Induction of benzo[a]pyrene Mono-oxygenase in liver cell culture by the photochemical generation of active oxygen species. Evidence for the involvement of singlet oxygen and the formation of a stable inducing intermediate

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Abstract
The photochemical generation of excited states of oxygen in [rat] liver cell culture by the mild illumination of culture medium containing riboflavin, results in stimulation of benzo[a]pyrene 3-mono-oxygenase [EC 1.14.14.2], a cytochrome P-450-linked mono-oxygenase. The same large increase in mono-oxygenase activity was found when medium containing riboflavin was illuminated in the absence of cells and then stored in the dark for 24 h before contact with the cells. From this it may be inferred that stimulation is due to the formation of a stable inducer in the culture medium. Further experiments indicate that the stable inducer is due to the photo-oxidation of an amino acid. Evidence that singlet oxygen is responsible for initiating the stimulation of the mono-oxygenase is based on the use of molecules that scavenge particular active oxygen species. Of all the scavengers tested, only those that scavenge single oxygen inhibited the stimulation. A hypothesis is developed to relate the stimulation of the mono-oxygenase by singlet oxygen in cultured cells to the regulation of the cytochrome P-450 enzyme system in vivo. Singlet oxygen generation within cells may be a common factor linking the many structurally diverse inducers of the enzyme system.