Biosynthesis of porphyrins and related macrocycles. Part VI. Nature of the rearrangement process leading to the natural type III porphyrins

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Abstract
Logical analysis of the problem posed by the constant formation in nature of type III porphyrins, e.g. protoporphyrin-IX (6), focuses attention on C–C bond making and bond breaking around the carbon atoms from which the interpyrrolic bridges are built (at C-5, C-10, C-15, and C-20). An approach to the study of such processes is outlined, based on double [13C] labelling combined with n.m.r. spectroscopy. [2,11-13C2]Porphobilinogen (17) has been prepared and applied (a) to determine the size of 13C–13C coupling for directly bonded carbon atoms in the porphyrin macrocycle and (b) to indicate that a biosynthetically significant 13C–13C coupling occurs between carbon atoms separated by three bonds. A coupled enzyme system has been developed from chicken blood cells and beef liver mitochondria which produces sufficient protoporphyrin-IX for spectroscopic and chemical work (20–30 mg); a similar system from Euglena gracilis is also described. [2,11-13C2] Porphobilinogen diluted with unlabelled porphobilinogen has been converted enzymically into protoporphyrin-IX; the 13C spectra of the dimethyl ester (i) as such, (ii) with a praseodymium shift reagent, and (iii) after chemical modification establish that the formation of type III porphyrins by both enzyme systems (avian and algal) is characterised by the same three precise features which are described.