Inactivation of catalase by phenylhydrazine. Formation of a stable aryl-iron heme complex

A closer look into NADPH oxidase inhibitors: Validation and insight into their mechanism of action

NADPH-oxidases (NOXs) purposefully produce reactive-oxygen-species (ROS) and are found in most kingdoms of life. The seven human NOXs are each characterized by a specific expression profile and a fine regulation to spatio-temporally tune ROS concentration in cells and tissues. One of the best known roles for NOXs is in host protection against pathogens but ROS themselves are important second messengers involved in tissue regeneration and the modulation of pathways that induce and sustain cell proliferation. As such, NOXs are attractive pharmacological targets in immunomodulation, fibrosis and cancer. We have studied an extensive number of available NOX inhibitors, with the specific aim to identify bona fide ligands versus ROS-scavenging molecules. Accordingly, we have established a comprehensive platform of biochemical and biophysical assays. Most of the investigated small molecules revealed ROS-scavenging and/or assay-interfering properties to various degrees. A few compounds, however, were also demonstrated to directly engage one or more NOX enzymes. Diphenylene iodonium was found to react with the NOXs' flavin and heme prosthetic groups to form stable adducts. We also discovered that two compounds, VAS2870 and VAS3947, inhibit NOXs through the covalent alkylation of a cysteine residue. Importantly, the amino acid involved in covalent binding was found to reside in the dehydrogenase domain, where the nicotinamide ring of NADPH is bound. This work can serve as a springboard to guide further development of bona fide ligands with either agonistic or antagonistic properties toward NOXs.

Catalase and its mysteries

Catalase is one of the firsts in every realm of biological sciences. At the same time it also has a number of unusual features. It has one of the highest turnover numbers of all enzymes. It is essential for neutralizing the noxious hydrogen peroxide both in the nature and the various industries such as dairy, textile and pharmaceutics. It also has the merit of being one of the first protein crystals to be isolated. Ironically its three-dimensional structure was discerned some forty years later. However through the times this senile enzyme has continued to intrigue the scientists by surprising facts and phenomena, such as peculiar interweaving of subunits and remarkable thermal stability. It is also known for suicide inactivation by its own substrate. Catalase is known to be implicated in various medical scenarios and its levels have served as a marker in that capacity. It has even been incorporated into several pharmaceuticals. This review strives to clarify these perspectives. It also draws attention to the biophysical contributions offered by thermodynamics and kinetics in these discoveries. The ultimate aim of this review, however, is to state that the venerable catalase will continue to bewilder us with its mysteries well into the twenty-first century.

Discovery and evaluation of inhibitors to the immunosuppressive enzyme indoleamine 2,3-dioxygenase 1 (IDO1): Probing the active site-inhibitor interactions

High expression of the immunosuppressive enzyme, indoleamine 2,3-dioxygenase 1 (IDO1) for a broad range of malignancies is associated with poor patient prognosis, and the enzyme is a validated target for cancer intervention. To identify novel IDO1 inhibitors suitable for drug development, 1597 compounds in the National Cancer Institute Diversity Set III library were tested for inhibitory activity against recombinant human IDO1. We retrieved 35 hits that inhibited IDO1 activity >50% at 20 μM. Five structural filters and the PubChem Bioassay database were used to guide the selection of five inhibitors with IC₅₀ between 3 and 12 μM for subsequent experimental evaluation. A pyrimidinone scaffold emerged as being the most promising. It showed excellent cell penetration, negligible cytotoxicity and passed four out of the five structural filters applied. To evaluate the importance of Ser167 and Cys129 residues in the IDO1 active site for inhibitor binding, the entire NCI library was subsequently screened against alanine-replacement mutant enzymes of these two residues. The results established that Ser167 but not Cys129 is important for inhibitory activity of a broad range of IDO1 inhibitors. Structure-activity-relationship studies proposed substituents interacting with Ser167 on four investigated IDO1 inhibitors. Three of these four Ser167 interactions associated with an increased IDO1 inhibition and were correctly predicted by molecular docking supporting Ser167 as an important mediator of potency for IDO1 inhibitors.

