Peroxidases: a role in the metabolism and side effects of drugs

The endoplasmic reticulum participated in drug metabolic toxicity

Covalent binding of reactive metabolites formed by drug metabolic activation with biological macromolecules is considered to be an important mechanism of drug metabolic toxicity. Recent studies indicate that the endoplasmic reticulum (ER) could play an important role in drug toxicity by participating in the metabolic activation of drugs and could be a primarily attacked target by reactive metabolites. In this article, we summarize the generation and mechanism of reactive metabolites in ER stress and their associated cell death and inflammatory cascade, as well as the systematic modulation of unfolded protein response (UPR)-mediated adaptive pathways.

Advances on Delivery of Cytotoxic Enzymes as Anticancer Agents

Cancer is one of the most serious human diseases, causing millions of deaths worldwide annually, and, therefore, it is one of the most investigated research disciplines. Developing efficient anticancer tools includes studying the effects of different natural enzymes of plant and microbial origin on tumor cells. The development of various smart delivery systems based on enzyme drugs has been conducted for more than two decades. Some of these delivery systems have been developed to the point that they have reached clinical stages, and a few have even found application in selected cancer treatments. Various biological, chemical, and physical approaches have been utilized to enhance their efficiencies by improving their delivery and targeting. In this paper, we review advanced delivery systems for enzyme drugs for use in cancer therapy. Their structure-based functions, mechanisms of action, fused forms with other peptides in terms of targeting and penetration, and other main results from in vivo and clinical studies of these advanced delivery systems are highlighted.

Reactive metabolites of the anticonvulsant drugs and approaches to minimize the adverse drug reaction

Several generations of antiepileptic drugs (AEDs) are available in the market for the treatment of seizures, but these are amalgamated with acute to chronic side effects. The most common side effects of AEDs are dose-related, but some are idiosyncratic adverse drug reactions (ADRs) that transpire due to the formation of reactive metabolite (RM) after the bioactivation process. Because of the adverse reactions patients usually discontinue the medication in between the treatment. The AEDs such as valproic acid, lamotrigine, phenytoin etc., can be categorized under such types because they form the RM which may prevail with life-threatening adverse effects or immune-mediated reactions. Hepatotoxicity, teratogenicity, cutaneous hypersensitivity, dizziness, addiction, serum sickness reaction, renal calculi, metabolic acidosis are associated with the metabolites of drugs such as arene oxide, N-desmethyldiazepam, 2-(1-hydroxyethyl)-2-methylsuccinimide, 2-(sulphamoy1acetyl)-phenol, E-2-en-VPA and 4-en-VPA and carbamazepine-10,11-epoxide, etc. The major toxicities are associated with the moieties that are either capable of forming RM or the functional groups may itself be too reactive prior to the metabolism. These functional groups or fragment structures are typically known as structural alerts or toxicophores. Therefore, minimizing the bioactivation potential of lead structures in the early phases of drug discovery by a modification to low-risk drug molecules is a priority for the pharmaceutical companies. Additionally, excellent potency and pharmacokinetic (PK) behaviour help in ensuring that appropriate (low dose) candidate drugs progress into the development phase. The current review discusses about RMs in the anticonvulsant drugs along with their mechanism vis-a-vis research efforts that have been taken to minimize the toxic effects of AEDs therapy.

Therapeutic drug monitoring of clozapine in adults with schizophrenia: a review of challenges and strategies

INTRODUCTION

Clozapine (CLZ) is the superior drug in treatment of schizophrenia. Serum concentration of CLZ is associated with clinical response and dose-dependents side effects, where generalized tonic-clonic seizures are most critical. Thus, therapeutic drug monitoring (TDM) of CLZ may guide individual dosing to reach target exposure and prevent dose-dependent side effects. However, current TDM methods are not capable of predicting the risk of agranulocytosis, which is a dose-independent side effect restricting use of CLZ to treatment-resistant schizophrenia (TRS).

AREAS COVERED

The article provides an overview of clinical, pharmacological, and toxicological aspects of CLZ, and the role of TDM as a tool for dose titration and follow-up in patients with TRS. Main focus is on current challenges and strategies in CLZ TDM, including future perspectives on potential identification/analysis of CLZ metabolite biomarkers reflecting the risk of granulocyte toxicity.

EXPERT OPINION

The association between CLZ serum concentration, clinical response and risk of seizures is indisputable. TDM should therefore always guide CLZ dose titration. Development of advanced TDM methods, including biomarkers predicting the risk of granulocyte toxicity might extend TDM to be a tool for deciding which patients that can be treated safely with CLZ, potentially increasing its utility beyond TRS.

The many roles of myeloperoxidase: From inflammation and immunity to biomarkers, drug metabolism and drug discovery

This review provides a practical guide to myeloperoxidase (MPO) and presents to the reader the diversity of its presence in biology. The review provides a historical background, from peroxidase activity to the discovery of MPO, to its role in disease and drug development. MPO is discussed in terms of its necessity, as specific individuals lack MPO expression. An underlying theme presented throughout brings up the question of the benefit and burden of MPO activity. Enzyme structure is discussed, including accurate masses and glycosylation sites. The catalytic cycle of MPO and its corresponding pathways are presented, with a discussion of the importance of the redox couples of the different states of MPO. Cell lines expressing MPO are discussed and practically summarized for the reader, and locations of MPO (primary and secondary) are provided. Useful methods of MPO detection are discussed, and how these can be used for studying disease processes are implied through the presentation of MPO as a biomarker. The presence of MPO in neutrophil extracellular traps is presented, and the activators of the former are provided. Lastly, the transition from drug metabolism to a target for drug development is where the review concludes.

