Hepatic microsomal dealkylations. Inhibition by a tyrosine-copper (II) complex provided with superoxide dismutase activity

Looking Back at the Early Stages of Redox Biology

The beginnings of redox biology are recalled with special emphasis on formation, metabolism and function of reactive oxygen and nitrogen species in mammalian systems. The review covers the early history of heme peroxidases and the metabolism of hydrogen peroxide, the discovery of selenium as integral part of glutathione peroxidases, which expanded the scope of the field to other hydroperoxides including lipid hydroperoxides, the discovery of superoxide dismutases and superoxide radicals in biological systems and their role in host defense, tissue damage, metabolic regulation and signaling, the identification of the endothelial-derived relaxing factor as the nitrogen monoxide radical (more commonly named nitric oxide) and its physiological and pathological implications. The article highlights the perception of hydrogen peroxide and other hydroperoxides as signaling molecules, which marks the beginning of the flourishing fields of redox regulation and redox signaling. Final comments describe the development of the redox language. In the 18th and 19th century, it was highly individualized and hard to translate into modern terminology. In the 20th century, the redox language co-developed with the chemical terminology and became clearer. More recently, the introduction and inflationary use of poorly defined terms has unfortunately impaired the understanding of redox events in biological systems.

Changing paradigms in thiology from antioxidant defense toward redox regulation

The history of free radical biochemistry is briefly reviewed in respect to major trend shifts from the focus on radiation damage toward enzymology of radical production and removal and ultimately the role of radicals, hydroperoxides, and related fast reacting compounds in metabolic regulation. Selected aspects of the chemistry of radicals and hydroperoxides, the enzymology of peroxidases, and the biochemistry of adaptive responses and regulatory phenomena are compiled and discussed under the perspective of how the fragments of knowledge can be merged to biologically meaningful concepts of regulation. It is concluded that (i) not radicals but H(2)O(2), hydroperoxides, and peroxynitrite are the best candidates for oxidant signals, (ii) peroxidases of the GPx and Prx family or functionally equivalent proteins have the chance to specifically sense hydroperoxides and to transduce the oxidant signal, (iii) redox signaling proceeds via reactions known from thiol peroxidase and redoxin chemistry, (iv) proximal targets are proteins that are modified at SH groups, and (v) redoxins are documented signal transducers but also used as terminators. The importance of kinetics for forward signaling and for sensitivity modulation by competition is emphasized and ways to restore resting conditions are discussed. Research needs to validate emerging concepts are outlined.

Differential effect of copper (II) on the cytochrome P450 enzymes and NADPH-cytochrome P450 reductase: inhibition of cytochrome P450-catalyzed reactions by copper (II) ion

Inhibitory effects of Cu(2+) on the cytochrome P450 (P450)-catalyzed reactions of liver microsomes and reconstituted systems containing purified P450 and NADPH-P450 reductase (NPR) were seen. However, Zn(2+), Mg(2+), Mn(2+), Ca(2+), and Co(2+) had no apparent effects on the activities of microsomal P450s. Cu(2+) inhibited the reactions catalyzed by purified P450s 1A2 and 3A4 with IC(50) values of 5.7 and 8.4 microM, respectively. Cu(2+) also inhibited reduction of cytochrome c by NPR (IC(50) value of 5.8 microM). Copper caused a decrease in semiquinone levels of NPR, although it did not disturb the rate of formation of semiquinone. P450 reactions supported by an oxygen surrogate, tert-butyl hydroperoxide, instead of NPR and NADPH, were inhibited by the presence of Cu(2+). The results indicate that Cu(2+) inhibits the P450-catalyzed reactions by affecting both P450s and NPR. It was also found that the inhibition of catalytic activities of P450s by Cu(2+) involves overall conformational changes of P450s and NPR, investigated by CD and intrinsic fluorescence spectroscopy. These results suggest that the inhibitory effect of Cu(2+) on the P450-catalyzed reactions may come from the inability of an efficient electron transfer from NPR to P450 and also the dysfunctional conformation of NPR and P450.

