Vanderbilt University

2003 Highlights

•The Roles of Detoxication Enzymes in Exposure to Industrial Chemicals
• Preventing Cellular Damage at the Molecular Level: Development of New Antioxidants

The Roles of Detoxication Enzymes in Exposure to Industrial Chemicals
Work has continued on chemicals from a group known as the halogenated hydrocarbons, some of which are known to be toxic in mammals. Many halogenated hydrocarbons are frequently found in waste dump sites and may pose an environmental threat to public health. One of these chemicals, methylene chloride, is of particular interest due to its high volume industrial use in solvents such as paint stripper. There are ongoing debates regarding safety levels of this chemical in the workplace, especially given methylene chloride's ability to cause tumors in mice. Certain catalysts known as detoxication enzymes are part of any organism's ability to resist chemical insult and are involved in metabolizing foreign molecules. One family of enzymes known as the glutathione transferases is involved in this process of detoxication in response to toxic exposure but actually activates halogenated hydrocarbons such as methylene chloride. Profs. Armstrong and Guengerich have performed detailed kinetic analyses on the rat, human, and bacterial enzymes that work to catalyze reactions with these chemicals. Their findings explain the lack of dose saturation observed in mammals (i.e., the continuing increase in cancer with higher doses). Additional work led to the analysis of DNA modifications relevant to the mutations and cancer. Prof. Armstrong's work on bacterial systems has relevance for bioremediation, a recent technology that utilizes naturally-occurring microorganisms for environmental clean-up. These studies have also led to fruitful searches for similar enzymes in bacteria that may shed light on the mechanisms of antibiotic resistance. Recently, Prof. Guengerich has identified similar chemical mechanisms for halocarbon activation in the enhancement of mutations by a DNA repair protein known as O6-alkylguanine DNA-alkyl transferase.

Selected Publications

  • Guengerich FP, McCormick WA, Wheeler JB. 2003 Nov. Analysis of the kinetic mechanism of haloalkane conjugation by mammalian theta-class glutathione transferases. Chem Res Toxicol16 (11):1493-9.
  • Liu L, Hachey DL, Valadez G, Williams KM, Guengerich FP, Loktionova NA, Kanugula S, Pegg AE. 2004 Feb 6. Characterization of a Mutagenic DNA Adduct Formed from 1,2-Dibromoethane by O6-Alkylguanine-DNA Alkyltransferase. J Biol Chem279 (6):4250-9.
  • Marsch GA, Botta S, Martin MV, McCormick WA, Guengerich FP. 2004 Jan. Formation and mass spectrometric analysis of DNA and nucleoside adducts by S-(1-acetoxymethyl)glutathione and by glutathione S-transferase-mediated activation of dihalomethanes. Chem Res Toxicol17 (1):45-54.
  • Rife CL, Parsons JF, Xiao G, Gilliland GL, Armstrong RN. 2003 Dec 1. Conserved structural elements in glutathione transferase homologues encoded in the genome of Escherichia coli. Proteins53 (4):777-82.
  • Stourman NV, Rose JH, Vuilleumier S, Armstrong RN. 2003 Sep 23. Catalytic mechanism of dichloromethane dehalogenase from Methylophilus sp. strain DM11. Biochemistry42 (37):11048-56.

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Preventing Cellular Damage at the Molecular Level: Development of New Antioxidants
A number of different mechanisms can cause damage to living cells. Free radicals are atoms or groups of atoms with an odd number of electrons that can be formed when oxygen interacts with certain molecules. Having one unpaired electron makes these radicals highly reactive, and they can start a chain reaction that damages the cell membrane or other important components in the cell. The process by which free radicals react with lipids in the cell membrane is known as lipid peroxidation. As a result of this reaction, cell function may be impaired, or the cell may die. To defend itself against free radical damage, the body utilizes dietary nutrients known as antioxidants. Antioxidants are molecules that are able to terminate the free radical chain reaction before molecules in the cell are damaged. 

Studies on the mechanisms and analysis of lipid peroxidation are continuing in the laboratories of Profs. Marnett, Porter, Rizzo, and Montine (former Center Investigator). Excessive lipid peroxidation has been implicated in a number of human disorders, including atherosclerosis, neurodegenerative disorders such as Alzheimer's disease, and various autoimmune conditions. Antioxidants such as vitamin E and vitamin C play critical roles in lipid peroxidation, stopping or inhibiting the critical free radical chain reaction. Understanding several chemical aspects of the destruction of lipids, particularly the fatty acid arachidonic acid, is important because of the potentially dangerous products that are produced, including hydroperoxides and malondialdehyde (which the Marnett laboratory has shown to be mutagenic in human cells). In addition to basic knowledge, this work has led to the successful development of new antioxidants, some of which are even more powerful than vitamin E. Other work has shown the beneficial effects of antioxidants (as judged by biomarkers) in nervous tissues. These studies collectively show (i) the dangers of lipid peroxidation, which can be exacerbated in a variety of situations, (ii) the usefulness of good biomarkers in environmental health, and (iii) the potential to develop even better intervention measures based on chemistry and pharmacology.

Selected Publications

  • Montine TJ, Montine KS, Reich EE, Terry ES, Porter NA, Morrow JD. 2003 Feb 15. Antioxidants significantly affect the formation of different classes of isoprostanes and neuroprostanes in rat cerebral synaptosomes. Biochem Pharmacol65 (4):611-7.
  • Pratt DA, Mills JH, Porter NA. 2003 May 14. Theoretical calculations of carbon-oxygen bond dissociation enthalpies of peroxyl radicals formed in the autoxidation of lipids. J Am Chem Soc125 (19):5801-10.
  • Wijtmans M, Pratt DA, Valgimigli L, DiLabio GA, Pedulli GF, Porter NA. 2003 Oct 20. 6-Amino-3-Pyridinols: Towards Diffusion-Controlled Chain-Breaking Antioxidants. Angew Chem Int Ed Engl42 (40):4847.
  • Yin H, Havrilla CM, Gao L, Morrow JD, Porter NA. 2003 May 9. Mechanisms for the formation of isoprostane endoperoxides from arachidonic acid. "Dioxetane" intermediate versus beta-fragmentation of peroxyl radicals. J Biol Chem278 (19):16720-5.

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