Voltage gated sodium channel inhibitors as anticonvulsant drugs: A systematic review on recent developments and structure activity relationship studies

Voltage-gated sodium channel blockers are one of the vital targets for the management of several central nervous system diseases, including epilepsy, chronic pain, psychiatric disorders, and spasticity. The voltage-gated sodium channels play a key role in controlling cellular excitability. This reduction in excitotoxicity is also applied to improve the symptoms of epileptic conditions. The effectiveness of antiepileptic drugs as sodium channel depends upon the reversible blocking of the spontaneous discharge without blocking its propagation. There are number of antiepileptic drug(s) which are in pipeline to flour the market to conquer abnormal neuronal excitability. They inhibit the seizures through the inhibition of complex voltage- and frequency-dependent ionic currents through sodium channels. Over the past decade, the sodium channel is one of the most explored targets to control or treat the seizure, but there has not been any game-changing discovery yet. Although there are large numbers of drugs approved for the treatment of epilepsy, however they are associated with several acute to chronic side effects. Many research groups have tirelessly worked for better therapeutic medication on this popular target to treat epileptic seizures. The review quotes briefly the developments of the approved examples of sodium channel blockers as anticonvulsant drugs. Medicinal chemists have tried the design and development of some more potent anticonvulsant drugs to minimize the toxicity that are discussed here, and an emphasis is given for their possible mechanism and the structure-activity relationship (SAR).

Progression of Repair and Injury in Human Liver Slices

Human liver slice function was stressed by daily dosing of acetaminophen (APAP) or diclofenac (DCF) to investigate injury and repair. Initially, untreated human liver and kidney slices were evaluated with the global human U133A array to assess the extended culture conditions. Then, drug induced injury and signals of repair in human liver slices exposed to APAP or DCF (1 mM) were evaluated via specific gene expression arrays. In culture, the untreated human liver and kidney slices remained differentiated and gene expression indicated that repair pathways were activated in both tissues. Morphologically the human liver slices exhibited evidence of repair and regeneration, while kidney slices did not. APAP and DCF exposure caused a direct multi-factorial response. APAP and DCF induced gene expression changes in transporters, oxidative stress and mitochondria energy. DCF caused a greater effect on heat shock and endoplasmic reticulum (ER) stress gene expression. Concerning wound repair, APAP caused a mild repression of gene expression; DCF suppressed the expression of matrix collagen genes, the remodeling metalloproteases, cell adhesion integrins, indicating a greater hinderance to wound repair than APAP. Thus, human liver slices are a relevant model to investigate the mechanisms of drug-induced injury and repair.

Peroxidase Activity of Human Hemoproteins: Keeping the Fire under Control

The heme in the active center of peroxidases reacts with hydrogen peroxide to form highly reactive intermediates, which then oxidize simple substances called peroxidase substrates. Human peroxidases can be divided into two groups: (1) True peroxidases are enzymes whose main function is to generate free radicals in the peroxidase cycle and (pseudo)hypohalous acids in the halogenation cycle. The major true peroxidases are myeloperoxidase, eosinophil peroxidase and lactoperoxidase. (2) Pseudo-peroxidases perform various important functions in the body, but under the influence of external conditions they can display peroxidase-like activity. As oxidative intermediates, these peroxidases produce not only active heme compounds, but also protein-based tyrosyl radicals. Hemoglobin, myoglobin, cytochrome c/cardiolipin complexes and cytoglobin are considered as pseudo-peroxidases. Рeroxidases play an important role in innate immunity and in a number of physiologically important processes like apoptosis and cell signaling. Unfavorable excessive peroxidase activity is implicated in oxidative damage of cells and tissues, thereby initiating the variety of human diseases. Hence, regulation of peroxidase activity is of considerable importance. Since peroxidases differ in structure, properties and location, the mechanisms controlling peroxidase activity and the biological effects of peroxidase products are specific for each hemoprotein. This review summarizes the knowledge about the properties, activities, regulations and biological effects of true and pseudo-peroxidases in order to better understand the mechanisms underlying beneficial and adverse effects of this class of enzymes.

The use of structural alerts to avoid the toxicity of pharmaceuticals

In order to identify compounds with potential toxicity problems, particular attention is paid to structural alerts, which are high chemical reactivity molecular fragments or fragments that can be transformed via bioactivation by human enzymes into fragments with high chemical reactivity. The concept has been introduced in order to reduce the likelihood that future candidate substances as pharmaceuticals will have undesirable toxic effects. A significant proportion (∼78–86%) of drugs characterized by residual toxicity contain structural alerts; there is also evidence indicating the formation of active metabolites as a causal factor for the toxicity of 62–69% of these molecules. On the other hand, the pharmacological action of certain drugs depends on the formation of reactive metabolites. Detailed assessment of the potential for the formation of active metabolites is recommended to characterize a biologically active compound. Although many prescribed drugs frequently contain structural alerts and form reactive metabolites, the vast majority of these drugs are administered in low daily doses. Avoiding structural alerts has become almost a norm in new drug design. An in-depth review of the biochemical reactivity of these structural alerts for new drug candidates is critical from a safety point of view and is currently being monitored in the discovery of drugs. The chemical strategies applied to structural alerts in molecules to limit the toxicity are:

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partial replacement or full replacement of the structural alert;

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reduction of electronic density;

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introduction of a structural element of metabolic interest (metabolic switching);

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multiple approaches.