Organometallic myoglobins: Formation of Fe-carbon bonds and distal pocket effects on aryl ligand conformations

Bioorganometallic Fe-C bonds are biologically relevant species that may result from the metabolism of natural or synthetic hydrazines. The molecular structures of four new sperm whale mutant myoglobin derivatives with Fe-aryl moieties, namely H64A-tolyl-m, H64A-chlorophenyl-p, H64Q-tolyl-m, and H64Q-chlorophenyl-p, have been determined at 1.7-1.9Å resolution. The structures reveal conformational preferences for the substituted aryls resulting from attachment of the aryl ligands to Fe at the site of net -NHNH2 release from the precursor hydrazines, and show distal pocket changes that readily accommodate these bulky ligands.

O-alkylhydroxylamines as rationally-designed mechanism-based inhibitors of indoleamine 2,3-dioxygenase-1

Indoleamine 2,3-dioxygenase-1 (IDO1) is a promising therapeutic target for the treatment of cancer, chronic viral infections, and other diseases characterized by pathological immune suppression. Recently important advances have been made in understanding IDO1's catalytic mechanism. Although much remains to be discovered, there is strong evidence that the mechanism proceeds through a heme-iron bound alkylperoxy transition or intermediate state. Accordingly, we explored stable structural mimics of the alkylperoxy species and provide evidence that such structures do mimic the alkylperoxy transition or intermediate state. We discovered that O-benzylhydroxylamine, a commercially available compound, is a potent sub-micromolar inhibitor of IDO1. Structure-activity studies of over forty derivatives of O-benzylhydroxylamine led to further improvement in inhibitor potency, particularly with the addition of halogen atoms to the meta position of the aromatic ring. The most potent derivatives and the lead, O-benzylhydroxylamine, have high ligand efficiency values, which are considered an important criterion for successful drug development. Notably, two of the most potent compounds demonstrated nanomolar-level cell-based potency and limited toxicity. The combination of the simplicity of the structures of these compounds and their excellent cellular activity makes them quite attractive for biological exploration of IDO1 function and antitumor therapeutic applications.

Discovery and characterisation of hydrazines as inhibitors of the immune suppressive enzyme, indoleamine 2,3-dioxygenase 1 (IDO1)

Screening of a fragment library identified 2-hydrazinobenzothiazole as a potent inhibitor of indoleamine 2,3-dioxygenase 1 (IDO1), an enzyme expressed by tumours that suppresses the immune system. Spectroscopic studies indicated that 2-hydrazinobenzothiazole interacted with the IDO1 haem and in silico docking predicted that the interaction was through hydrazine. Subsequent studies of hydrazine derivatives identified phenylhydrazine (IC50=0.25 ± 0.07 μM) to be 32-fold more potent than 2-hydrazinobenzothiazole (IC50=8.0 ± 2.3 μM) in inhibiting rhIDO1 and that it inhibited cellular IDO1 at concentrations that were noncytotoxic to cells. Here, phenylhydrazine is shown to inhibit IDO1 through binding to haem.

Animal models with enhanced erythropoiesis and iron absorption

The regulation of iron absorption is of considerable interest in mammals since excretion is minimal. Recent advances in iron metabolism have expounded the molecular mechanisms by which iron absorption is attuned to the physiological demands of the body. The pinnacle was the discovery and identification of hepcidin, a hepatic antimicrobial peptide that regulates absorption to maintain iron homeostasis. While the intricacies of its expression and regulation by HFE, transferrin receptor 2 and hemojuvelin are still speculative, hepcidin responsiveness has correlated negatively with iron absorption in different models and disorders of iron metabolism. Consequently, hepcidin expression is repressed to enhance iron absorption during stimulated erythropoiesis even in situations of elevated iron stores. Animal models have been crucial to the advances in understanding iron metabolism and the present review focuses on phenylhydrazine treated and hypotransferrinaemic rodents. These, respectively, experimental and genetic models of enhanced erythropoiesis highlight the shifting focus of iron absorption regulation from the marrow to the liver.

