Isolation and identification of photoproducts from UVA-irradiated mixture of N-nitrosoproline with 2'-deoxyadenosine

N-Nitrosoproline (NPRO) is known to form endogenously from sodium nitrite and the amino acid proline, and is thought to be non-mutagenic and non-carcinogenic. However, earlier studies in our laboratory showed that irradiated NPRO can be converted directly to a mutagenic compound upon UVA irradiation. We previously investigated the mutagenic spectrum of NPRO on M13mp2 DNA with UVA irradiation, and found a hot spot comprising AT to GC, where there might be a particular photoreaction of adenine residues with NPRO. In the present study, we investigated the reaction of NPRO with 2'-deoxyadenosine (dA) under UVA irradiation, and identified the chemical structures of the photoproducts. Photoproducts from NPRO and dA were isolated and analyzed with HPLC, MS and NMR. (R)-, (S)-2-(2-pyrrolidyl)-2’-deoxyadenosine (P1, P2) and deoxyinosine (dI) were identified from the mixture of dA and NPRO irradiated with UVA. Results suggest that UVA-activated NPRO produces pyrrolidyl cations which react with dA to generate pyrrolidyl-dA adducts. Production of P1 and P2 from irradiated NPRO with dA increased under N ₂ saturation compared to those under air saturation, while production of P1 and P2 from irradiated solution under O ₂ saturation was lower compared to that under air. These results suggest that a Type-I mechanism is involved in the photoproduction of P1 and P2. Furthermore, reactive oxygen species generated during photo-reaction in the presence of O ₂ may act to destroy the activated form of irradiated NPRO to reduce the formation of P1 and P2. The formation of dI in the irradiated mixture of NPRO with dA might mediate NO release from irradiated NPRO.