Therefore, chemical intervention strategies to eliminate bioactivation are often interactive processes; their success depends largely on a close working relationship between drug chemists, pharmacologists and researchers in metabolic science.

Exacerbated liver injury of antithyroid drugs in endotoxin-treated mice

Drug-induced liver injury is a major concern in clinical studies as well as in post-marketing surveillance. Previous evidence suggested that drug exposure during periods of inflammation could increase an individual's susceptibility to drug hepatoxicity. The antithyroid drugs, methimazole (MMI) and propylthiouracil (PTU) can cause adverse reactions in patients, with liver as a usual target. We tested the hypothesis that MMI and PTU could be rendered hepatotoxic in animals undergoing a modest inflammation. Mice were treated with a nonhepatotoxic dose of LPS (100 µg/kg, i.p) or its vehicle. Nonhepatotoxic doses of MMI (10, 25 and 50 mg/kg, oral) and PTU (10, 25 and 50 mg/kg, oral) were administered two hours after LPS treatment. It was found that liver injury was evident only in animals received both drug and LPS, as estimated by increases in serum alanine aminotransferase (ALT), lactate dehydrogenase (LDH), aspartate aminotransferase (AST), and TNF-α. An increase in liver myeloperoxidase (MPO) enzyme activity and tissue lipid peroxidation (LPO) in addition of liver glutathione (GSH) depletion were also detected in LPS and antithyroid drugs cotreated animals. Furthermore, histopathological changes including, endotheliitis, fatty changes, severe inflammatory cells infiltration (hepatitis) and sinusoidal congestion were detected in liver tissue. Methyl palmitate (2 g/kg, i.v, 44 hours before LPS), as a macrophage suppressor, significantly alleviated antithyroids hepatotoxicity in LPS-treated animals. The results indicate a synergistic liver injury from antithyroid drugs and bacterial lipopolysaccharide coexposure.

Liver Effects of Clinical Drugs Differentiated in Human Liver Slices

Drugs with clinical adverse effects are compared in an ex vivo 3-dimensional multi-cellular human liver slice model. Functional markers of oxidative stress and mitochondrial function, glutathione GSH and ATP levels, were affected by acetaminophen (APAP, 1 mM), diclofenac (DCF, 1 mM) and etomoxir (ETM, 100 μM). Drugs targeting mitochondria more than GSH were dantrolene (DTL, 10 μM) and cyclosporin A (CSA, 10 μM), while GSH was affected more than ATP by methimazole (MMI, 500 μM), terbinafine (TBF, 100 μM), and carbamazepine (CBZ 100 μM). Oxidative stress genes were affected by TBF (18%), CBZ, APAP, and ETM (12%–11%), and mitochondrial genes were altered by CBZ, APAP, MMI, and ETM (8%–6%). Apoptosis genes were affected by DCF (14%), while apoptosis plus necrosis were altered by APAP and ETM (15%). Activation of oxidative stress, mitochondrial energy, heat shock, ER stress, apoptosis, necrosis, DNA damage, immune and inflammation genes ranked CSA (75%), ETM (66%), DCF, TBF, MMI (61%–60%), APAP, CBZ (57%–56%), and DTL (48%). Gene changes in fatty acid metabolism, cholestasis, immune and inflammation were affected by DTL (51%), CBZ and ETM (44%–43%), APAP and DCF (40%–38%), MMI, TBF and CSA (37%–35%). This model advances multiple dosing in a human ex vivo model, plus functional markers and gene profile markers of drug induced human liver side-effects.

Concurrent Inflammation Augments Antimalarial Drugs-Induced Liver Injury in Rats

Purpose: Accumulating evidence suggests that drug exposure during a modest inflammation induced by bacterial lipopolysaccharide (LPS) might increase the risk of drug-induced liver injury. The current investigation was designed to test if antimalarial drugs hepatotoxicity is augmented in LPS‑treated animals. Methods: Rats were pre-treated with LPS (100 µg/kg, i.p). Afterward, non-hepatotoxic doses of amodiaquine (25, 50 and 100 mg/kg, oral) and chloroquine (25, 50 and 100 mg/kg, oral) were administered. Results: Interestingly, liver injury was evident only in animals treated with both drug and LPS as estimated by pathological changes in serum biochemistry (ALT, AST, LDH, and TNF-α), and liver tissue (severe hepatitis, endotheliitis, and sinusoidal congestion). An increase in liver myeloperoxidase enzyme activity, lipid peroxidation, and protein carbonylation, along with tissue glutathione depletion were also detected in LPS and drug co-treated animals. Conclusion: Antimalarial drugs rendered hepatotoxic in animals undergoing a modest inflammation. These results indicate a synergistic liver injury from co-exposure to antimalarial drugs and inflammation.

