Office: PHSC 326A Phone: (405) 325-2924 Email: mashby@ou.edu Complete Publication List	Michael T. Ashby Professor B.S. (University of Arizona) 1982 Ph.D. (University of Arizona) 1986 Alexander von Humboldt Research Fellow (University of Wurzburg, Germany) 1986-1987 Postdoctoral Research Associate, (University of Chicago) 1988-1989
Division: Inorganic chemistry Research Interests Inorganic and bioinorganic synthesis and mechanisms; reactive sulfur species; the microbiology of inorganic compounds.

Research Description

Our current research is encompassed by two general themes, Inorganic Antimicrobials and Inorganic Antioxidants, although research in these two areas often overlaps.  While the progenitors of the chemistry that interests us are reactiveinorganic compounds, the subsequent chemistry is frequently organic and biological.

Inorganic Antimicrobials:  Our studies focus on two endogenous inorganic human defense factors, hypochlorite (OCl^-) and hypothiocyanite (OSCN^-).  These biocides are generated in vivo via peroxidase-catalyzed oxidation of chloride (Cl^-) and thiocyanate (SCN^-), respectively.  Hypochlorite is an archetypal indiscriminant biocide, whereas hypothiocyanite is a less powerful, but more selective, biocide.  Together, the hypohalites play a significant role in human health (e.g., Figure 1).  Hypochlorite is also implicated in the host tissue damage that accompanies numerous chronic inflammatory diseases such as atherosclerosis, cystic fibrosis, and periodontitis (see below). Our current work is based in part upon our observation of the facile non-enzymic reaction of HOCl with SCN^- to produce OSCN^- (see our recent paper on this subject).  This observation substantiates the hypothesis that SCN^- serves as a redox buffer by governing the lifetime, and consequently the potential autotoxicity, of HOCl (see below). Furthermore, this reaction preserves the oxidizing equivalents of HOCl by transferring them to OSCN^-, a more discriminate biocide that is not lethal to mammalian cells.  Remarkably little is known about the biocidal mechanisms of the hypohalites (especially hypothiocyanite), due in part to their extraordinary reactivities and the fact that they produce cascades of derivative reactive intermediates with largely unknown physiologic properties.  Recent studies have demonstrated distinctive genetic responses to chemical insult of bacteria by the HOCl and OSCN^- systems, which is consistent with the general recognition that HOCl exhibits promiscuous reaction chemistry (see our recent paper on this subject), whereas we have observed OSCN^- reacts exclusively with sulfhydryl groups vis-à-vis sulfenyl thiocyanates (RSSCN, see our recent paper on this subject).  Alkylsulfenyl thiocyanates are only stable under very acidic conditions.  Furthermore, the hydrolysis of alkylsulfenylthiocyanites yield unstable sulfenic acids, and thereby produce a cascade of reactive sulfur species (RSS)  A significant facet of our work involves the study of both inorganic and organic RSS. These studies generally involve synthetic, spectroscopic, and kinetic components (see another recent publication that concerns RSS).  Knowledge gained from our investigation of small molecules and simplified reaction systems are frequently extended to in vitro experiments involving biomolecules and in vivo experiments with whole cells. 

Inorganic Antioxidants: Atherosclerosis is the leading cause of morbidity and mortality in the United States and most other Western countries. The oxidative modification hypothesis of atherosclerosis envisages that low-density lipoprotein (LDL) oxidation contributes to atherogenesis.  Buttressing this hypothesis are the observations that oxidized LDL supports foam cell formation in vitro, lipid in human lesions is considerably oxidized, there is evidence for the presence of oxidized LDL in vivo, oxidized LDL has many potentially proatherogenic activities, and several structurally unrelated antioxidants inhibit atherosclerosis in animal models.  Paradoxically, despite the availability of abundant circumstantial data that indicate a causal role of oxidative stress in atherogenesis and the progression of the disease, clinical trials of dietary antioxidant supplements have failed to demonstrate marked beneficial effects, which indicates specific oxidation pathways rather than generic oxidation mechanisms.  A recent review of the role of oxidative modifications in atherosclerosis points out that current antioxidant strategies "presuppose a detailed knowledge of the relevant oxidants involved" when in fact relatively little information is available regarding the nature of these oxidants in a complex biological setting [Stocker and Keaney, Physiol. Rev., 2004, 84, 1381].  Most of the antioxidants that have been investigated to date (e.g., probucol, vitamin E, and butylated hydroxyl toluene) have been chosen because they inhibit LDL oxidation in vitro by 1e-oxidants (e.g. radical species).  However, there is considerable anecdotal evidence that 2e-oxidants such as the reactive oxygen species (ROS) hypochlorous acid (HOCl) and reactive nitrogen species (RNS) peroxynitrite (ONOO^-) may play significant roles in LDL damage.  To further complicate this issue of which reactants may be involved in LDL damage, as discussed above, we have recently demonstrated that HOCl reacts rapidly with thiocyanate (SCN^-) to give the reactive sulfur species (RSS) hypothiocyanite (OSCN^-), an antimicrobial agent that has been shown to exclusively target sulfhydryl groups (see our recent paper on this subject).  The latter non-enzymic transfer of the oxidizing equivalents of HOCl to SCN^- may serve to govern the lifetime of more powerful oxidants such as HOCl, thereby moderating their potential autotoxicity.  Given the significant ties that exist between oxidative stress and atherosclerosis, and the dearth of quantitative information regarding the chemical dynamics of reactive oxidants in physiologic fluids, we are developing quantitative kinetic models that rationalize oxidative modifications in terms of the production of reactive oxidants, their interconversion reactions, and their terminal reactions to produce potential biomarkers (e.g., Scheme 1).  Furthermore, these kinetic models for the partitioning of atherogenic oxidants in plasma are being experimentally tested using natural and synthetic plasma solutions.

