Urinary excretion of position isomers type III in normal man and a series of different types of porphyrias.

The excretion of position isomers of type III porphyrins, not only in porphyric disorders in which the enzymatic decarboxylation of uroporphyrinogen III to coproporphyrinogen III is known to or suspected to play a role, but also in normal man and in conditions like erythropoietic protoporphyria and lead-intoxication, in which no such changes have ever been reported, or would be expected for theoretical reasons, has been described. The implications of these and other findings have been discussed and it is suggested that the enzymatic decarboxylation of uroporphyrinogen III to coproporphyrinogen III is normally a random process. It is, however, not excluded that one of the many routes which could be followed might be energetically more favourable than the others and that most of the coproporphyrinogen III which is formed under physiological conditions might be formed via such a 'prefered' pathway.