Copper neurotoxicity is dependent on dopamine-mediated copper uptake and one-electron reduction of aminochrome in a rat substantia nigra neuronal cell line

The mechanism of copper (Cu) neurotoxicity was studied in the RCSN-3 neuronal dopaminergic cell line, derived from substantia nigra of an adult rat. The formation of a Cu-dopamine complex was accompanied by oxidation of dopamine to aminochrome. We found that the Cu-dopamine complex mediates the uptake of (64)CuSO(4) into the Raúl Caviedes substantia nigra-clone 3 (RCSN3) cells, and it is inhibited by the addition of excess dopamine (2 m M) (63%, p < 0.001) and nomifensine (2 microM) (77%, p < 0.001). Copper sulfate (1 m M) alone was not toxic to RCSN-3 cells, but was when combined with dopamine or with dicoumarol (95% toxicity; p < 0.001) which inhibits DPNH and TPNH (DT)-diaphorase. Electron spin resonance (ESR) spectrum of the 5,5-dimethylpyrroline-N-oxide (DMPO) spin trap adducts showed the presence of a C-centered radical when incubating cells with dopamine, CuSO(4) and dicoumarol. A decrease in the expression of CuZn-superoxide dismutase and glutathione peroxidase mRNA was observed when RCSN-3 cells were treated with CuSO(4), dopamine, or CuSO(4) and dopamine. However, the mRNA expression of glutathione peroxidase remained at control levels when the cells were treated with CuSO(4), dopamine and dicoumarol. The regulation of catalase was different since all the treatments with CuSO(4) increased the expression of catalase mRNA. Our results suggest that copper neurotoxicity is dependent on: (i) the formation of Cu-dopamine complexes with concomitant dopamine oxidation to aminochrome; (ii) dopamine-dependent Cu uptake; and (iii) one-electron reduction of aminochrome.

Down-regulation of NADPH-diaphorase (nitric oxide synthase) may account for the pharmacological activities of Cu(II)2 (3,5-diisopropylsalicylate)4

Purposes of this work were to develop an enzyme system as an in vitro model of the NADPH-dependent component of nitric oxide synthase (NOS) and examine the plausible down-regulation of this system and brain NOS by copper (II)2(3,5-diisopropylsalicylate)4[Cu(II)2(3,5-DIPS)4] as a mechanism accounting for its analgesic, anticonvulsant, and other pharmacological activities. Porcine heart diaphorase (PHD) was found to oxidize 114 microM NADPH with the corresponding reduction of an equivalent amount of 2,6-dichlorophenolindophenol (DCPIP). Addition of Cu(II)2(3,5-DIPS)4 to the reaction mixture decreased the reduction of DCPIP without substantially affecting the oxidation of NADPH. The IC50 for Cu(II)2(3,5-DIPS)4 in inhibiting the reduction of DCPIP was 1.5 microM. Mechanistically, this inhibition of DCPIP reduction was found to be due to the ability of Cu(II)2(3,5-DIPS)4 to serve as a catalytic electron acceptor for reduced PHD, which was enhanced by the presence of a large concentration of DCPIP and inhibited by a large concentration of NADPH. Oxidation of NADPH by PHD in the absence of DCPIP was linearly related to the concentration of Cu(II)2(3,5-DIPS)4 through the concentration range of 5-25 microM Cu(II)2(3,5-DIPS)4 with 50% recovery of NADPH oxidation by PHD at a concentration of 16 microM Cu(II)2(3,5-DIPS)4. Whole rat brain tissue sections incubated in medium containing an NADPH-generating system and nitroblue tetrazolium chloride (NBT) were less intensely stained when Cu(II)2(3,5-DIPS)4 was added to the medium. It is concluded that Cu(II)2(3,5-DIPS)4 serves as an electron acceptor in down-regulating PHD reduction of DCPIP and in down-regulating NOS in brain tissue sections. A decrease in NO synthesis in animal models of seizure, pain, and other disease states with Cu(II)2(3,5-DIPS)4 may account for the anticonvulsant, analgesic, and other pharmacological activities of this complex.