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

Current safety screening of drug candidates or new chemical entities for reactive metabolite formation focuses on the role of cytochrome P450. However, peroxidases also have a major role in drug metabolism, and peroxidase-catalyzed drug oxidation could lead to reactive metabolite formation, resulting in oxidative stress and cytotoxicity. Here, the different classes of human peroxidases are summarized and the molecular mechanisms of peroxidase-catalyzed drug metabolism are discussed. In addition, evidence is presented that indicates a role of these enzymes in drug toxicity.

The role of thyroid hormone on phenylhydrazine hydrochloride mediated inhibitory effects on blood acetylcholinesterase: An in vivo and in vitro study

A novel phenomenon of protective counteraction by thyroid hormone has been demonstrated in phenylhydrazine hydrochloride (PHH) induced insult on blood acetylcholinesterase (AChE, EC 3.1.1.7) activity, in both, in vivo and in vitro conditions. Injection of PHH (20 microg/g) to juvenile male rats for three consecutive days caused a 48% decrease (p < 0.001) in the total blood AChE activity on the third day (i.e. 24 h after injections for three consecutive days) in comparison to the control animals. Simultaneous injections of thyroxine (T4) 1 or 2 microg/g with PHH (20 microg/g) showed a recovery in AChE activity by 27% (p < 0.02) and 55% (p < 0.001), respectively, in comparison to the only PHH-injected animals. T4 at 1, 2 and 4 microg/g doses showed unchanged levels in comparison to the untreated controls. In our in vitro system, incubations of the RBCs in PHH (2 mM) containing medium also showed an inhibition of 44% (p < 0.001) of the RBC membrane AChE activity in comparison to the control conditions. A recovery of 23-81% of the enzyme activity was observed after simultaneous use of T4 (1 nM-100 nM) or T3 (0.1 nM-100 nM), or triiodothyroacetic acid (TRIAC) (100 nM) with PHH (2 mM) in a dose-dependent manner with a potency profile of T3 > T4 > TRIAC. Incubation of RBCs only with T4, T3, or TRIAC at 0.1-100 nM concentration did not cause any alteration in the membrane AChE activity in comparison to control conditions. Thus, thyroid hormone distinctly demonstrated a counteraction or protective nature of action on the PHH-induced inhibition of total blood and RBC membrane AChE activity.

Identification of catalase in human livers as a factor that enhances phenytoin dihydroxy metabolite formation by human liver microsomes

We have reported previously that the formation of a 3',4'-dihydroxylated metabolite of phenytoin (3',4'-diHPPH) by human liver microsomal cytochrome P450 (P450) is enhanced by the addition of human liver cytosol [Komatsu et al., Drug Metab Dispos 2000;28:1361-8]. The enhancing factor was determined in this study. The addition of cytosolic proteins precipitated by 50% ammonium sulfate to incubation mixtures increased the rate of microsomal 3',4'-diHPPH formation. This fraction was separated further by diethylaminoethyl-, carboxymethyl-, and hydroxyapatite-column chromatography. The amino acid sequence of the purified protein of approximately 55kDa by electrophoresis revealed this protein to be a catalase. The addition of purified or authentic catalase to the incubation mixtures increased the rates of microsomal 3',4'-diHPPH formation from 3'- and 4'-hydroxylated metabolites and from phenytoin in a concentration-dependent manner. In reconstituted systems containing CYP2C9, CYP2C19, and CYP3A4, the formation of 3',4'-diHPPH was also enhanced by catalase to different extents. This is the first report that catalase in livers enhances drug oxidation activities catalyzed by P450 in human liver microsomes.