Modulatory effect of luteolin on redox homeostasis and inflammatory cytokines in a mouse model of liver cancer

Liver cancer is one of the leading causes of cancer-associated mortality worldwide. Due to changes in lifestyle and daily exposure to various chemicals, which may lead to chemical intoxication, liver cancer has become a prominent disease in humans. Chemical-induced carcinogenesis in experimental animals has become a reliable model for the investigation of liver cancer-associated biological alterations that may mimic human hepatic cancer. Liver cancer in BALB/c mice was induced by administering diethylnitrosamine (DN) in drinking water for 6 weeks. Luteolin (LUT) is a flavone that is found in the leaves of the majority of spice-associated plants. In the present study, 20 µg/kg of body weight LUT was administered intraperitoneally every alternate day to treat the DN-induced liver cancer in mice. LUT improved the host system by modifying the levels of α-fetoprotein, enzymatic antioxidants, such as superoxide dismutase and catalase, marker enzymes, such as AST and ALT, and lipid peroxides in the plasma or liver tissue. LUT also reduced the levels of glutathione and the inflammatory cytokines interleukin-2 and interferon-γ in the plasma or liver tissue. These findings augmented the treatment against liver cancer and supported the effective anticancer activity of LUT against DN-induced liver carcinogenesis in mice.

Caralluma umbellata Peroxidase: Biochemical Characterization and Its Detoxification Potentials in Comparison with Horseradish Peroxidase

Caralluma umbellata peroxidase (CUP) is an acidic heme-containing protein having a molecular weight of ~42 kDa and is specific to guaiacol. It is not a glycoprotein. It was purified to 12.5-fold purity with 6.16 % yield. Its activity is dependent on hydrogen peroxide and has an optimum pH and temperature of 6.2 and 45 °C respectively. It can decolorize dyes, viz., Aniline Blue, Reactive Black 5, and Reactive Blue 19 but not Congo Red, while HRP can decolorize Congo Red also. It has lignin-degrading potentiality as it can decompose veratryl alcohol. Detoxification of phenol was more by CUP compared to HRP while with p-nitrophenol HRP has a greater detoxification rate. Based on our results, CUP was identified to be capable of oxidizing a variety of hazardous substances and also a lignin-degrading plant biocatalyst.

An overview on the proposed mechanisms of antithyroid drugs-induced liver injury

Drug-induced liver injury (DILI) is a major problem for pharmaceutical industry and drug development. Mechanisms of DILI are many and varied. Elucidating the mechanisms of DILI will allow clinicians to prevent liver failure, need for liver transplantation, and death induced by drugs. Methimazole and propylthiouracil (PTU) are two convenient antithyroid agents which their administration is accompanied by hepatotoxicity as a deleterious side effect. Although several cases of antithyroid drugs-induced liver injury are reported, there is no clear idea about the mechanism(s) of hepatotoxicity induced by these medications. Different mechanisms such as reactive metabolites formation, oxidative stress induction, intracellular targets dysfunction, and immune-mediated toxicity are postulated to be involved in antithyroid agents-induced hepatic damage. Due to the idiosyncratic nature of antithyroid drugs-induced hepatotoxicity, it is impossible to draw a specific conclusion about the mechanisms of liver injury. However, it seems that reactive metabolite formation and immune-mediated toxicity have a great role in antithyroids liver toxicity, especially those caused by methimazole. This review attempted to discuss different mechanisms proposed to be involved in the hepatic injury induced by antithyroid drugs.

Flutamide-induced cytotoxicity and oxidative stress in an in vitro rat hepatocyte system

Flutamide (FLU) is a competitive antagonist of the androgen receptor which has been reported to induce severe liver injury in some patients. Several experimental models suggested that an episode of inflammation during drug treatment predisposes animals to tissue injury. The molecular cytotoxic mechanisms of FLU in isolated rat hepatocytes using an in vitro oxidative stress inflammation system were investigated in this study. When a nontoxic hydrogen peroxide (H₂O₂) generating system (glucose/glucose oxidase) with peroxidase or iron(II) [Fe(II)] (to partly simulate in vivo inflammation) was added to the hepatocytes prior to the addition of FLU, increases in FLU-induced cytotoxicity and lipid peroxidation (LPO) were observed that were decreased by 6-N-propyl-2-thiouracil or deferoxamine, respectively. N-Acetylcysteine decreased FLU-induced cytotoxicity in this system. Potent antioxidants, for example, Trolox ((±)-6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid), resveratrol (3,5,4′-trihydroxy-trans-stilbene), and DPPD (N,N′-diphenyl-1,4-phenylenediamine) also significantly decreased FLU-induced cytotoxicity and LPO and increased mitochondrial membrane potential (MMP) and glutathione (GSH) levels in the H₂O₂ generating system with peroxidase. TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl), a known reactive oxygen species (ROS) scavenger and superoxide dismutase mimetic, also significantly decreased toxicity caused by FLU in this system. These results raise the possibility that the presence or absence of inflammation may be another susceptibility factor for drug-induced hepatotoxicity.

