Protein Adduct Formation by Reactive Electrophiles: Identifying Mechanistic Links with Benzene-Induced Hematotoxicity

Abstract

The modification of proteins by xenobiotic and endogenous electrophilic species produced in cells undergoing oxidative stress contributes to cellular toxicity and disease processes. Many xenobiotics are themselves reactive electrophiles; however non-reactive compounds may become reactive towards proteins and DNA following metabolism. Identifying actual sites of adduction on target proteins is critical for determining the structural and functional consequences associated with the modification. 1,4-benzoquinone (BQ) is a reactive quinone and environmental toxicant, formed from oxidative metabolism of benzene, an aromatic hydrocarbon found in gasoline and other fuels. Although environmental and occupational exposure to benzene is associated with the development of aplastic anemia and leukemia, the mechanism of toxicity remains elusive. Due to the electrophilic nature of BQ, it reacts with glutathione to form quinol-thioether (QT) conjugates that retain the ability to redox cycle between the reduced (HQ) and oxidized (BQ) forms. BQ and its QT metabolites are reactive, and can produce cellular necrosis through oxidative stress and protein modification. One further consequence of oxidative stress is the elevation of cellular membrane lipid peroxidation, resulting in the formation of reactive lipid-aldehydes such as 4-hydroxynonenal (4HNE). Adduction of critical amino acid residues in target bone marrow proteins by 4HNE and QTs following exposure to benzene could contribute to its hematotoxic effects. This dissertation builds upon the foundation of proteins targeted by electrophilic adduction by outlining techniques to pinpoint the specific amino acids targeted and furthermore predict the functional releavance of adduction. For the first time, protein targets of reactive endogenous lipid aldehydes are reported in the bone marrow of chemically treated rats. Furthermore, novel sites of adduction by aldehydes and benzene-glutathione conjugates are reported within functional regions of topoisomerase II. Inhibition of bone marrow DNA topoisomerase II by benzene metabolites is implicated as a potential mechanism of benzene-induced hematotoxicity and acute-myeloid leukemia. The strong inhibitory effect of these compounds on topoisomerase II activity suggests that their presence in the bone marrow may play a role in benzene-induced myelotoxicity.