Chemical, spectroscopic and structural investigation of the substrate-binding site in ascorbate peroxidase

The interaction of recombinant ascorbate peroxidase (APX) with its physiological substrate, ascorbate, has been studied by electronic and NMR spectroscopies, and by phenylhydrazine-modification experiments. The binding interaction for the cyanide-bound derivative (APX-CN) is consistent with a 1:1 stoichiometry and is characterised by an equilibrium dissociation binding constant. Kd, of 11.6 +/- 0.4 microM (pH 7.002, mu = 0.10 M, 25.0 degrees C). Individual distances between the non-exchangeable substrate protons of APX-CN and the haem iron were determined by paramagnetic-relaxation NMR measurements, and the data indicate that the ascorbate binds 0.90-1.12 nm from the haem iron. The reaction of ferric APX with the suicide substrate phenylhydrazine yields predominantly (60%) a covalent haem adduct which is modified at the C20 carbon, indicating that substrate binding and oxidation is close to the exposed C20 position of the haem, as observed for other classical peroxidases. Molecular-modelling studies, using the NNM-derived distance restraints in conjunction with the crystal structure of the enzyme [Patterson, W. R. & Poulos, T. L. (1995) Biochemistry 34, 4331-4341], are consistent with binding of the substrate close to the C20 position and a possible functional role for alanine 134 (proline in other class-III peroxidases) is implicated.

Possible involvement of catalase in the protective effect of interleukin-6 against 6-hydroxydopamine toxicity in PC12 cells

We examined the effects of various neurotrophic factors and cytokines on 6-hydroxydopamine (6-OHDA)-induced toxicity in PC12 cells. Exposure of PC12 cells to 6-OHDA resulted in a concentration- and time-dependent cell death, as evidenced by the release of lactate dehydrogenase into the culture medium. Addition of catalase, but not superoxide dismutase, to the culture medium protected PC12 cells from the 6-OHDA-induced toxicity. Interleukin (IL)-6 provided a dose-dependent protection against the 6-OHDA toxicity, as did nerve growth factor (NGF). In addition, basic fibroblast growth factor and dibutyryl cyclic AMP partially protected PC12 cells from 6-OHDA toxicity. Neither IL-1alpha, IL-2, IL-4, transforming growth factor-beta, nor leukemia inhibitory factor had any effect. The protective effect of IL-6 was attenuated by 3-amino-1,2,4-triazole, an inhibitor of catalase. These results suggest that IL-6 may protect PC12 cells against the 6-OHDA toxicity by activating free radical detoxifying mechanisms, such as catalase activity.

Horseradish peroxidase His-42 --> Ala, His-42 --> Val, and Phe-41 --> Ala mutants. Histidine catalysis and control of substrate access to the heme iron

Polyhistidine-tagged horseradish peroxidase (hHRP) and its F41A, H42A, and H42V mutants have expressed in an insect cell system. Kinetic studies show that the rates of Compound I formation and peroxidative catalysis are greatly decreased by the His-42 mutation. Furthermore, Compound II is not detected during turnover of the His-42 mutants. Compounds I and II are the two- and one-electron oxidized intermediates, respectively, of hHRP. In peroxygenative catalysis, the F41A and H42A mutants catalyze thioanisole sufoxidation 100 and 10 times faster, respectively, than hHRP. Styrene epoxidation is catalyzed by both the Phe-41 and His-42 mutants but not by wild-type hHRP. The higher peroxygenase activity of the mutants reflects increased accessibility of the ferryl species. This is indicated by the finding that, contrary to the reaction with wild-type hHRP, reaction of phenyldiazene with the F41A mutant yields a new and unidentified product, and the same reaction with the His-42 mutants yields phenyl-iron complexes. Phe-41 and His-42 thus shield the iron-centered catalytic species, and His-42 plays a key catalytic role in the formation of Compound I. The peroxygenase activities of the Phe-41 and His-42 mutants approach those of cytochrome P450.