Naturally-occurring tetrahydro-β-carboline alkaloids derived from tryptophan are oxidized to bioactive β-carboline alkaloids by heme peroxidases

β-Carbolines are indole alkaloids that occur in plants, foods, and endogenously in mammals and humans, and which exhibit potent biological, psychopharmacological and toxicological activities. They form from naturally-occurring tetrahydro-β-carboline alkaloids arising from tryptophan by still unknown way and mechanism. Results in this research show that heme peroxidases catalyzed the oxidation of tetrahydro-β-carbolines (i.e. 1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid and 1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid) into aromatic β-carbolines (i.e. norharman and harman, respectively). This oxidation followed a typical catalytic cycle of peroxidases through redox intermediates I, II, and ferric enzyme. Both, plant peroxidases (horseradish peroxidase, HRP) and mammalian peroxidases (myeloperoxidase, MPO and lactoperoxidase, LPO) catalyzed the oxidation in an efficient manner as determined by kinetic parameters (VMAX and KM). Oxidation of tetrahydro-β-carbolines was inhibited by peroxidase inhibitors such as sodium azide, ascorbic acid, hydroxylamine and excess of H2O2. The formation of aromatic β-carbolines by heme peroxidases can help to explain the presence and activity of these compounds in biological systems.

Protective Effects of N-acetylcysteine Against the Statins Cytotoxicity in Freshly Isolated Rat Hepatocytes

PURPOSE

Hepatotoxicity is one of the most important side effects of the statins therapy as lipid-lowering agents. However, the mechanism(s) of hepatotoxicity induced by these drugs is not clearly understood yet, and no hepatoprotective agent has been developed against this complication.

METHODS

The protective effect of N-acetylcysteine (NAC) against statins-induced cytotoxicity was evaluated by using freshly isolated rat hepatocytes. Hepatocytes were prepared by the method of collagenase enzyme perfusion via portal vein. This technique is based on liver perfusion with collagenase after removal of calcium ion (Ca2+) with a chelator (ethylene glycol tetra acetic acid (EGTA) 0.5 mM). The level of parameters such as cell death, ROS formation, lipid peroxidation, mitochondrial membrane potential (MMP) in the statins-treated hepatocytes were determined. Additionally, the mentioned markers were assessed in the presence of NAC.

RESULTS

Incubation of hepatocytes with the statins resulted in cytotoxicity characterized by an elevation in cell death, increasing ROS generation and consequently lipid peroxidation and impairment of mitochondrial function. Administration of NAC caused reduction in amount of ROS formation, lipid peroxidation and finally, cell viability and mitochondrial membrane potential (MMP) were improved.

CONCLUSION

This study confirms that oxidative stress and consequently mitochondrial dysfunction is one of the mechanisms underlying the statins-induced liver injury and treating hepatocytes by NAC (200 μM) attenuates this cytotoxicity.

Molecular mechanism of 17α-ethinylestradiol cytotoxicity in isolated rat hepatocytes

17α-Ethinylestradiol (17-EE) is used in formulations of contraceptives and hormone replacement therapy because it is an estradiol derivative. However, it has been associated with an increase in the risk of liver cancers and injury. The carcinogenic properties of 17-EE are similar to that of other estrogens, but the molecular mechanism of liver injury is still unclear. It is important to identify any secondary toxic mechanisms that can be used to prevent or treat the toxicity. The LC50 of 17-EE toward isolated rat hepatocytes was determined to be 150 ± 8 μmol/L. Accelerated cytotoxicity mechanism screening (ACMS) techniques using isolated rat hepatocytes showed that CYP1A inhibitors decreased cytotoxicity, whereas tyrosinase increased toxicity; this suggests that the toxic mechanism involved is the oxidation of 17-EE. A hepatocyte inflammation model also increased 17-EE-induced mitochondrial toxicity, as well as the formation of ROS and H2O2. Cytotoxicity was increased when inhibitors of quinone reduction, catechol-O-methylation, glucuronidation, glutathione conjugation, and sulfation were co-incubated with 17-EE. The hepatocytes could be rescued with antioxidants and quinone trapping agents, thereby suggesting a role for quinoid moiety induced oxidative stress in 17-EE induced cytotoxicity. These mechanisms for 17-EE hepatotoxicity could provide a new perspective for the treating 17-EE-induced liver injury.

Metabolite profile resulting from the activation/inactivation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and 2-methyltetrahydro-β-carboline by oxidative enzymes

Metabolic enzymes are involved in the activation/deactivation of the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyiridine (MPTP) neurotoxin and its naturally occurring analogs 2-methyltetrahydro-β-carbolines. The metabolic profile and biotransformation of these protoxins by three enzymes, monoamine oxidase (MAO), cytochrome P450, and heme peroxidases (myeloperoxidase and lactoperoxidase), were investigated and compared. The metabolite profile differed among the enzymes investigated. MAO and heme peroxidases activated these substances to toxic pyridinium and β-carbolinium species. MAO catalyzed the oxidation of MPTP to 1-methyl-4-phenyl-2,3-dihydropyridinium cation (MPDP(+)), whereas heme peroxidases catalyzed the oxidation of MPDP(+) to 1-methyl-4-phenylpyridinium (MPP(+)) and of 2-methyltetrahydro-β-carboline to 2-methyl-3,4-dihydro-β-carbolinium cation (2-Me-3,4-DH β C(+)). These substances were inactivated by cytochrome P450 2D6 through N-demethylation and aromatic hydroxylation (MPTP) and aromatic hydroxylation (2-methyltetrahydro-β-carboline). In conclusion, the toxicological effects of these protoxins might result from a balance between the rate of their activation to toxic products (i.e., N-methylpyridinium-MPP(+) and MPDP(+)- and N-methyl--β--carbolinium- βC(+)-) by MAO and heme peroxidases and the rate of inactivation (i.e., N-demethylation, aromatic hydroxylation) by cytochrome P450 2D6.

