Thiol-dependent degradation of protoporphyrin IX by plant peroxidases

Protoporphyrin IX (PP) is the last porphyrin intermediate in common between heme and chlorophyll biosynthesis. This pigment normally does not accumula te in plants because its highly photodynamic nature makes it toxic. While t he steps leading to heme and chlorophylls are well characterized, relativel y little is known of the metabolic fate of excess PP in plants, We have dis covered that plant peroxidases can rapidly degrade this pigment in the pres ence of thiol-containing substrates such as glutathione and cysteine, This thiol-dependent degradation of PP by horseradish peroxidase consumes oxygen and is inhibited by ascorbic acid. (C) 1999 Federation of European Biochem ical Societies.