Neuronal nitric oxide synthase. Expression in Escherichia coli, irreversible inhibition by phenyldiazene, and active site topology

A gene coding for rat neuronal nitric oxide synthase (nNOS) has been cloned into pCWori and the vector has been expressed in Escherichia coli. The expressed enzyme has been purified with a final yield of purified protein of approximately 1 mg/g of wet cells. The recombinant protein reconstituted with calmodulin and Ca2+ exhibits spectroscopic and catalytic properties identical to those reported in the literature for nNOS. Reaction of recombinant nNOS with phenyldiazene produces a phenyl-iron (Fe.Ph) complex with a maximum at 470 nm. Formation of this complex is paralleled by inactivation of the enzyme and is inhibited by arginine, the natural substrate of the enzyme. Phenyl-iron complex formation does not alter the rate of electron transfer from the flavin domain to cytochrome c. Addition of ferricyanide triggers migration of the phenyl residue from the iron to the porphyrin nitrogens. The N-phenylprotoporphyrin isomers with the phenyl on the nitrogens of pyrrole rings B, A, C, and D are formed in, respectively, approximately a 14:20:21:45 ratio. The regioisomer pattern indicates that the active site of NOS is open to some extent above all four pyrrole rings but more so above pyrrole ring D. Arylhydrazines are thus not only a new class of inhibitors of nNOS but provide useful information on the active site topology of the enzyme.

(Z)-3-(4-bromophenyl)-3-(3-pyridyl)allylamine as substrate for studies of myeloperoxidase activity

(Z)-3-(4-Bromophenyl)-3-(3-pyridyl)allylamine (CPP 200) is transformed to the corresponding chloroimine by hypochlorite ion (ClO-) formed in the presence of myeloperoxidase. A scheme for this transformation is given. The influence of various compounds on this process has been studied. Cysteamine, cysteine and 6-chloro-3-hydrazino-pyridazine inhibited the transformation of CPP 200, while some p-hydroxyphenyl derivatives increased the rate of transformation of CPP 200. The increase seen on addition of the p-hydroxyphenyl derivatives is not a chloride-dependent reaction. Various mechanisms for the inhibiting effect as well as for the activating effect on the transformation of CPP 200 are discussed.

Arylhydrazines as probes of hemoprotein structure and function

The reactions of arylhydrazines (ArNHNH2) or aryldiazenes (ArN = NH) with simple iron porphyrins or with hemoproteins that have relatively open active sites, including hemoglobin, myoglobin, cytochrome P450, chloroperoxidase, catalase, prostaglandin synthase, and indoleamine-2,3-dioxygenase yield sigma-bonded aryl-iron complexes. Denaturation of the protein complexes under aerobic, acidic conditions shifts the aryl group to the porphyrin nitrogens and produces mixtures of the four possible N-arylprotoporphyrin IX regioisomers. The regioisomers are obtained in approximately equal amounts if the iron-to-nitrogen shift occurs outside of the protein but the ratio of isomers differs if the rearrangement is controlled by the protein. Only in the case of cytochrome P450 enzymes can the shift be induced to occur without denaturation of the protein. The isomer ratios obtained when the shift occurs in the intact active site provide direct experimental information on the active site topology and dynamics. Topological information has thus been obtained for cytochromes P450 1A1, 1A2, 2B1, 2B2, 2B4, 2B10, 2B11, 2E1, 11A1, 51, 101, 102, and 108. In contrast to hemoproteins with open active sites, conventional peroxidases react with arylhydrazines to give delta-meso-aryl adducts and covalent protein adducts. Reaction with the delta-meso edge but not the heme iron provides key evidence that restricting access of substrates to the ferryl oxygen helps direct the reaction towards peroxidase rather than peroxygenase catalysis.

Reactions of prostaglandin H synthase with monosubstituted hydrazines and diazenes. Formation of iron(II)-diazene and iron(III)-sigma-alkyl or iron(III)-sigma-aryl complexes

