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Abstract
The main porphyrins found in nature apart from protoporphyrin and derivatives of this compound are uroporphyrin and coproporphyrin, having 8 and 4 carboxylic groups respectively. The relation of these porphyrins to heme biosynthesis has been debated for some time, and in 1950 Radin, Rittenberg and Shemin & Neuberger, Muir & Gray suggested that protoporphyrin was formed from uroporphyrin by stepwise de-carboxylation and dehydrogenation. However, the experimental evidence for such a mechanism was rather incomplete.
A coproporphyrin precursor behaving chromatographically as a 3 carboxylic porphyrin has been found in extracts from rabbit reticulocytes after incubation of the cells under hypoxic conditions.
The nature of this precursor has been discussed, and evidence that we are dealing with a partially oxidized coproporphyrinogen has been presented.
It is suggested that the first step in the conversion of coproporphyrinogen into protoporphyrin is a partial oxidation of the por-phyrinogen prior to the final decarboxylation and dehydrogenation into protoporphyrin.