Photo-oxidation and Photoreduction of Catechols by Chlorophyll Metabolites and Methylene Blue

While plant-derived oxidants can protect cells from oxidative damage, limited research has examined the role of dietary chlorophyll. Photoreduction of ubiquinone by chlorophyll metabolites and red light has been reported in vitro and in animal models. Herein we examined photo-oxidation and photoreduction reactions of catechols, dopamine and hydrocaffeic acid. Photo-oxidation of dopamine by methylene blue and the chlorophyll metabolites pheophorbide A, chlorin e6 and sodium copper chlorophyllin was studied by monitoring aminochrome, the cyclized product of the dopamine o-quinone with its amine. Singlet oxygen scavengers including sodium azide, ascorbate and glutathione decreased aminochrome formation by methylene blue and pheophorbide A. Addition of EDTA, a tertiary amine electron donor, to the reaction of dopamine, photosensitizer and red light decreased aminochrome formation. Photoreduction of the dopamine o-quinone produced by mushroom tyrosinase was achieved by both methylene blue and pheophorbide A only when an electron donor was included. Due to limited solubility, photo-oxidation and photoreduction reactions by pheophorbide A required 5-7.5% dimethylformamide for optimal reactivity. Catalytic photoreduction of 2,3-dimethoxy-5-methyl-p-benzoquinone by methylene blue or pheophorbide A and tertiary amine electron donors was observed. Among the chlorophyll metabolites, pheophorbide A was more effective than chlorin e6 or sodium copper chlorophyllin in photo-oxidation of dopamine and photoreduction reactions. Singlet oxygen inhibited lactate dehydrogenase A activity, and higher molecular weight protein cross-links were observed on SDS-PAGE. Hydrocaffeic acid competed with lactate dehydrogenase A for reaction with singlet oxygen produced by methylene blue; however, no protection by hydrocaffeic acid (HCA) was observed when pheophorbide A was used. Cysteine modification of lactate dehydrogenase A by the o-quinone of hydrocaffeic acid was detected using a redox cycling stain. Inclusion of an electron donor decreased protein labeling.