Submitted to: Biochemical and Biophysical Research Communications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/12/1999
Publication Date: N/A
Interpretive Summary: Protoporphyrin is involved in heme and chlorophyll biosynthesis. However, accumulation of protoporphyrin in animal and plant tissues is toxic. We have previously shown that the enzyme horseradish peroxidase may protect plants against this accumulation by degrading protoporphyrin. We have found that a similar protection exists in animals, with the enzyme bovine lactoperoxidase catalyzing a similar reaction. Vitamin C was found to inhibit this process. This finding has broad implications in photodynamic cancer therapy and for treatment of certain human diseases involving abnormal heme metabolism.
Technical Abstract: Protoporphyrin, an intermediate in heme and chlorophyll biosynthesis, can accumulate 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 deuteroporphyrin are rapidly oxidized by horseradish peroxidase in the presence of some thiols, especially glutathione. This study reports that bovine lactoperoxidase, but not leucocyte myeloperoxidase, can also catalyze this reaction and that Tween and ascorbic acid are inhibitors. Exogenous hydrogen peroxide is not required and cannot replace glutathione. Deuteroporphyrin was oxidized to a unique green chlorin product with two oxygen functions added directly to the characteristic# reduced pyrrole ring of the chlorin.# Spectroscopic and chromatographic results suggest that protoporphyrin was oxidized 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 non-natural decarboxylic porphyrins, with ethyl or hydroxyethyl instead of vinyl side chains, are not substrates or products for this# enzymatic conversion.