The Monitoring of Exposure to Carcinogens by the GC-MS Determination of Alkylated Amino Acids in Hemoglobin and of Alkylated Nucleic Acid Bases in Urine

Exposure to alkylating carcinogens results in the covalent binding of the active genotoxic species to cellular macromolecules. Human exposure to these alkylating agents could satisfactorily be monitored by determination of the extent of this binding, ideally at the biologically significant site in deoxyribonucleic acid (DNA). However, in practice the nature of this site is not normally known with certainty, and the acquisition of sufficient carcinogen-DNA adducts for chemical determination presents considerable difficulty. For human monitoring, one is restricted to readily accessible biological media (e.g., blood) and the use of hemoglobin adducts as an indicator of the formation of carcinogen-DNA adducts has recently become established [1]. Examples will be given in this chapter of methods that we have developed, using capillary gas chromatography-mass spectrometry (GC-MS) for the determination of adducts of several simple alkylating agents (e.g., methylating, ethylating, hydroxyethylating, and hydroxypropylating) with cysteine or histidine residues in hemoglobin. Our recent work on exposure of animals to acrylamide will also be discussed. For some alkylating agents, nucleic acid adducts may be monitored by quantitation of excreted N-7-alkylated guanines. N-7-substitution of guanine (and N-3-substitution of adenine) in nucleic acids yields adducts which are unstable and which decompose to liberate the free alkylated bases. For example, the extent of the excretion of 7-alkylguanine has been shown to be directly related to exposure dose for aflatoxin B₁ [2] and for dimethylnitrosamine [3] (liberated from the in vivo nitrosation of aminopyrine). We are currently comparing the extent of urinary excretion of 7-alkylguanine with the amount of hemoglobin amino acid alkylation following exposure of animals to carcinogens. Comparison of the extent of reaction of a carcinogen at different nucleophilic sites may allow predictions to be made of its reaction at the biologically significant DNA site, and hence of the risk associated with the exposure.