Submitted to: The Federation of European Biochemical Societies
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 9, 1999
Publication Date: N/A
Interpretive Summary: The fate of porphyrins and chlorophylls in plants is essentially unknown. We have discovered that plant peroxidases, such as horseradish peroxidase, rapidly break down the pigment protoporphyrin IX. This reaction requires the presence of reducing thiol substrates, such as glutathione and cysteine, that are quite abundant in nature. The oxidative degradation of protoporphyrin IX consumes oxygen and this process is inhibited by ascorbic acid. This new mechanism may play a significant role in the resistance of certain weed species to herbicides, such as the diphenyl ethers, that cause protoporphyrin IX to accumulate to toxic levels in plants. Plants that can metabolize protoporphyrin IX rapidly may be more resistant to this class of herbicide than plants that have low level of porphyrin degradation activity.
Technical Abstract: Abstract Protoporphyrin IX (PP) is the last compound in common between heme and chlorophyll biosynthesis. This pigment normally does not accumulate in plants because its highly photodynamic nature makes it toxic. While the steps leading to heme and chlorophylls are well characterized, relatively little is known of the metabolic fate of excess PP in plants. We have discovered that plant peroxidases can rapidly degrade this pigment in the presence 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.