Hydrogen peroxide-mediated inactivation of microsomal cytochrome P450 during monooxygenase reactions

Cytochrome P450 can undergo inactivation following monooxygenase reactions in liver microsomes of untreated, phenobarbital and 3-methylcholanthrene-treated rats and rabbits. The acceleration of cytochrome P450 loss in the presence of catalase inhibitors (sodium azide, hydroxylamine) indicates that hydrogen peroxide is involved in hemoprotein degradation. It was revealed that cytochrome P450 is inactivated mainly by H2O2 formed through peroxy complex breakdown, whereas H2O2 formed via the dismutation of superoxide anions produces a slight inactivating effect. The hydrogen peroxide added outside or formed by a glucose-glucose oxidase system has less of an inactivating effect than H2O2 produced within the cytochrome P450 active center. Self-inactivation of cytochrome P450 during oxygenase reactions is highly specific. Other components of the monooxygenase system, such as cytochrome b5, NADH- and NADPH-specific flavoproteins, undergo no inactivation. The alterations in phospholipid content and in the rate of lipid peroxidation were not observed as well. The inactivation of cytochrome P450 by H2O2 is the result of heme loss or destruction without cytochrome P420 formation. Such a mechanism operates with different substrates and cytochrome P450 species catalyzing the partially coupled monooxygenase reactions.

Isoelectric focusing of cytochrome P450: isolation of six phenobarbital-inducible rat liver microsomal isoenzymes

A procedure for the isolation of native proteins from membranes by isoelectric focusing is described. It was used to resolve into six components the major fraction of cytochrome P450, obtained from liver microsomes of phenobarbital-treated rats, after chromatography on DE-52 cellulose. When eluted from the gel, these proteins are in a native form as shown by (a) the light absorption spectra of the Soret region of their reduced carbonyl derivatives, all characterized by maxima around 450 nm, and (b) their enzymatic activities toward three different substrates. Characterization by a monoclonal antibody and partial sequence analysis of tryptic peptides reveal that three of the IEF-purified proteins have P450IIB1 character, whereas the other three are related to P450IIB2.

Mechanism of monoclonal antibody inhibition/stimulation of reactions catalyzed by cytochrome P450IIB1

We describe two monoclonal antibodies (MAbs) against rat cytochrome P450IIB1 and investigate the mechanisms by which they influence P450IIB1-mediated catalysis. MAb ce9 partially inhibits the activities toward p-nitroanisole, 7-ethoxycoumarin, and benzphetamine as well as NADPH oxidation. These findings can be explained by the observation that ce9 cross-links P450 to form large aggregates resulting in the inhibition of the functional interaction with NADPH cytochrome P450 reductase. Binding of ce9 to P450IIB1 does not affect the spin state of the P450 heme, as revealed by comparing the magnetic circular dichroism (MCD) spectra of free and antibody-bound P450IIB1. On the other hand, the second antibody tested, MAb 14E10, induces a remarkable low to high spin transition upon binding to P450IIB1, as shown by MCD difference spectroscopy. This MAb stimulates activities toward p-nitroanisole and 7-ethoxycoumarin without affecting the rate of NADPH oxidation. This observation indicates that MAb 14E10 may increase the efficiency of electron utilization by P450IIB1. Benzphetamine metabolism remains unchanged in the presence of MAb 14E10.

