Profiling Cys34 adducts of human serum albumin by fixed-step selected reaction monitoring

DNA and protein adducts in human tissues resulting from exposure to tobacco smoke

Tobacco smoke contains a variety of genotoxic carcinogens that form adducts with DNA and protein in the tissues of smokers. Not only are these biochemical events relevant to the carcinogenic process, but the detection of adducts provides a means of monitoring exposure to tobacco smoke. Characterization of smoking-related adducts has shed light on the mechanisms of smoking-related diseases and many different types of smoking-derived DNA and protein adducts have been identified. Such approaches also reveal the potential harm of environmental tobacco smoke (ETS) to nonsmokers, infants and children. Because the majority of tobacco-smoke carcinogens are not exclusive to this source of exposure, studies comparing smokers and nonsmokers may be confounded by other environmental sources. Nevertheless, certain DNA and protein adducts have been validated as biomarkers of exposure to tobacco smoke, with continuing applications in the study of ETS exposures, cancer prevention and tobacco product legislation. Our article is a review of the literature on smoking-related adducts in human tissues published since 2002.

Biomarkers intersect with the exposome

The exposome concept promotes use of omic tools for discovering biomarkers of exposure and biomarkers of disease in studies of diseased and healthy populations. A two-stage scheme is presented for profiling omic features in serum to discover molecular biomarkers and then for applying these biomarkers in follow-up studies. The initial component, referred to as an exposome-wide-association study (EWAS), employs metabolomics and proteomics to interrogate the serum exposome and, ultimately, to identify, validate and differentiate biomarkers of exposure and biomarkers of disease. Follow-up studies employ knowledge-driven designs to explore disease causality, prevention, diagnosis, prognosis and treatment.

Cys34 adducts of reactive oxygen species in human serum albumin

Long-term exposures to reactive oxygen species (ROS) have been linked to scores of chronic diseases. This has motivated interest in oxidation products of the only free cysteine residue (Cys34) of human serum albumin (HSA) as possible biomarkers of ROS exposure. However, Cys34 oxidation products have not been detected in human serum or plasma. Using liquid chromatography-high resolution tandem mass spectrometry, we report accurate masses and molecular compositions of Cys34 oxidation products in the 2432 Da peptide resulting from tryptic digestion of HSA. Peptides containing the expected sulfinic (Cys-SO(2)H) and sulfonic (Cys-SO(3)H) acids, as well as an adduct representing addition of one oxygen atom and loss of two hydrogen atoms, were detected in four archived samples of human plasma and one fresh sample of human serum. We speculate that this latter adduct is a sulfinamide formed by intramolecular reaction between either the Cys34 sulfenic acid (Cys-SOH) or sulfinic acid (Cys-SO(2)H) and the adjacent glutamine residue (Gln33). All three Cys34 adducts were measured in the five human samples with levels decreasing in the order sulfinic acid > (proposed) sulfinamide > sulfonic acid. Parallel measurements of a negative control detected only small amounts of the Cys34 sulfonic acid and the (proposed) sulfinamide and did not detect the sulfinic acid.

Benzene, the exposome and future investigations of leukemia etiology

Benzene exposure is associated with acute myeloid leukemia (AML), myelodysplastic syndromes (MDS), and probably lymphoma and childhood leukemia. Biological plausibility for a causal role of benzene in these diseases comes from its toxicity to hematopoietic stem cells (HSC) or progenitor cells, from which all leukemias and related disorders arise. The effect of this toxicity is manifest as lowered blood counts (hematotoxicity), even in individuals occupationally exposed to low levels of benzene. Benzene can induce AML/MDS via several well-characterized pathways associated with these diseases. Through its metabolites, benzene induces multiple alterations that likely contribute to the leukemogenic process, and appears to operate via multiple modes of action. To improve mechanistic understanding and for risk assessment purposes, it may be possible to measure several of the key events in these modes of action in an in vitro model of the bone marrow stem cell niche. Even though benzene is leukemogenic at relatively low occupational levels of exposure, it seems unlikely that it is a major cause of leukemia in the general population exposed to benzene in the ppb range. Other established non-genetic causes of AML, e.g. smoking, ionizing radiation and cancer chemotherapy, also only explain about 20% of AML incidence, leaving ∼80% unexplained. The question arises as to how to find the causes of the majority of de novo AMLs that remain unexplained. We propose that we should attempt to characterize the 'exposome' of human leukemia by using unbiased laboratory-based methods to find the unknown 'environmental' factors that contribute to leukemia etiology.

Adductomics: characterizing exposures to reactive electrophiles

To understand environmental causes of disease, unbiased methods are needed to characterize the human exposome, which represents all toxicants to which people are exposed from both exogenous and endogenous sources. Because they directly modify DNA and important proteins, reactive electrophiles are probably the most important constituents of the exposome. Exposures to reactive electrophiles can be characterized by measuring adducts from reactions between circulating electrophiles and blood nucleophiles. We define an 'adductome' as the totality of such adducts with a given nucleophilic target. Because of their greater abundance and residence times in human blood, adducts of hemoglobin (Hb) and human serum albumin (HSA) are preferable to those of DNA and glutathione for characterizing adductomes. In fact, the nucleophilic hotspot represented by the only free sulfhydryl group in HSA (HSA-Cys(34)) offers particular advantages for adductomic experiments. Although targeted adducts of HSA-Cys(34) have been monitored for decades, an unbiased method has only recently been reported for visualizing the HSA-Cys(34) 'subadductome'. The method relies upon a novel mass spectrometry application, termed fixed-step selected reaction monitoring (FS-SRM), to profile Cys(34) adducts in tryptic digests of HSA. Here, we selectively review the literature regarding the potential of adductomics to partially elucidate the human exposome, with particular attention to the HSA-Cys(34) subadductome.