Involvement of Singlet Oxygen and the Formation of a Stable Inducing Intermediate

1. The photochemical generation of excited states of oxygen in liver cell culture by the mild illumination of culture medium containing riboflavin, results in stimulation of benzo[a]pyrene 3-mono-oxygenase, a cytochrome P-450-linked mono-oxygenase. 2. 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 24h 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. 3. Evidence that singlet oxygen is responsible for initiating the stimulation of the monooxygenase is based on the use of molecules that scavenge particular active oxygen species. Of all the scavengers tested, only those that scavenge singlet oxygen inhibited the stimulation. 4. A hypothesis is developed to relate the stimulation of the mono-oxygenase by singlet oxygen in cultured cells to the reguilation of the cytochrome P.450 enzyme system in vivo. It is suggested thatsinglet oxygen generation within cells may be acommon factor linking the many structurally diverse inducers of the enzyme system.