Evaluation of drug-induced injury and human response in precision-cut tissue slices

1.Drug induced organ injury is multifaceted, encompassing a spectrum of cell types and numerous networks reflecting cell-cell and cell-matrix interactions. Characterization of drug induced side effects and human response can be addressed in organ slice models. 2.The application of human tissue to various organ slice models including liver, intestine, kidney, liver-blood co-cultures and thyroid enhances our ability to focus on the clinical relevance of side effects identified in animal studies for human, and to evaluate potential biomarkers of the side effects. Dose-response relationships can help discern drug concentrations which alter organ function or affect morphology, to identify drug concentrationswhich could pose a risk for humans. 3.Insight into pathways of organ injury, by incorporating gene and protein expression profiling, with functional measurements and morphology, aid to define species differences and sensitivity. 4.Human organ slice studies are valuable for bridging the extrapolation of animal derived data and for identifying mechanisms relevant for humans, thereby expanding the scope of translational research for drug safety assessment.

Identification of the ortho-benzoquinone intermediate of 5-O-caffeoylquinic acid in vitro and in vivo: comparison of bioactivation under normal and pathological situations

5-O-Caffeoylquinic acid (5-CQA) is one of the major bioactive ingredients in some Chinese herbal injections. Occasional anaphylaxis has been reported for these injections during their clinical use, possibly caused by reactive metabolites of 5-CQA. This study aimed at characterizing the bioactivation pathway(s) of 5-CQA and the metabolic enzyme(s) involved. After incubating 5-CQA with GSH and NADPH-supplemented human liver microsomes, two types of GSH conjugates were characterized: one was M1-1 from the 1,4-addition of GSH to ortho-benzoquinone intermediate; the other was M2-1 and M2-2 from the 1,4-addition of GSH directly to the α,β-unsaturated carbonyl group of the parent. The formation of M1-1 was cytochrome P450 (P450)-mediated, with 3A4 and 2E1 as the principal catalyzing enzymes, whereas the formation of M2-1 and M2-2 was independent of NADPH and could be accelerated by cytosolic glutathione transferase. In the presence of cumene hydroperoxide, M1-1 formation increased 6-fold, indicating that 5-CQA can also be bioactivated by P450 peroxidase under oxidizing conditions. Furthermore, M1-1 could be formed by myeloperoxidase in activated human leukocytes, implying that 5-CQA bioactivation is more likely to occur under inflammatory conditions. This finding was supported by experiments on lipopolysaccharide-induced inflammatory rats, where a greater amount of M1-1 was detected. In S-adenosyl methionine- and GSH-supplemented human S9 incubations, M1-1 formation decreased by 80% but increased after tolcapone-inhibited catechol-O-methyltransferase (COMT) activity. In summary, the high reactivities of the ortho-benzoquinone metabolite and α,β-unsaturated carbonyl group of 5-CQA to nucleophiles have been demonstrated. Different pathological situations and COMT activities in patients may alter the bioactivation extent of 5-CQA.

Green tea (Camellia sinesis) ameliorates female Schistosoma mansoni-induced changes in the liver of Balb/C mice

This study was designed to assess the effect of green tea, an aqueous extract of Camellia sinensis, on the oxidative stress, antioxidant defense system and liver pathology of Schistosoma mansoni-infected mice. Green tea at concentration of 3% (w/v) was given orally to treated mice as sole source of drinking water from the end of the 4th week to the end of 10th week post-infection; untreated mice were allowed to drink normal water. The data of the studied S. mansoni-infected mice exhibited a suppression of hepatic total antioxidant capacity, superoxide dismutase (SOD), catalase (CAT) activity and glutathione content. The liver lipid peroxidation was deleteriously elevated in S. mansoni-infected mice. The hepatic total protein content, AST and ALT activities were profoundly decreased in the S. mansoni-infected mice. Most hepatocytes were damaged and showed abnormal microscopic appearance with aggressive necrosis. Both total protein and glycogen levels have been greatly reduced as indicated by histochemical examination. The treatment of S. mansoni-infected mice with green tea succeeded to suppress oxidative stress by decreasing the lipid peroxides but failed to significantly enhance the antioxidant defense system and deteriorated changes owing to liver damage and necrosis. In consistence with biochemical data, histopathological and histochemical data indicated that treatment of S. mansoni-infected mice with green tea could ameliorate hepatocytes thus reduce cellular necrosis and partially restore both total protein and glycogen levels. Thus, the study concluded that the green tea suppresses the oxidative stress through its constituent with free radicals scavenging properties rather than through the endogenous antioxidant defense system.

Bioactivation of drugs: risk and drug design

Bioactivation through drug metabolism is frequently suspected as an initiating event in many drug toxicities. The CYP450 and peroxidase enzyme systems are generally considered the most important groups of enzymes involved in bioactivation, producing either electrophilic or radical metabolites. Drug design efforts routinely consider these factors, and a number of structural alerts for bioactivation have been identified. Among the most frequently encountered structural alerts are aromatic systems with electron-donating substituents and some five-membered heterocycles. Metabolism of these groups can lead to chemically reactive electrophiles. Strategies that have been used to minimize the associated risk involve replacing the structural-alert moiety, blocking or making metabolism less favorable, and incorporating metabolic soft spots to facilitate metabolism away from the structural-alert substituent. The metabolism of drugs to radicals usually leads to cellular oxidative stress. The formation of radical metabolites can be minimized through the use of similar approaches but remains an area less frequently considered in drug design.

