Maillard reaction-like lysine modification by a lipid peroxidation product: immunochemical detection of protein-bound 2-hydroxyheptanal in vivo

Protein Adducts Generated from Products of Lipid Oxidation: Focus on HNE and ONE

Modification of proteins in conditions of oxidative stress can contribute to protein dysfunction or tissue damage and disease progression. Bifunctional, most often alpha,beta-unsaturated carbonyl compounds such as 4-hydroxy-2-nonenal (HNE), 4-oxo-2-nonenal (ONE), and acrolein, generated from oxidation of polyunsaturated fatty acids (PUFAs), readily bind to protein nucleophiles. Modification by bifunctional aldehydes can also lead to intramolecular or intermolecular protein crosslinking. Model studies are revealing the structure of adducts that can then be more readily identified in mass spectrometric studies on proteins exposed to the various pure aldehydes or to peroxidized PUFAs. Although simple Michael and Schiff base adducts are often formed initially, only some of these adducts, such as the HNE- and ONE-derived Michael adducts on Cys and His residues, are found to survive the conditions of proteolysis and HPLC-MS analysis. Reversibly formed adducts, such as the HNE-Lys Michael adduct, can be found on proteolytic peptides only if a NaBH4-reduction step is used prior to proteolysis. Initial adducts can evolve by tautomerization, oxidation, cyclization, dehydration, and sometimes condensation with a second aldehyde molecule (the same or different), to give stable advanced lipoxidation end products (ALEs) that can be found by mass spectrometry. These include the HNE-Lys-derived 2-pentylpyrrole, the ONE-Lys-derived 4-ketoamide, the ONE-derived His-Lys pyrrole crosslink, and a Lys-derived 3-formyl-4-pentylpyrrole that results from combined action of ONE and acrolein. Michael adducts of alpha,beta-unsaturated aldehydes such as HNE and ONE can be derivatized by 2,4-dinitrophenylhydrazine (DNPH) and can thus constitute significant DNPH-detectable protein-bound carbonyl activity that serves as a key indicator of oxidative stress in tissues. It appears that lipid oxidation is a more important contributor to such activity than metal-catalyzed oxidation of protein side-chains.

New analytical strategies in studying drug metabolism

Identification and elucidation of the structures of metabolites play major roles in drug discovery and in the development of pharmaceutical compounds. These studies are also important in toxicology or doping control with either pharmaceuticals or illicit drugs. This review focuses on: new analytical strategies used to identify potential metabolites in biological matrices with and without radiolabeled drugs; use of software for metabolite profiling; interpretation of product spectra; profiling of reactive metabolites; development of new approaches for generation of metabolites; and detection of metabolites with increased sensitivity and simplicity. Most of the new strategies involve mass spectrometry (MS) combined with liquid chromatography (LC).

Aldehydic lipid peroxidation products in human brain astrocytomas

Among mediators of oxidative stress, highly reactive secondary aldehydic lipid peroxidation products can initiate the processes of spontaneous mutagenesis and carcinogenesis and can also act as a growth-regulating factors and signaling molecules. We explored whether these aldehydes and histone H3 mRNA levels could serve as biomarkers of malignancy and predictive factor in human brain astrocytomas. Histone H3 mRNA, a biomarker of cellular proliferation, was analyzed by QRT-PCR (TaqMan). Aldehydic lipid peroxidation products were determined as their dinitrophenylhydrazone derivatives in specimens obtained from 26 adult patients with brain astrocytomas. RP-HPLC with diode array detector and MSMS spectrometer were used for the analysis. H3 mRNA, 2-hydroxyhexanal, and 4-hydroxynonenal levels were higher in high-grade astrocytomas compared to low-grade astrocytomas and showed negative correlation with survival. Higher levels of 2-hydroxyhexanal and 4-hydroxynonenal, and lower levels of n-hexanal were associated with poorer patient prognosis. Our data suggest that tissue concentrations of aldehydic lipid peroxidation products can assist grading and predicting the clinical outcome in patients with astrocytic brain tumors. Possibly, this parameter will enhance optimal selection of patients for individualized treatment protocols, tailored to unique biochemical and molecular profile of the tumor.

Structural characteristics of a lipid peroxidation product, trans-2-nonenal, that favour inhibition of membrane-associated phosphotyrosine phosphatase activity

Protein-tyrosine phosphatases (PTPs) are very susceptible to oxidation by reactive oxygen species (ROS), which induce the oxidation of catalytic cysteines, thereby inactivating these PTPs. PTPs are also inactivated by treatment with different aldehydes (such as trans-2-nonenal), produced after tissue damage by ROS. However, the molecular mechanisms behind such aldehyde-due inactivation remain unknown. Using commercially available compounds, we examined the structural characteristics of trans-2-nonenal that allow the inhibition of platelet membrane-associated PTP activity, as well as how these compounds affect the dynamics of SH-, CO- and NH2- protein groups on the membranes. PTP was effectively inhibited by physiological amounts of trans-2-nonenal (1-10 microM). Incubation with trans-2-nonene (10 microM) also decreased PTP activity, although to a lower extent. Treatment with nonyl aldehyde almost eliminated PTP inhibition. Decreases in protein thiols were visible after trans-2-nonenal and trans-2-nonene treatments. Both the latter compounds also increased protein carbonyls (although trans-2-nonenal was more effective) and decreased protein amino groups to an equal extent. Collectively, our data indicate that alpha,beta unsaturation (and not a double bond in another position) is the most important structural determinant for PTP inhibition, the alkenal with 9-carbon atoms being the most effective in eliciting such inhibition. The data allow us to predict the modification of sulfhydryls and/or the formation of addition products with lysyl or histidyl residues, and hence the kind of specific antibodies that it would be necessary to generate in order to test such modifications directly.

Application of stable isotope labeled glutathione and rapid scanning mass spectrometers in detecting and characterizing reactive metabolites

The formation of reactive metabolites from a number of compounds was studied in vitro using a mixture of non-labeled and stable isotope labeled glutathione (GSH) as a trapping agent. GSH was labeled by incorporating [1,2-(13)C(2),(15)N]glycine into the tripeptide to give an overall increase of 3 Da over the naturally occurring substance. Detection and characterization of reactive metabolites was greatly facilitated by using the data-dependent scanning features of the linear ion trap mass spectrometers to give complimentary and confirmatory data in a single analytical run. A comparison was made by analyzing the samples simultaneously on a triple-stage quadrupole mass spectrometer operated in the constant neutral loss mode. The compounds studied included 2-acetamidophenol, 3-acetamidophenol, 4-acetamidophenol (acetaminophen), and flufenamic acid. GSH adducts for each of these compounds produced a characteristic pattern of 'twin ions' separated by 3 Da in the mass spectral data. This greatly facilitated the detection and characterization of any GSH-related adducts present in the microsomal extracts. Furthermore, characterization of these adducts was greatly facilitated by the rapid scanning capability of linear ion trap instruments that provided full-scan, MS/MS and MS(3) data in one single analysis. This method of detecting and characterizing reactive metabolites generated in vitro was found to be far superior to any of the existing methods previously employed in this laboratory. The combination of two techniques, stable isotope labeled glutathione and linear ion traps, provided a very sensitive and specific method of identifying compounds capable of producing reactive metabolites in a discovery setting. The complimentary set of mass spectral data (including full-scan, MS/MS and MS(3) mass spectra), obtained rapidly in a single analysis with the linear ion trap instruments, greatly accelerated identification of metabolically bioactivated soft spots on the molecules. This in turn enabled chemists to rapidly design out the potential metabolic liability from the back-up compounds by making appropriate structural modifications.