Three models of free radical-induced cell injury

Three models of free radical-induced cell injury are presented in this review. Each model is described by the mechanism of action of few prototype toxic molecules. Carbon tetrachloride and monobromotrichloromethane were selected as model molecules for alkylating agents that do not induce GSH depletion. Bromobenzene and allyl alcohol were selected as prototypes of GSH depleting agents. Paraquat and menadione were presented as prototypes of redox cycling compounds. All these groups of toxins are converted, during their intracellular metabolism, to active species which can be radical species or electrophilic intermediates. In most cases the activation is catalyzed by the microsomal mixed function oxidase system, while in other cases (e.g. allyl alcohol) cytosolic enzymes are responsible for the activation. Radical species can bind covalently to cellular macromolecules and can promote lipid peroxidation in cellular membranes. Of course both phenomena produce cell damage as in the case of CCl4 or BrCCl3 intoxication. However, the covalent binding is likely to produce damage at the molecular site where it occurs; lipid peroxidation, on the other hand, besides causing loss of membrane structure, also gives rise to toxic products such as 4-hydroxyalkenals and other aldehydes which in principle can move from the site of origin and produce effects at distant sites. Electrophilic intermediates readily reacts with cellular nucleophiles, primarily with GSH. The result is a severe GSH depletion as in the case of bromobenzene or allyl alcohol intoxication. When the depletion reaches some threshold values lipid peroxidation develops abruptly and in an extensive way. This event is accompanied by cellular death. The reason for which lipid peroxidation develops in a cell severely depleted of GSH remains to be clarified. Probably the loss of the defense systems against a constitutive oxidative stress is not compatible with cellular life. Some free radicals generated by one-electron reduction can react with oxygen to give superoxide anions which can be converted to other more dangerous reactive oxygen species. This is the case of paraquat and menadione. Damage to cellular macromolecules is due to the direct action of these oxygen radicals and, at least in the menadione-induced cytotoxicity, lipid peroxidation is not involved. All these initial events affect the protein sulfhydryl groups in the membranes. Since some protein thiols are essential components of the molecular arrangement responsible for the Ca2+ transport across cellular membranes, loss of such thiols can affect the calcium sequestration activity of subcellular compartments, that is the capacity of mitochondria and microsomes to regulate the cytosolic calcium level.(ABSTRACT TRUNCATED AT 400 WORDS)

Glutathione depleting agents and lipid peroxidation

The mechanisms by which glutathione (GSH) depleting agents produce cellular injury, particularly liver cell injury have been reviewed. Among the model molecules most thoroughly investigated are bromobenzene and acetaminophen. The metabolism of these compounds leads to the formation of electrophilic reactants that easily conjugate with GSH. After substantial depletion of GSH, covalent binding of reactive metabolites to cellular macromolecules occurs. When the hepatic GSH depletion reaches a threshold level, lipid peroxidation develops and severe cellular damage is produced. According to experimental evidence, the cell death seems to be more strictly related to lipid peroxidation rather than to covalent binding. Loss of protein sulfhydryl groups may be an important factor in the disturbance of calcium homeostasis which, according to several authors, leads to irreversible cell injury. In the bromobenzene-induced liver injury loss of protein thiols as well as impairment of mitochondrial and microsomal Ca2+ sequestration activities are related to lipid peroxidation. However, some redox active compounds such as menadione and t-butylhydroperoxide produce direct oxidation of protein thiols.

The C-C bond cleavage of a lignin model compound, 1,2-diarylpropane-1,3-diol, with a heme-enzyme model catalyst tetraphenylporphyrinatoiron(III)chloride in the presence of tert-butylhydroperoxide

The catalytic C-C bond cleavage of a lignin model compound was investigated by use of tetraphenylporphyrinatoiron(III)chloride as a model for enzymic degradation of lignin. The C-C bond of the lignin model compound 1,2-bis(4-ethoxy-3-methoxyphenyl) propane-1,3-diol was oxidatively cleaved by catalysis of iron-porphyrins in the presence of tert-butylhydroperoxide or iodosylbenzene at a room temperature. The products formed after complete oxidation of the substrate were identified as 4-O-ethylvanillin, alpha-hydroxy-4-ethoxy-3-methoxyacetophenone, 4-O-ethylvanillic acid, 4-ethoxy-3-methoxyphenylglycol, 4-ethoxy-3-methoxy-alpha-(4-ethoxy-3-methoxyphenyl)-beta-hydroxypropi ophenone and formaldehyde.