The reaction of p-chlorophenylhydrazine with prostaglandin H synthase (PGHS) Fe(III) under aerobic conditions leads to a partial destruction of the heme and to a new complex absorbing at 436 nm. This complex is also obtained by reaction of p-chlorophenyldiazene (pClPhN = NH) with PGHS Fe(III) under anaerobic conditions and by oxidation of the PGHS Fe(II)(pClPhN = NH) diazene complex by Fe(CN)6K3. The similarity between those reactions and those of arylhydrazines and aryldiazenes with other hemoproteins such as cytochrome P450 and hemoglobin and myoglobin, as well as the similarities between the spectroscopic and chemical properties of this complex and those of the sigma-aryl complexes of other hemoproteins such as hemoglobin and myoglobin, strongly suggested a PGHS Fe(III)-pClPh structure for this complex. It was completely established after the extraction of its heme, by butan-2-one at 0 degree C under neutral or acidic conditions, which led to the sigma-aryl PGHS-Fe(III)-pClPh complex and to N-phenylprotoporphyrin IX, respectively. A mechanism is proposed for the formation of the PGHS Fe(III) pClPh complex; it includes the reduction of PGHS Fe(III) into PGHS Fe(II) with formation of the diazene pClPhN = NH. This diazene can bind to PGHS Fe(II) or be oxidized with formation of pClPh free radicals. These radicals can react with PGHS Fe(II) to form the PGHS Fe(III)-pClPh complex or with the protein, or may initiate free radical oxidations which could lead to destruction of the heme or of the protein. Other alkylhydrazines or arylhydrazines also react with PGHS Fe(III) under aerobic conditions with the formation of PGHS Fe(III)-R or aryl (Ar) complexes and heme destruction. Alkylhydrazines such as methylhydrazine, which lead to very reactive alkyl radicals, lead to very low amounts of PGHS Fe(III)-R complex and high amounts of heme destruction, whereas arylhydrazines bearing electron-withdrawing substituents such as 3,4-dichlorophenylhydrazine, which lead to stabilized aryl radicals, lead to a high amounts of PGHS Fe(III)-Ar complex and low amounts of heme destruction.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of various compounds on the degradation of hyaluronic acid by a myeloperoxidase system

Myeloperoxidase in the presence of 0.7 mM hydrogen peroxide degrades hyaluronic by a mechanism which involves iron. Degradation is enhanced in the presence of chloride ion, which is attributed to the formation of hypochlorous acid. Myeloperoxidase-dependent degradation of hyaluronic acid is inhibited by superoxide dismutase, desferrioxamine, iodide ion, bromide ion, mannitol, histidine and various antiinflammatory agents. The destructing agent is presumably the hydroxyl radical.

Phenylhydrazones as new good substrates for the dioxygenase and peroxidase reactions of prostaglandin synthase: formation of iron(III)-sigma-phenyl complexes

Phenylhydrazones of various aromatic and aliphatic aldehydes or ketones act as good substrates of the dioxygenase reaction of prostaglandin synthase (PGHS). Corresponding alpha-azo hydroperoxides are formed as intermediates with maximum initial rates of O2 consumption between 8 and 230 mol (mol of PGHS)-1 s-1 for benzophenone and hexanal phenylhydrazone, respectively. The Km values for these reactions vary from 100 to 300 microM. These alpha-azo hydroperoxides are then converted to the corresponding alpha-azo alcohols by the peroxidase reaction of PGHS. During such oxidations of phenylhydrazones by PGHS, a new complex of this hemeprotein characterized by peaks at 438 and 556 nm is formed. This complex was obtained both by direct reaction of PGHS Fe(III) with phenyldiazene and by reaction of PGHS Fe(III) with phenylhydrazine in the presence of O2. By analogy to results previously reported for hemoglobin, myoglobin, catalase, and cytochrome P450, this species should be a sigma-phenyl PGHS FeIII-Ph complex. The PGHS FeIII-Ph complex should derive from an oxidation of the intermediate alpha-azo alcohol by PGHS Fe(III), cleavage of the resulting alkoxy radical with formation of a ketone (or aldehyde) and Ph*, and combination of PGHS Fe(II) with Ph*. Such an oxidation of alpha-azo alcohols by lipoxygenase-FeIII with formation of Ph* was reported previously. The formation of Ph* and of PGHS FeIII-Ph is likely the cause of the inhibitory effects previously reported for arylhydrazones toward PGHS.

