suspension) she was able 

 to show some incorporation into all three compounds and we attributed the formation 

 of coprohaem from the leuco-compound to its preliminary oxidation. However, in 

 view of Neilands' findings of the very rapid incorporation of iron into the partially 

 reduced porphyrin it seems worth while to consider the structure of these intermediate 

 reduction products. Although these compounds are very badly defined and have not 

 been obtained crystalline, it is generally accepted that there is a hexahydro-, a tetra- 

 hydro- and a dihydro-compound. Now the dihydro-compound probably has a 



structure with two NnH groups and two \n groups, that is, the same ligands as a 



porphyrin. It is suggested that the possible non-planar structure of this compound 

 might allow a very rapid entry of iron. 



Lemberg : A dihydroporphyrin with reduction of two opposite methine bridges to CHj 

 would be tilted at these bridges and might be suitable for incorporation of iron into 

 the bis-dipyrromethene system. Such a compound should have a spectrum similar to 

 that of dipyrromethenes with an absorption band in the blue-green and be of yellow 

 colour. 



Barrett: In connexion with Neve's studies on the enzymic incorporation of iron into 

 protoporphyrin, I should like to refer to the question of haem a^ biosynthesis. In 

 non-enzymic systems the rate of incorporation of iron — and of metals generally — into 

 a chlorin is very much slower than for a porphyrin with similar side-chains. The study 

 of the enzymic incorporation of iron into chlorin 02 should be very interesting since 

 one would expect a greater difference between the rates for the enzymic and non- 

 enzymic systems. 



A further point is relevant to Falk's evidence from studies with avian red cell 

 haemolysates, suggesting that a reduced form of protoporphyrin not protoporphyrin 

 itself, is the true precursor of haem. Reduction of chlorin a.^ to a leuco-compound 

 would be expected, through introduction of symmetry into the molecule, to facilitate 

 greatly entry of iron into the tetrapyrrole ring. 



Lemberg: We must not forget that in vitro incorporation of iron into copro- and uro- 

 porphyrin is certainly no more difficult than into protoporphyrin. Yet a careful 

 search by Lockwood for uro- or coprohaem has failed to reveal their presence in 

 several tissues. This strongly supports the role of enzymes in iron incorporation 



Biosynthesis and Metabolism of Cytochrome c 



By D. L. Drabkin (Philadelphia) 



Drabkin : The systematic studies of the metabolism and biosynthesis of cytochrome c in 

 the writer's laboratory have been previously reviewed (Drabkin, 1951a, 1955). The 

 research findings have permitted the development of a number of major concepts: 



(1) The independent biosynthesis of the different haemin proteins is a general 

 property of living, aerobic cells (Drabkin, 1951b; Marsh and Drabkin, 1957). This 

 was based on studies of the labelling of cytochrome c by means of [2-^^C] glycine and 

 [2-^^C] lysine. A similar conclusion was reached independently by Theorell et al. 

 (1951) in a study of liver and red cell catalases by means of ^'Fe. In our in vivo studies 

 (Drabkin, 1951b) the technique of liver regeneration after partial hepatectomy 

 (Crandall and Drabkin, 1946) renders unequivocal the conclusion that new cytochrome 



