(S)-preferential detoxification of 4-hydroxy-2(E)-nonenal enantiomers by hepatic glutathione S-transferase isoforms in guinea-pigs and rats

Mass Spectrometry in Advancement of Redox Precision Medicine

Redox (portmanteau of reduction-oxidation) reactions involve the transfer of electrons between chemical species in biological processes fundamental to life. It is of outmost importance that cells maintain a healthy redox state by balancing the action of oxidants and antioxidants; failure to do so leads to a multitude of diseases including cancer, diabetes, fibrosis, autoimmune diseases, and cardiovascular and neurodegenerative diseases. From the perspective of precision medicine, it is therefore beneficial to interrogate the redox phenotype of the individual-similar to the use of genomic sequencing-in order to design tailored strategies for disease prevention and treatment. This chapter provides an overview of redox metabolism and focuses on how mass spectrometry (MS) can be applied to advance our knowledge in redox biology and precision medicine.

Proteomics and Beyond: Cell Decision-Making Shaped by Reactive Electrophiles

Revolutionary proteomic strategies have enabled rapid profiling of the cellular targets of electrophilic small molecules. However, precise means to directly interrogate how these individual electrophilic modifications at low occupancy functionally reshape signaling networks have until recently been largely limited. We highlight here new methods that transcend proteomic platforms to forge a quantitative link between protein target-selective engagement and downstream signaling. We focus on recent progress in the study of non-enzyme-assisted signaling mechanisms and crosstalk choreographed by native reactive electrophilic species (RES). Using this as a model, we offer a long-term vision of how these toolsets together with fundamental biochemical knowledge of precision electrophile signaling may be harnessed to assist covalent ligand-target matching and ultimately amend disease-specific signaling dysfunction.

Effects of Local Heart Irradiation in a Glutathione S-Transferase Alpha 4-Null Mouse Model

Glutathione S-transferase alpha 4 (GSTA4-4) is one of the enzymes responsible for the removal of 4-hydroxynonenal (4-HNE), an electrophilic product of lipid peroxidation in cellular membranes during oxidative stress. 4-HNE is a direct activator of nuclear factor (erythroid-derived 2)-like 2 (Nrf2), a transcription factor with many target genes encoding antioxidant and anti-electrophile enzymes. We have previously shown that Gsta4-null mice on a 129/Sv background exhibited increased activity of Nrf2 in the heart. Here we examined the sensitivity of this Gsta4-null mouse model towards cardiac function and structure loss due to local heart irradiation. Male Gsta4-null and wild-type mice were exposed to a single X-ray dose of 18 Gy to the heart. Six months after irradiation, immunohistochemical staining for respiratory complexes 2 and 5 indicated that radiation exposure had caused most pronounced alterations in mitochondrial morphology in Gsta4-null mice. On the other hand, wild-type mice showed a decline in cardiac function and an increase in plasma levels of troponin-I, while no such changes were observed in Gsta4-null mice. Radiation-induced Nrf2-target gene expression only in Gsta4-null mice. In conclusion, although loss of GSTA4-4 led to enhanced susceptibility of cardiac mitochondria to radiation-induced loss of morphology, cardiac function was preserved in Gsta4-null mice. We propose that this protection against cardiac function loss may occur, at least in part, by upregulation of the Nrf2 pathway.

Glutathionylated 4-hydroxy-2-(E)-alkenal enantiomers in rat organs and their contributions toward the disposal of 4-hydroxy-2-(E)-nonenal in rat liver