Myeloperoxidase-derived oxidation: mechanisms of biological damage and its prevention

There is considerable interest in the role that mammalian heme peroxidase enzymes, primarily myeloperoxidase, eosinophil peroxidase and lactoperoxidase, may play in a wide range of human pathologies. This has been sparked by rapid developments in our understanding of the basic biochemistry of these enzymes, a greater understanding of the basic chemistry and biochemistry of the oxidants formed by these species, the development of biomarkers that can be used damage induced by these oxidants in vivo, and the recent identification of a number of compounds that show promise as inhibitors of these enzymes. Such compounds offer the possibility of modulating damage in a number of human pathologies. This reviews recent developments in our understanding of the biochemistry of myeloperoxidase, the oxidants that this enzyme generates, and the use of inhibitors to inhibit such damage.

Selenium toxicosis assessment (in vivo and in vitro) and the protective role of vitamin B12 in male quail (Coturnix Coturnix)

The present study was undertaken to elucidate the toxicity induced by sodium selenite in male quail through in vivo and in vitro studies and the role played by vitamin B12 in alleviating selenium toxicity. Administration of selenite orally for 1 month induced hepatic oxidative damage. Selenite decreased body weight gain and increased relative liver weight. Selenite reduced hemoglobin and iron concentrations and elevated total bilirubin concentration. Serum transaminases and alkaline phosphatase activities were increased in selenium-intoxicated quails. Total protein concentration was decreased associated with the appearance of prealbumin fraction, an increased γ-globulin and a decreased α- and β-globulins. The highest level of selenium was found in liver followed by kidney, testis, faeces and blood. Supplementation of vitamin B12 orally for 1 month simultaneously with selenite caused less marked biological alteration in the investigated parameters. In vitro study using isolated quail hepatocytes incubated with sodium selenite showed a dose-dependent response for toxicity markers. These results suggest that selenosis can be reduced by vitamin B12 supplementation.

Mammalian heme peroxidases: from molecular mechanisms to health implications

A marked increase in interest has occurred over the last few years in the role that mammalian heme peroxidase enzymes, primarily myeloperoxidase, eosinophil peroxidase, and lactoperoxidase, may play in both disease prevention and human pathologies. This increased interest has been sparked by developments in our understanding of polymorphisms that control the levels of these enzymes, a greater understanding of the basic chemistry and biochemistry of the oxidants formed by these species, the development of specific biomarkers that can be used in vivo to detect damage induced by these oxidants, the detection of active forms of these peroxidases at most, if not all, sites of inflammation, and a correlation between the levels of these enzymes and a number of major human pathologies. This article reviews recent developments in our understanding of the enzymology, chemistry, biochemistry and biologic roles of mammalian peroxidases and the oxidants that they generate, the potential role of these oxidants in human disease, and the use of the levels of these enzymes in disease prognosis.

Cytotoxic effects of the dietary flavones chrysin and apigenin in a normal trout liver cell line

Many flavonoids have been shown to possess prooxidant properties, capable of causing oxidative stress, especially at larger doses. Here, we examined the potential cell toxicity caused by exposure to the hydroxylated flavones chrysin, apigenin, luteolin and quercetin in comparison to the methylated flavones 5,7-dimethoxyflavone and 3',4'-dimethoxyflavone in normal Rainbow trout hepatocytes. The hydroxylated flavones, especially chrysin, demonstrated cell toxicity and inhibition of DNA synthesis at very low (2 microM) concentrations. The cytotoxicity of chrysin may partially be due to its metabolism by myeloperoxidase, which was shown to be present in these normal trout liver cells (164pmol/(min mg protein)). In contrast, methylated flavones showed no significant metabolism by myeloperoxidase and no signs of toxicity, even at much higher concentrations. These results may be useful for further investigations of cytotoxicity of dietary flavonoids.

The biochemistry of drug metabolism--an introduction: Part 2. Redox reactions and their enzymes

This review continues a general presentation of the metabolism of drugs and other xenobiotics started in a recent issue of Chemistry & Biodiversity. This Part 2 presents the numerous oxidoreductases involved, their nomenclature, relevant biochemical properties, catalytic mechanisms, and the very diverse reactions they catalyze. Many medicinally, environmentally, and toxicologically relevant examples are presented and discussed. Cytochromes P450 occupy a majority of the pages of Part 2, but a large number of relevant oxidoreductases are also considered, e.g., flavin-containing monooxygenases, amine oxidases, molybdenum hydroxylases, peroxidases, and the innumerable dehydrogenases/reductases.