The toxicology of molecular oxygen

Molecular oxygen, itself not very reactive, can be converted by photosensitization to electronically excited singlet states, and by partial reduction to the superoxide and hydroxyl free radicals and to hydrogen peroxide. The very considerable toxicity of oxygen, which is due primarily to the properties of these derivatives, is ordinarily overlooked because aerobes have evolved an elaborate system of defenses which is reasonably adequate under ambient conditions. This toxicity becomes all too apparent when these defenses are overwhelmed at elevated pO2 or through the action of compounds which increase the conversion of oxygen to its more reactive derivatives. We will here consider the threat posed by oxygen and the defenses which make aerobic life possible.

The influence of cholesterol incorporation and removal on lipid-bilayer viscosity and electron transfer in rat-liver microsomes

The incorporation and removal of cholesterol from rat liver microsomes was used as a methodical approach to investigate the molecular organization of microsomal redox-chains. It was shown that the incorporation of cholesterol in microsomes increases and removal of cholesterol decreases lipid bilayer viscosity as indicated from the rate of fluorescent probe-pyrene eximerisation in cholesterol-enriched and cholesterol-depleted microsomes. The increase of membrane viscosity slows down the initial rates and decreases the rate constants of cytochrome b5 reduction by NAD(P)H, whereas the decrease of membrane viscosity enhances the initial rates and increases the rate constants of these reactions. The rates of cytochrome P450 reduction by reduced pyridine nucleotides do not depend on the viscosity of lipid bilayer. The incorporation and removal of cholesterol from microsomes was not followed by any essential changes in the rates of dimethylaniline N-demethylation, aniline p-hydroxylation, p-nitroanisole O-demethylation, oxygen consumption, oxidation of NADH and NADPH. Thus the reduction of cytochrome b5 by NADH and NADPH is the diffusion-dependent reaction in the redox-chains of microsomes only.

Microsomal and photochemical oxidation and reduction of 1-piperidinoanthraquinone

In microsomes of control Wistar rats, NADPH-dependent reduction of 1-piperidinoanthraquinone (1-PA) to the corresponding hydroquinone, in the absence of oxygen, has been observed. Two facts ((i) inhibition of the formation of 1-piperidinoanthrahydroquinone (1-PAH) by metyrapone and antibodies to cytochrome P-450, and (ii) increase in the rate of 1-PAH formation upon induction of rats by phenobarbital) indicate that cytochrome P-450 participates in the reduction of 1-PA. Since 1-PA is a substrate of cytochrome P-450 and is oxidized in microsomes to (N-anthraquinonyl-1)-delta-aminovaleric acid (AAV), model experiments have been conducted to examine whether the reduced forms of 1-PA are involved in its oxidation. During photochemical generation of 1-PAH and its subsequent oxidation (N-anthraquinonyl-1)-delta-aminovaleric aldehyde (AAVal) is formed. However, this product is formed without participation of activated form of the substrate and oxygen. AAVal is a substrate in photochemical systems, apparently, is a precursor of AAV in microsomal oxidation of 1-PA. AAVal is substrate of cytochrome P-450 (the Type 1 of binding) and is oxidized quantitatively in microsomal systems to yield AAV. The date obtained enable us to propose a possible mechanism of enzyme oxidation of 1-PA.

Structure of Cu2(indomethacin)4 and the reaction with superoxide in aprotic systems

The copper complex of indomethacin (1-(p-chlorobenzoyl)-5-methoxy-2-methyl-indole acetate), a common anti-inflammatory drug, was prepared and characterized. Crystal structure determination revealed the dimeric form of the 1 : 2 complex, namely Cu2(indomethacin)4 x L2, in the unit cell. Surprisingly, the copper-copper distance (263 pm) was very close to metallic copper (256 pm). The two coordination sites in the copper-copper axis can be readily replaced by superoxide. An intriguing similarity to Cu2(acetate)4 was seen. Due to the lipophilic nature of the indomethacin ligand, this copper complex reacted with superoxide in aprotic solvents. the superoxide dismutating activity was successfully demonstrated in Me2SO/water and acetonitrile/water mixtures using the nitro-blue tetrazolium assay and pulse radiolysis. The second-order rate constant of 6 x 10(9) M-1 x s-1 in strictly aqueous systems dropped only slightly to 1.1 x 10(9) M-1 x s-1 when aprotic solvents were used. This is the fastest rate constant ever observed for a copper-dependent dismutation of superoxide. The KO2-induced lipid peroxidation in both erythrocytes and liver microsomes was suppressed by 70% in the presence of 1 x 10(-10) mol x ml-1 of Cu2(indomethacin)4. The inhibitory action dropped to 25% when Cu2Zn2superoxide dismutase was employed. The formation of copper x indomethacin in rat serum after administration of indomethacin was shown in vitro and vivo.