The major route for elimination of 4-hydroxy-2-(E)-nonenal (4-HNE) has long been considered to be through glutathionylation and eventual excretion as a mercapturic acid conjugate. To better quantitate the glutathionylation process, we developed a sensitive LC-MS/MS method for the detection of glutathione (GSH) conjugates of 4-hydroxy-2-(E)-alkenal enantiomers having a carbon skeleton of C5 to C12. The newly developed method enabled us to quantify 4-hydroxy-2-(E)-alkenal-glutathione diastereomers in various organs, i.e., liver, heart, and brain. We identified the addition of iodoacetic acid as a critical step during sample preparation to avoid an overestimation of glutathione-alkenal conjugation. Specifically, we found that in the absence of a quenching step reduced GSH and 4-hydroxy-2-(E)-alkenals react very rapidly during the extraction and concentration steps of sample preparation. Rat liver perfused with d11-4-hydroxy-2-(E)-nonenal (d11-4-HNE) revealed enantioselective conjugation with GSH and transportation out of the liver. In the d11-4-HNE-perfused rat livers, the amount of d11-(S)-4-HNE-GSH released from the rat liver was higher than that of d11-(R)-4-HNE-GSH, and more d11-(R)-4-HNE-GSH than d11-(S)-4-HNE-GSH remained in the perfused liver tissues. Overall, the glutathionylation pathway was found to account for only 8.7% of the disposition of 4-HNE, whereas catabolism to acetyl-CoA, propionyl-CoA, and formate represented the major detoxification pathway.

New substrates and activity of Phanerochaete chrysosporium Omega glutathione transferases

Omega glutathione transferases (GSTO) constitute a family of proteins with variable distribution throughout living organisms. It is notably expanded in several fungi and particularly in the wood-degrading fungus Phanerochaete chrysosporium, raising questions concerning the function(s) and potential redundancy of these enzymes. Within the fungal families, GSTOs have been poorly studied and their functions remain rather sketchy. In this study, we have used fluorescent compounds as activity reporters to identify putative ligands. Experiments using 5-chloromethylfluorescein diacetate as a tool combined with mass analyses showed that GSTOs are able to cleave ester bonds. Using this property, we developed a specific activity-based profiling method for identifying ligands of PcGSTO3 and PcGSTO4. The results suggest that GSTOs could be involved in the catabolism of toxic compounds like tetralone derivatives. Biochemical investigations demonstrated that these enzymes are able to catalyze deglutathionylation reactions thanks to the presence of a catalytic cysteine residue. To access the physiological function of these enzymes and notably during the wood interaction, recombinant proteins have been immobilized on CNBr Sepharose and challenged with beech wood extracts. Coupled with GC-MS experiments this ligand fishing method allowed to identify terpenes as potential substrates of Omega GST suggesting a physiological role during the wood-fungus interactions.

Trans-4-oxo-2-nonenal potently alters mitochondrial function

Alzheimer disease elevates lipid peroxidation in the brain and data indicate that the resulting lipid-aldehydes are pathological effectors of lipid peroxidation. The disposition of 4-substituted nonenals derived from arachidonate (20:4, n-6) and linoleate (18:2, n-6) oxidation is modulated by their protein adduction targets, their metabolism, and the nature of the 4-substitutent. Trans-4-oxo-2-nonenal (4-ONE) has a higher toxicity in some systems than the more commonly studied trans-4-hydroxy-2-nonenal (HNE). In this work, we performed a structure-function analysis of 4-hydroxy/oxoalkenal upon mitochondrial endpoints. We tested the hypotheses that 4-ONE, owing to a highly reactive nature, is more toxic than HNE and that HNE toxicity is enantioselective. We chose to study freshly isolated brain mitochondria because of the role of mitochondrial dysfunction in neurodegenerative disorders. Whereas there was little effect related to HNE chirality, our data indicate that in the mitochondrial environment, the order of toxic potency under most conditions was 4-ONE>HNE. 4-ONE uncoupled mitochondrial respiration at a concentration of 5μM and inhibited aldehyde dehydrogenase 2 (ALDH2) activity with an IC(50) of approximately 0.5μM. The efficacy of altering mitochondrial endpoints was ALDH2 inhibition>respiration=mitochondrial swelling=ALDH5A inhibition>GSH depletion. Thiol-based alkenal scavengers, but not amine-based scavengers, were effective in blocking the effects of 4-ONE upon respiration. Quantum mechanical calculations provided insights into the basis for the elevated reactivity of 4-ONE>HNE. Our data demonstrate that 4-ONE is a potent effector of lipid peroxidation in the mitochondrial environment.