Topological analysis of the active sites of cytochromes P450IIB4 (rabbit), P450IIB10 (mouse), and P450IIB11 (dog) by in situ rearrangement of phenyl-iron complexes

The reaction of phenyldiazene with purified, phenobarbital-inducible rabbit cytochrome P450IIB4, mouse cytochrome P450IIB10, and dog cytochrome P450IIB11 yields complexes with absorbance maxima at 480 nm. Treatment of the cytochrome P450 complexes with K3Fe(CN)6 results in disappearance of the 480-nm absorption. Extraction of the prosthetic group from the proteins after these reactions yields the two isomers of N-phenylprotoporphyrin IX with the N-phenyl group on pyrrole rings A and D as the major products and the regioisomer with the N-phenyl on pyrrole ring C as a minor product. The A:C:D arylated pyrrole ring ratio is 3:2:3 for rabbit P450IIB4, 3:1:3 for mouse P450IIB10, and 4:1:2 for dog P450IIB11. Formation of the A and D regioisomers is consistent with the results obtained previously for rat isozymes IA1, IIB1, IIB2, and IIE1, but the rabbit, mouse, and dog P450IIB enzymes differ from the four rat enzymes in that a substantial amount of the isomer with the N-phenyl on pyrrole ring C is also formed. The results indicate that the region over pyrrole ring B is masked by protein residues in all the active sites and suggest that the region over pyrrole ring C is more hindered by protein residues in the rat than in the rabbit, mouse, or dog enzymes so far examined.

The dual oxygenase and peroxidase activities of porphobilinogen oxygenase and horseradish peroxidase: a study using the reaction with phenylhydrazine

Porphobilinogen oxygenase and horseradish peroxidase show dual oxygenase and peroxidase activities. By treating porphobilinogen oxygenase with phenylhydrazine in the presence of H2O2 both activities were inhibited. When horseradish peroxidase was treated in the same manner only the peroxidase activity was lost while its oxygenase activity toward porphobilinogen remained unchanged. The phenylhydrazine treatment alkylated the prosthetic heme group of porphobilinogen oxygenase and N-phenylheme as well as N-phenylprotoporphyrin IX were isolated from the treated hemoprotein. In horseradish peroxidase the modified heme was mainly 8-hydroxymethylheme. The apoproteins of the alkylated enzymes were isolated and recombined with hemin IX. The oxygenase and peroxidase activities of porphobilinogen oxygenase were entirely recovered in the reconstituted enzyme, while the reconstituted horseradish peroxidase regained 75% of its peroxidase activity.

Inactivation of lignin peroxidase by phenylhydrazine and sodium azide

Lignin peroxidase (LiP) is rapidly inactivated in a concentration-dependent manner by H2O2 and either phenylhydrazine or sodium azide. Full inactivation of isozyme 2b (H8) requires approximately 50 eq of phenylhydrazine or 80 eq of sodium azide. Anaerobic incubation of isozyme 2b with [14C]phenylhydrazine and H2O2 results in 77% loss of catalytic activity and covalent binding of 0.45 mol radiolabel/mol of enzyme. Comparable but not identical results are obtained with an isozyme mixture. A lag period is observed before the peroxidative activity can be measured when an aliquot of an incubation with sodium azide is diluted into the mixture used to assay residual catalytic activity. This lag is associated with reversible accumulation of a catalytically inert species with a Compound III-like spectrum. No meso-phenyl, iron-phenyl, or N-phenyl adducts are formed with phenylhydrazine but a low yield of what appears to be delta-meso-azidoheme is obtained with sodium azide. LiP is thus less susceptible to meso heme additions and more susceptible to oxidative heme degradation than horseradish peroxidase. The data suggest that the active of LiP resembles the closed structure of horseradish peroxidase more than it does the open structure of the globins, catalase, chloroperoxidase, or cytochrome P450.

