PEROXYLATED AND HYDROXYLATED UROPORPHYRINS - A STUDY OF THEIR PRODUCTION IN-VITRO IN ENZYMATIC AND CHEMICAL-MODEL SYSTEMS

In previous work certain hydroxylated and peroxylated derivatives of u roporphyrin (URO) have been isolated from the urine of patients suffer ing from porphyria. We have now investigated the mechanism of producti on of these oxygenated derivatives of URO, using both enzymic and chem ical model systems and also the effect of exposure to light during reo xidation of uroporphyrinogen (URO'gen). When URO'gen was incubated wit h haemolysates, peaks with the same retention times as peroxyacetic ac id URO, meso-hydroxy URO and beta-hydroxypropionic acid URO were all d etected. The first of these was formed in sufficient amounts to allow its characterization by mass spectrometry. Under these conditions, per oxyacetic acid derivatives of heptacarboxylate and pentacarboxylate po rphyrins could also be produced from the corresponding porphyrinogens, but no peroxylated product could be obtained from coproporphyrinogen (COPRO'gen, where no acetic acid side chains are present) or from the fully oxidized URO. Similar results were obtained on re-oxidation of U RO'gen in the xanthine oxidase-xanthine system and in the presence of hydrogen peroxide/Fe-EDTA (ethylenediamine-tetraacetic acid) and here again no peroxylated product could be detected from either COPRO'gen o r URO. Finally, formation of peroxyacetic acid URO could be demonstrat ed during photo-oxidation of URO'gen and this was followed by light-in duced loss of both URO and its peroxylated derivative. It is concluded that the oxygenated derivatives arise from the action of reactive oxy gen species on the porphyrinogens (rather than the porphyrins), with o ne of the acetic acid side chain serving as the preferential (or exclu sive target) for peroxylation.