Stereoselective effects of 4-hydroxynonenal in cultured mouse hepatocytes

4-Hydroxynonenal (HNE) is produced from arachidonic acid or linoleic acid during oxidative stress. Although HNE is formed in tissues as a racemate, enantiospecific HNE effects have not been widely documented, nor considered. Therefore, a panel of cellular responses was compared after treatment with (R)-HNE, (S)-HNE, or racemic HNE. The phosphorylation status of Jun kinase (JNK) or Akt increased 28-fold or 2-3-fold, respectively, after treatment with 100 μM (S)-HNE and racemic HNE compared to (R)-HNE. In contrast, the increase in phosphorylation of MAPK was greatest for (R)-HNE. Caspase-3-dependent cleavage of the glutamate cysteine ligase (GCL) catalytic subunit and focal adhesion kinase (FAK) were greater in cells treated with (S)-HNE at 48 h. (S)-HNE also caused a greater number of subG1 nuclei, a hallmark of apoptosis, at 30 h after treatment. Together, the results demonstrate different dose- and time-dependent responses to (R)-HNE and (S)-HNE. The results further suggest that HNE enantiomers could differentially contribute to the progression of different diseases or contribute by different mechanisms.

Oxidative stress and covalent modification of protein with bioactive aldehydes

The term "oxidative stress" links the production of reactive oxygen species to a variety of metabolic outcomes, including insulin resistance, immune dysfunction, and inflammation. Antioxidant defense systems down-regulated due to disease and/or aging result in oxidatively modified DNA, carbohydrates, proteins, and lipids. Increased production of hydroxyl radical leads to the formation of lipid hydroperoxides that produce a family of alpha,beta-unsaturated aldehydes. Such reactive aldehydes are subject to Michael addition reactions with the side chains of lysine, histidine, and cysteine residues, referred to as "protein carbonylation." Although not widely appreciated, reactive lipids can accumulate to high levels in cells, resulting in extensive protein modification leading to either loss or gain of function. The use of mass spectrometric methods to identify the site and extent of protein carbonylation on a proteome-wide scale has expanded our view of how oxidative stress can regulate cellular processes.

The stereochemical course of 4-hydroxy-2-nonenal metabolism by glutathione S-transferases

4-Hydroxy-2-nonenal (HNE) is a toxic aldehyde generated during lipid peroxidation and has been implicated in a variety of pathological states associated with oxidative stress. Glutathione S-transferase (GST) A4-4 is recognized as one of the predominant enzymes responsible for the metabolism of HNE. However, substrate and product stereoselectivity remain to be fully explored. The results from a product formation assay indicate that hGSTA4-4 exhibits a modest preference for the biotransformation of S-HNE in the presence of both enantiomers. Liquid chromatography mass spectrometry analyses using the racemic and enantioisomeric HNE substrates explicitly demonstrate that hGSTA4-4 conjugates glutathione to both HNE enantiomers in a completely stereoselective manner that is not maintained in the spontaneous reaction. Compared with other hGST isoforms, hGSTA4-4 shows the highest degree of stereoselectivity. NMR experiments in combination with simulated annealing structure determinations enabled the determination of stereochemical configurations for the GSHNE diastereomers and are consistent with an hGSTA4-4-catalyzed nucleophilic attack that produces only the S-configuration at the site of conjugation, regardless of substrate chirality. In total these results indicate that hGSTA4-4 exhibits an intriguing combination of low substrate stereoselectivity with strict product stereoselectivity. This behavior allows for the detoxification of both HNE enantiomers while generating only a select set of GSHNE diastereomers with potential stereochemical implications concerning their effects and fates in biological tissues.