Free radical modification of prosthetic heme groups

Hemoproteins catalyze reductive and oxidative one-electron transformations. Not infrequently, the radicals produced by these one-electron reactions add to the prosthetic heme group of the enzyme and modify or terminate its catalytic function. Reactions of the radicals with the heme group include additions to the iron atom, pyrrole nitrogens, pyrrole carbons, vinyl groups, and meso carbons. The radicals involved in these reactions derive from the oxidizing agent, the substrate, or the amino acid residues of the catalytic site. The mechanism by which the radicals are generated, their steric and electronic properties, and the extent to which they have access to the heme group determine the nature and regiospecificity of the reaction. The reaction of heme prosthetic groups with radicals is relevant to the inhibition of hemoprotein enzymes, the normal and pathological degradation of heme, and our understanding of hemoprotein function.

Fragmentation of human hemoglobin by oxidative stress produced by phenylhydrazine

Exposure of purified human hemoglobin to phenylhydrazine induces the oxidation of hemoglobin and the generation of acid soluble peptides. The extent of protein fragmentation depends on the concentration of phenylhydrazine, incubation time and temperature. The fragments, excluded by gel filtration chromatography on Sephadex G-15, are partially degraded by leucine aminopeptidase and are totally converted to amino acids by acid hydrolysis. The addition of inhibitors for serine proteinases (phenylmethylsulfonylfluoride), cysteine proteinases (leupeptin), aspartic proteinases (pepstatin A) and metalloproteinases (EDTA) does not alter the formation of acid soluble peptides, thus excluding the involvement of erythrocyte proteinases in the generation of peptides. It is suggested that oxygen and phenylhydrazine free radicals produced in the course of hemoglobin oxidation might be responsible for protein fragmentation. We also discuss a possible relationship between the fragmentation of oxidized hemoglobin and the ATP-independent proteolysis stimulated by oxidative agents.

In vivo formation of sigma-methyl- and sigma-phenyl-ferric complexes of hemoglobin and liver-cytochrome P-450 upon treatment of rats with methyl- and phenylhydrazine

Ferric sigma-phenyl complexes of hemoglobin and liver cytochrome P-450 are formed in vivo upon administration of C6H5NHNH2 to rats. Small amounts of the sigma-methyl complex of hemoglobin were also detected in vivo upon treatment of rats with CH3NHNH2. At the doses used for CH3NHNH2 (25 and 50 mg/kg) the states and levels of hemoglobin in the blood and spleen, and of cytochrome P-450 in the liver were almost unchanged. On the contrary, C6H5NHNH2 (25-100 mg/kg) led to a decrease of the HbO2 blood level (10-50%), together with an increase in the HbFe(III) level and the appearance of the HbFe(III)-C6H5 complex. The concentration of this complex reaches its maximum value (2 mM) 1 h after C6H5NHNH2 administration (20% of total hemoglobin). At the same time large amounts of HbO2, HbFe(III) and HbFe(III)-C6H5 appeared in the spleen, and remained high up to 24 h after treatment. Treatment of rats with C6H5NHNH2 (25-100 mg/kg) led to a significant decrease in the level of liver cytochrome P-450 (a 70% decrease 2 h after treatment with 100 mg/kg C6H5NHNH2). About 15% of the remaining cytochrome P-450 existed as a cyt.-P-450-Fe(III)-C6H5 complex, a new example of cytochrome P-450-Fe-metabolite complex which is stable in vivo.

Spin-trapping of methyl radical in the oxidative metabolism of 1,2-dimethylhydrazine

A carbon-centered free radical formed during oxidative metabolism of 1,2-dimethylhydrazine has been spin-trapped with alpha-(4-pyridyl-1-oxide)N-tert-butyl nitrone and 2-methyl-2-nitrosopropane. In the horseradish peroxidase/H2O2 catalyzed oxidation, the trapped species was identified as the methyl radical by the characteristic 1:3:3:1 quartet pattern of the 2-methyl-2-nitroso propane adduct. A carbon-centered radical is also formed during microsomal oxidation of 1,2-dimethylhydrazine in the presence of NADPH. However, the alpha-(4-pyridyl-1-oxide)N-tert-butyl nitrone trapped radical has not been unambiguously identified in this latter instance. These results may be of importance in regard to both carcinogenic and antitumor properties of 1,2-disubstituted hydrazine derivatives.