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.