Effect of Docosahexaenoic Acid Ingestion on Temporal Change in Urinary Excretion of Mercapturic Acid in ODS Rats

We hypothesized a suppressive mechanism for docosahexaenoic acid (22:6n-3; DHA)-induced tissue lipid peroxidation in which the degradation products, especially aldehydic compounds, are conjugated with glutathione through catalysis by glutathione S-transferases, and then excreted into urine as mercapturic acids. In the present study, ascorbic acid-requiring ODS rats were fed a diet containing DHA (3.6% of total energy) for 31 days. Lipid peroxides including degradation products and their scavengers in the liver and kidney were determined, and the temporal change in the urinary excretion of mercapturic acids was also measured. The activity of aldehyde dehydrogenase, which catalyzes the oxidation and detoxification of aldehydes, tended to be higher in the liver of DHA-fed rats. The levels of lipid peroxides as measured by thiobarbituric acid-reactive substances and aldehydic compounds were higher and that of alpha-tocopherol was lower in the liver, and the pattern of temporal changes in the urinary excretion of mercapturic acids was also different between the n-6 linoleic acid and DHA-fed rats. Accordingly, we presume from these results that after dietary DHA-induced lipid peroxidation, a proportion of the lipid peroxidation-derived aldehydic degradation products is excreted into urine as mercapturic acids.

Enantioselective metabolism of trans-4-hydroxy-2-nonenal by brain mitochondria

Trans-4-hydroxy-2-nonenal (HNE) is a product of lipid peroxidation with many cellular effects. HNE possesses a stereogenic center at the C4 carbon that influences the metabolism and alkylation targets of HNE. We tested the hypothesis that rat brain mitochondria metabolize HNE in an enantioselective manner after exposure to racemic HNE. The study of HNE chirality, however, is hindered by the lack of facile methods to chromatographically resolve (R)-HNE and (S)-HNE. We used a chiral hydrazine, (S)-carbidopa, as a derivatization reagent to form diastereomers with (R)-HNE and (S)-HNE that were separated by reverse-phase HPLC. After exposure to racemic HNE, rat brain mitochondria metabolized HNE enantioselectively with a higher rate of (R)-HNE metabolism. By using the purified enantiomers of HNE, we found that this enantioselective metabolism of HNE was the result of higher rates of enzymatic oxidation of (R)-HNE by aldehyde dehydrogenases compared to (S)-HNE. Conjugation of HNE to glutathione was a minor metabolic pathway and was not enantioselective. These studies demonstrate that the chirality of HNE affects its mitochondrial metabolism and potentially other processes in the central nervous system.

Fate of 4-hydroxynonenal in vivo: disposition and metabolic pathways

Due to the cytotoxicity of 4-hydroxynonenal (HNE), and to the fact that this major product of lipid peroxidation is a rather long-living compound compared with reactive oxygen species, the capability of organisms to inactivate and eliminate HNE has received increasing attention during the last decade. Several recent in vivo studies have addressed the issue of the diffusion, kinetics, biotransformation and excretion of HNE. Part of these studies are primarily concerned with the toxicological significance of HNE biotransformation and more precisely with the metabolic pathways by which HNE is inactivated and eliminated. The other aim of in vivo metabolic study is the characterisation of end-metabolites, especially in urine, in order to develop specific and non-invasive biomarkers of lipid peroxidation. When HNE is administered intravenously or intraperitoneally, it is mainly excreted into urine and bile as conjugated metabolites, in a proportion that is dependent on the administration route. However, biliary metabolites undergo an enterohepatic cycle that limits the final excretion of faecal metabolites. Only a very low amount of metabolites is found to be bound to macromolecules. The main urinary metabolites are represented by two groups of compounds. One comes from the mercapturic acid formation from (i) 1,4 dihydroxynonene-glutathione (DHN-GSH) which originates from the conjugation of HNE with GSH by glutathione-S-transferases and the subsequent reduction of the aldehyde by a member of aldo-keto reductase superfamily; (ii) the lactone of 4-hydroxynonanoic-GSH (HNA-lactone-GSH) which originates from the conjugation of HNE followed by the oxidation of the aldehyde by aldehyde dehydrogenase; (iii) HNA-GSH which originates from the hydrolysis of the corresponding lactone. The other one is a group of metabolites issuing from the omega-hydroxylation of HNA or HNA-lactone by cytochromes P450 4A, followed eventually, in the case of omega-oxidized-HNA-lactone, by conjugation with GSH and subsequent mercapturic acid formation. Biliary metabolites are GSH or mercapturic acid conjugates of DHN, HNE and HNA. Stereochemical aspects of HNE metabolism are also discussed.