Glutathione-dependent oxidative modification of protoporphyrin and other dicarboxylic porphyrins by mammalian and plant peroxidases

Protoporphyrin, an intermediate in heme and chlorophyll biosynthesis, can a ccumulate in human and plant tissues under certain pathological conditions and is a photosensitizer used in cancer phototherapy. We previously showed that protoporphyrin and the related non-natural dicarboxylic porphyrin deut eroporphyrin are rapidly oxidized by horseradish peroxidase in the presence of some thiols, especially glutathione. This study reports that bovine lac toperoxidase, but not leucocyte myeloperoxidase, can also catalyze this rea ction and that Tween and ascorbic acid are inhibitors. Exogenous hydrogen p eroxide is not required and cannot replace glutathione. Deuteroporphyrin wa s oxidized to a unique green chlorin product with two oxygen functions adde d directly to the characteristic reduced pyrrole ring of the chlorin. Spect roscopic and chromatographic results suggest that protoporphyrin was oxidiz ed not to a green chlorin, but to a much more polar red porphyrin modified by oxidative addition to the two vinyl side chains. Two related nonnatural dicarboxylic porphyrins, with ethyl or hydroxyethyl instead of vinyl side c hains, are not substrates or products for this enzymatic conversion. (C) 19 99 Academic Press.