Acknowledgement:  We are very grateful to the National Science Foundation (CHE-0503984), the National Institutes of Health (5 P20 RR018741-02), the Petroleum Research Fund (PRF#42850-AC4), the American Heart Association (0555677Z), the Oklahoma Center for the Advancement of Science and Technology (HR02-019), and the Department of Education (GAANN) for their financial support.

Recent Results

Brief examples of some additional recent results from our laboratory are available by following these links:

Redox Buffering of Physiologic Fluids by Inorganic Compounds

Aqueous Chemistry of Sulfenyl Thiocyanates

Chloramine Derivatives of Cystine

Diffusion-Controlled Insult of Prokaryotes

Changes in the Permeability of Gram Negative Bacteria

Selected Recent Publications

P. Nagy and M. T. Ashby, “Kinetics and Mechanism of the Oxidation of Glutathione Dimer by Hypochlorous Acid and Catalytic Reduction of the Chloroamine Product by Glutathione Reductase”, Chem. Res. Toxicol., 20, 79-87, 2007.

M. T. Ashby, “Appreciating Formal Similarities in the Kinetics of Homogeneous, Heterogeneous, and Enzyme Catalysis”, J. Chem. Educ., 84, 1515-1519, 2007.

P. Nagy, K. Lemma, and M. T. Ashby, "Kinetics and Mechanism of the Comproportionation of Hypothiocyanous Acid and Thiocyanate to Give Thiocyanogen in Acidic Aqueous Solution."  Inorganic Chemistry, 46,  285-292, 2007.

P. Nagy, S. S. Alguindigue, and M. T. Ashby, “Lactoperoxidase-Catalyzed Oxidation of Thiocyanate by Hydrogen Peroxide: A Reinvestigation of Hypothiocyanite by Nuclear Magnetic Resonance and Optical Spectroscopy”, Biochemistry, 45, 12610-12616, 2006.

P. Nagy, J. C. Beal, and M. T. Ashby, “Thiocyanate is an Efficient Endogenous Scavenger of the Phagocytic Killing Agent Hypobromous Acid”, Chem. Res. Toxicol., 19, 587-593, 2006.

R. C. Mallo and M. T. Ashby "AqpZ-Mediated Water Permeability in Escherichia coli Measured by Stopped-Flow Spectroscopy", J. Bacteriol., 188, 820-822, 2006.

P. Nagy and M. T. Ashby, "Reactive Sulfur Species: Kinetics and Mechanism of the Oxidation of Cystine by Hypochlorous Acid to Give N,N'-Dichlorocystine", Chem Res. Toxicol., 18, 919-923, 2005.

M. T. Ashby, A. C. Carlson, and M. J. Scott "Redox Buffering of Hypochlorous Acid by Thiocyanate in Physiologic Fluids" J. Am. Chem. Soc.,126; 15976-15977, 2004.

M. T. Ashby and H. Aneetha "Reactive Sulfur Species: Aqueous Chemistry of Sulfenyl Thiocyanates", J. Am. Chem. Soc., 126, 10216 - 10217, 2004.

J. D. Schwane and M. T. Ashby, "FTIR Investigation of the Intermediates Formed in the Reaction of Nitroprusside and Thiolates", J. Am. Chem. Soc., 124, 6822-6823, 2002.