Inhibition of the microsomal stearoyl coenyme A desaturation by divalent copper and its chelates

Divalent copper and copper complexes of tyrosine, histidine and lysine inhibited at low concentrations the stearoyl-CoA desaturation reaction in both chicken liver microsomes and in a purified system consisting of chicken liver delta 9 terminal desaturase, cytochrome b5, ascorbate and liposome. Although the copper chelates lowered the steady-state level of ferrocytochrome b5 by 20%, and partially inhibited the NADH-ferricyanide reductase activity, the availability of the ferrocytochrome b5 during the time course of desaturation was not affected, indicating that the site of inhibition of desaturation was at the terminal step, i.e., on the delta 9 terminal desaturase. The presence of chalates during catalysis was essential for the observed inhibition. Based on the observation that O2 is involved in the desaturation and that there is an initial electron reduction of desaturase iron, it is plausible that the copper chelates are inhibiting by acting as superoxide scavengers.

The induction of benzo[a]pyrene-3-mono-oxygenase by singlet oxygen in liver cell culture is mediated by oxidation products of histidine

The photochemical generation of excited states of oxygen by the mild illumination of culture medium containing 15 microM riboflavin results in a typical induction of benzo[a]pyrene-3-mono-oxygenase in cell lines derived from liver. However, the induction of the mono-oxygenase is not due to an excited state of oxygen directly activating the inducing mechanism inside the cell but is due to the oxidation of a component of the culture medium forming a stable inducer. The present work unequivocably shows that the component oxidised is histidine. The mild illumination of culture medium containing riboflavin therefore converts a physiological component of the medium which is not normally an inducer of the mono-oxygenase into a compound which is as effective an inducer as the classical inducer. The finding that singlet oxygen will oxidise a cell constituent into a powerful inducer is compatible with the hypothesis that excited states of oxygen and their oxidation products may play a central role in the induction of cytochrome P-450 and associated enzyme activities by many chemically unrelated inducers.

Functional aspects of the superoxide dismutative action of Cu-penicillamine

The superoxide dismutative action of Cu-penicillamine was examined by pulse radiolysis. The second order rate constand of the reaction wpith superoxide was 0.4 +/- o.2.10(9) M-1.s-1, comparable to the action of Fe and Mn-superoxide dismutases. No marked pH-dependence was seen. Neither ethylene diamine tetraacetic acid nor cyanide affected the catalytic action of Cu-penicillamine. The cyanide resistant reactivity as well as further X-ray photoelectron spectrometric measurements supported the suggestion of a Cu(I) stabilized sulphur radical being the active species involved in the catalysis of superoxide dismutation.

Involvement of superoxide in the catalytic cycle of diamine oxidase

The reaction of pig kidney diamine oxidase (amine:oxygen oxidoreductase (deaminating) (pyridoxal-containing), EC 1.4.3.6) could be significantly inhibited by superoxide dismutase active copper chelates but not by native 2Cu,2Zn-superoxide dimutase (cuprein). The ligands alone as well as Cd2+, a heavy metal of similar toxicity to Cu2+, showed no inhibition whatsoever. This indicates that .O-2 participates in the catalytic cycle and is produced at a site scarcely accessible to such a large molecule as cuprein. A mechanism for the second, aerobic step of the diamine oxidase reaction is suggested.