Involvement of enzymes other than CYPs in the oxidative metabolism of xenobiotics

Although the majority of oxidative metabolic reactions are mediated by the CYP superfamily of enzymes, non-CYP-mediated oxidative reactions can play an important role in the metabolism of xenobiotics. The (major) oxidative enzymes, other than CYPs, involved in the metabolism of drugs and other xenobiotics are: the flavin-containing monooxygenases, the molybdenum hydroxylases (aldehyde oxidase and xanthine oxidase), the prostaglandin H synthase, the lipoxygenases, the amine oxidases (monoamine, polyamine, diamine and semicarbazide-sensitive amine oxidases) and the alcohol and aldehyde dehydrogenases. In a similar manner to CYPs, these oxidative enzymes can also produce therapeutically active metabolites and reactive/toxic metabolites, modulate the efficacy of therapeutically active drugs or contribute to detoxification. Many of them have been shown to be important in endobiotic metabolism, and, consequently, interactions between drugs and endogenous compounds might occur when they are involved in drug metabolism. In general, most non-CYP oxidative enzymes appear to be noninducible or much less inducible than the CYP system, although some of them may be as inducible as some CYPs. Some of these oxidative enzymes exhibit polymorphic expression, as do some CYPs. It is possible that the contribution of non-CYP oxidative enzymes to the overall metabolism of xenobiotics is underestimated, as most investigations of drug metabolism in discovery and lead optimisation are performed using in vitro test systems optimised for CYP activity.

N-methyltetrahydro-beta-carboline analogs of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) neurotoxin are oxidized to neurotoxic beta-carbolinium cations by heme peroxidases

2-Methyl-1,2,3,4-tetrahydro-beta-carboline (2-Me-THbetaC) and 2,9-dimethyl-1,2,3,4-tetrahydro-beta-carboline (2,9-diMe-THbetaC) are naturally occurring analogs of the Parkinsonian neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), whereas their corresponding aromatic 2-methyl-beta-carbolinium cations resemble 1-methyl-4-phenylpyridinium (MPP(+)) and are considered potential toxins involved in Parkinson's disease (PD). To become toxicants, 2-methyltetrahydro-beta-carbolines need to be oxidized (aromatized) by human metabolic enzymes to pyridinium-like (beta-carbolinium) cations as occur with MPTP/MPP(+) model. In contrast to MPTP, human MAO-A or -B were not able to oxidize 2-Me-THbetaC to pyridinium-like cations. Neither, cytochrome P-450 2D6 or a mixture of six P450 enzymes carried out this oxidation in a significant manner. However, 2-Me-THbetaC and 2,9-diMe-THbetaC were efficiently oxidized by horseradish peroxidase (HRP), lactoperoxidase (LPO), and myeloperoxidase (MPO) to 2-methyl-3,4-dihydro-beta-carbolinium cations (2-Me-DHbetaC(+), 2,9-diMe-DHbetaC(+)) as the main products, and detectable amount of 2-methyl-beta-carbolinium cations (2-Me-betaC(+), 2,9-diMe-betaC(+)). The apparent kinetic parameters (k(cat), k(4)) were similar for HRP and LPO and higher for MPO. Peroxidase inhibitors (hydroxylamine, sodium azide, and ascorbic acid) highly reduced or abolished this oxidation. Although MPTP was not oxidized by peroxidases; its intermediate metabolite 1-methyl-4-phenyl-2,3-dihydropyridinium cation (MPDP(+)) was efficiently oxidized to MPP(+) by heme peroxidases. It is concluded that heme peroxidases could be key catalysts responsible for the aromatization (bioactivation) of endogenous and naturally occurring N-methyltetrahydro-beta-carbolines and related protoxins to toxic pyridinium-like cations resembling MPP(+), suggesting a role for these enzymes in toxicological and neurotoxicological processes.

Drug-induced mitochondrial toxicity

Mitochondria play a critical role in generating most of the cell's energy as ATP. They are also involved in other metabolic processes such as urea generation, haem synthesis and fatty acid beta-oxidation. Disruption of mitochondrial function by drugs can result in cell death by necrosis or can signal cell death by apoptosis (e.g., following cytochrome c release). Drugs that injure mitochondria usually do so by inhibiting respiratory complexes of the electron chain; inhibiting or uncoupling oxidative phosphorylation; inducing mitochondrial oxidative stress; or inhibiting DNA replication, transcription or translation. It is important to test for mitochondrial toxicity early in drug development as impairment of mitochondrial function can induce various pathological conditions that are life threatening or can increase the progression of existing mitochondrial diseases.

Superoxide-dependent Oxidation of Melatonin by Myeloperoxidase

Myeloperoxidase uses hydrogen peroxide to oxidize numerous substrates to hypohalous acids or reactive free radicals. Here we show that neutrophils oxidize melatonin to N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK) in a reaction that is catalyzed by myeloperoxidase. Production of AFMK was highly dependent on superoxide but not hydrogen peroxide. It did not require hypochlorous acid, singlet oxygen, or hydroxyl radical. Purified myeloperoxidase and a superoxide-generating system oxidized melatonin to AFMK and a dimer. The dimer would result from coupling of melatonin radicals. Oxidation of melatonin was partially inhibited by catalase or superoxide dismutase. Formation of AFMK was almost completely eliminated by superoxide dismutase but weakly inhibited by catalase. In contrast, production of melatonin dimer was enhanced by superoxide dismutase and blocked by catalase. We propose that myeloperoxidase uses superoxide to oxidize melatonin by two distinct pathways. One pathway involves the classical peroxidation mechanism in which hydrogen peroxide is used to oxidize melatonin to radicals. Superoxide adds to these radicals to form an unstable peroxide that decays to AFMK. In the other pathway, myeloperoxidase uses superoxide to insert dioxygen into melatonin to form AFMK. This novel activity expands the types of oxidative reactions myeloperoxidase can catalyze. It should be relevant to the way neutrophils use superoxide to kill bacteria and how they metabolize xenobiotics.