A CYCLIC N-7,C-8 GUANINE ADDUCT OF N-NITROSOPYRROLIDINE (NPYR) - FORMATION IN NUCLEIC-ACIDS AND EXCRETION IN THE URINE OF NPYR-TREATED RATS

N-Nitrosopyrrolidine (NPYR) is a well-established hepatocarcinogen tha t is present in the diet and tobacco smoke and may form endogenously i n humans. Biomarkers to assess NPYR exposure and metabolic activation in humans are needed. The cyclic N-7,C-8 guanine adduct 9-tetrahydro-9 -hydroxypyrido[2,1-f]purin-4(3H)-one (8), which is formed in tissues o f rats treated with NPYR, is one potential candidate for such a biomar ker. In this study, we evaluated the formation of this and other NPYR adducts in reactions of alpha-acetoxyNPYR with dGuo, Guo, DNA, and RNA and determined the extent of urinary excretion of adduct 8 in rats tr eated with NPYR. alpha-AcetoxyNPYR, a stable precursor to the major pr oduct of NPYR metabolic activation, was allowed to react with dGuo, Gu o, DNA, or RNA at 37 degrees C, pH 7. The most striking observation wa s that the cyclic N-7,C-8 guanine adduct 8 was formed 9 times more ext ensively in the reaction with Guo than with dGuo. It was also formed 2 .5 times more extensively in RNA than in DNA. In rats treated with NPY R, levels of the cyclic N-7,C-8 guanine adduct 8 were 2 times as high in RNA than in DNA. Rats treated with [C-14]adduct 8 excreted 51% of t his adduct unchanged in urine. Rats treated with [3,4-H-3]NPYR excrete d 0.000 04% of the dose as adduct 8. The major differences in product formation in reactions of alpha-acetoxyNPYR with dGuo versus Guo are u nusual for alkylating agents; potential mechanisms are discussed. The higher levels of adduct 8 in RNA than in DNA suggest that RNA may be s uperior as a source of adduct 8 as a biomarker.