Free radical involvement in long wavelength UV light activation of nitrosamines to mutagens

Nitrosamines are carcinogenic and mutagenic only after metabolic activation via endoplasmic reticulum bound mixed function oxidase enzyme systems. Rencently a new photochemical process has been discovered by which nitrosamines are converted into unknown mutagenic compounds by irradiation with long wavelength UV light (> 335 nm) in the presence of phosphate ion at neutral pH. The mutagenic activity is detected by Ames Salmonella Typhimurium strain TA100 in the absence of rat liver microsomes. We have shown that mutagen production with nitrosomorpholine is inhibited in the presence of light by various spin trapping agents (N-t-butyl-phenylnitrone, etc.). Concurrent with this inhibition a stable free radical signal has been detected whose kinetics of formation is similar to the time course of mutagen formation during irradiation in the absence of spin trap. The free radical signal is formed only when phosphate or similar ions are present in the reaction mixture. Monomethylphosphate and dimethylphosphate can substitute for phosphate ion but with small ESR signals and mutagen formation. Trimethylphosphate gives a weak, time independent ESR signal and does not cause mutagen formation. The ESR splitting constants (a^N and a^H) for signals generated with each of the different phosphate species show differences which suggest that these ions may be components of some intermediate free radical species that is involved in stable mutagen formation. Arsenate ion inhibits mutagen formation in the presence of phosphate but is able in the absence of phosphate to form a ESR signal similar to that observed with phosphate ion.