A comparison of two ultrasensitive methods for measuring 1,N6-etheno-2'-deoxyadenosine and 3,N4-etheno-2'-deoxycytidine in cellular DNA.

1,N6-Ethenodeoxyadenosine (edA) and 3,N4-ethenodeoxycytidine (edC) are two mutagenic adducts associated with exposure to ethyl carbamate (urethane) and vinyl chloride. We have recently developed two ultrasensitive methods for determining the molecular dose of these adducts in cellular DNA. In both methods, purified DNA was first enzymatically digested to 2c-deoxyribonucleotide 3c-monophosphates. Etheno-modified nucleotides were then separated from normal nucleotides in one of two ways: either by reverse phase, ion-pair HPLC coupled with 260 nm UV detection, or by immunoaffinity chromatography using reusable microcolumns containing specific monoclonal antibodies coupled to Protein A-Sepharose. Fractions enriched for the adducted nucleotides were labeled using T4 poly-nucleotide kinase and [32P]ATP, and individual nucleotides were subsequently resolved by two-dimensional TLC, visualized by autoradiography, and quantified by liquid scintillation counting. When used to analyze the same sample of etheno-modified calf thymus DNA, both assays produced similar results. However, when both methods were used to analyze rat liver DNA 'spiked' with known amounts of etheno nucleotide standards, the immuno-affinity/32P TLC procedure proved to be more sensitive and more reproducible than the HPLC/32P TLC method: while the detection limit of the immunoaffinity/32P TLC technique was < 4 etheno adducts/10(9) parent deoxynucleotides, the HPLC/32P TLC method often failed to detect adducts at concentrations < 2/10(8). In other experiments, the immunoaffinity/32P TLC method was used to demonstrate formation of edA and edC in cells treated with vinyl chloride monomer. Because of its exquisite sensitivity, the immunoaffinity/32P TLC method promises to be extremely useful for measuring both background and induced levels of etheno adducts, making it possible to examine the role of these adducts in inducing mutations and/or carcinogenesis.