Iron species of hepatic microsomes from control and phenobarbital-treated rats

Antioxidant activity of metallothionein compared with reduced glutathione

The antioxidant activity of metallothionein (MT) was investigated. Metallothionein scavenged hydroxyl radicals (HO.) to protect DNA from the oxidative attack by microsomes. This scavenging activity of MT was approximately 50 times greater than reduced glutathione (GSH) on a molar basis. However, MT did not inhibit DNA damage induced by microsomes in the presence of adriamycin (ADM)-Fe3+. Presumably, MT did not access DNA at the binding site of ADM-Fe3+ by steric hindrance. The MT also inhibited microsomal lipid peroxidation induced by ADP-Fe3+ and NADPH. The inhibitory activity of MT was about 10 times greater than GSH on a molar basis. In addition, MT effectively inhibited the degradation of R-phycoerythrin fluorescence induced by 2,2'-azobis-(2-amidino-propane)-dihydrochloride, suggesting that MT scavenges peroxyl radicals. This activity of MT was approximately 100 times greater than GSH on a molar basis. Presumably, the inhibitory activity of MT against lipid peroxidation is due to its ability to scavenge lipid peroxyl radicals on the membrane surface. The antioxidant activity of MT seems to be due to MT scavenging free HO. and peroxyl radicals.

Stimulation of NADH-dependent microsomal DNA strand cleavage by rifamycin SV

Rifamycin SV is an antibiotic anti-bacterial agent used in the treatment of tuberculosis. This drug can autoxidize, especially in the presence of metals, and generate reactive oxygen species. A previous study indicated that rifamycin SV can increase NADH-dependent microsomal production of reactive oxygen species. The current study evaluated the ability of rifamycin SV to interact with iron and increase microsomal production of hydroxyl radical, as detected by conversion of supercoiled plasmid DNA into the relaxed open circular state. The plasmid used was pBluescript II KS(-), and the forms of DNA were separated by agarose-gel electrophoresis. Incubation of rat liver microsomes with plasmid plus NADH plus ferric-ATP caused DNA strand cleavage. The addition of rifamycin SV produced a time- and concentration-dependent increase in DNA-strand cleavage. No stimulation by rifamycin SV occurred in the absence of microsomes, NADH or ferric-ATP. Stimulation occurred with other ferric complexes besides ferric-ATP, e.g. ferric-histidine, ferric-citrate, ferric-EDTA, and ferric-(NH4)2SO4. Rifamycin SV did not significantly increase the high rates of DNA strand cleavage found with NADPH as the microsomal reductant. The stimulation of NADH-dependent microsomal DNA strand cleavage was completely blocked by catalase, superoxide dismutase, GSH and a variety of hydroxyl-radical-scavenging agents, but not by anti-oxidants that prevent microsomal lipid peroxidation. Redox cycling agents, such as menadione and paraquat, in contrast with rifamycin SV, stimulated the NADPH-dependent reaction; menadione and rifamycin SV were superior to paraquat in stimulating the NADH-dependent reaction. These results indicate that rifamycin SV can, in the presence of an iron catalyst, increase microsomal production of reactive oxygen species which can cause DNA-strand cleavage. In contrast with other redox cycling agents, the stimulation by rifamycin SV is more pronounced with NADH than with NADPH as the microsomal reductant. Interactions between rifamycin SV, iron and NADH generating hydroxyl-radical-like species may play a role in some of the hepatotoxic effects associated with the use of this antibacterial antibiotic.

DNA strand cleavage as a sensitive assay for the production of hydroxyl radicals by microsomes: role of cytochrome P4502E1 in the increased activity after ethanol treatment

There is increasing interest in the role of reactive oxygen radicals in the hepatotoxicity associated with ethanol consumption. Reactive oxygen intermediates interact with DNA and can cause single-strand breaks of supercoiled DNA. Experiments were carried out to evaluate the utility of this system as a sensitive assay for the detection of potent oxidants generated by rat liver microsomes isolated from pair-fed control rats and rats treated chronically with ethanol. DNA strand cleavage was assayed by monitoring the migration of the supercoiled and open circular forms in agarose. Microsomes catalysed DNA strand breakage with either NADPH or NADH as cofactors; iron was required to catalyse the reaction and various ferric complexes were effective in promoting the reaction. DNA strand cleavage was prevented by catalase, superoxide dismutase, GSH and hydroxyl-radical-scavenging agents, suggesting that a hydroxyl-radical-like species was the oxidant responsible for the breakage. This assay system proved to be much more sensitive in detecting hydroxyl radicals than are other methods, such as e.s.r. spectroscopy or oxidation of chemical scavenging agents with respect to the amount of microsomal protein and the nature and concentration of the iron catalyst required. Microsomes from ethanol-treated rats were more reactive than control microsomes in catalysing the DNA strand cleavage with either NADPH or NADH; increased catalytic activity was observed with various ferric complexes and was sensitive to the above antioxidants. Compared with preimmune IgG, anti-(cytochrome P4502E1) IgG had no effect on DNA strand cleavage by the control microsomes, but completely prevented the NADPH- and the NADH-dependent increased activity found with microsomes from the ethanol-treated rats. Inhibitors of cytochrome P4502E1, such as diethyl dithiocarbamate and tryptamine, also lowered the extent of increase of DNA strand cleavage produced by microsomes from the ethanol-treated rats. These results indicate that DNA strand cleavage is a very sensitive assay for detecting the production of hydroxyl radicals by microsomes and to demonstrate increased activity by microsomes after chronic ethanol treatment. This increased activity with NADPH and NADH is due, at least in part, to induction of cytochrome P4502E1.

Ferritin-dependent inactivation of microsomal glucose-6-phosphatase

Glucose-6-phosphatase (G6Pase) is a microsomal enzyme which is very sensitive to inactivation by lipid peroxidation. Experiments were carried out to evaluate whether ferritin, which is the major storage form of iron within cells, could catalyze inactivation of G6Pase and to determine the mechanism responsible for this effect of ferritin. Incubation of microsomes with NADPH in the absence of ferritin led to decreased activity of G6Pase. Ferritin stimulated this inactivation of G6Pase in a time- and concentration-dependent manner. Ferritin did not stimulate G6Pase inactivation when NADH replaced NADPH as the microsomal reductant. Superoxide dismutase but not catalase or DMSO prevented the ferritin-stimulated inactivation of G6Pase suggesting a role for superoxide, but not H2O2 or hydroxyl radical, in the overall mechanism. Trolox, at concentrations which prevent lipid peroxidation, also prevented the ferritin-catalyzed inactivation of G6Pase. Inhibition of G6Pase by ferritin was further enhanced in the presence of ATP but was inhibited in the presence of EDTA or desferrioxamine; ferric-ATP stimulates, whereas ferric-EDTA inhibits microsomal lipid peroxidation. The redox cycling agent paraquat increased the ability of ferritin to inactivate G6Pase by a reaction prevented by superoxide dismutase, trolox, EDTA, and desferrioxamine, but not by catalase or DMSO. Ferritin stimulated microsomal light emission, a reaction reflecting lipid peroxidation, with time and concentration dependence, and sensitivity to scavengers (trolox, superoxide dismutase), iron chelators and paraquat, identical to the inactivation of G6Pase. These results indicate that one possible toxicological consequence of ferritin-catalyzed lipid peroxidation is inhibition of microsomal enzymes such as G6Pase.

Stimulation of microsomal chemiluminescence by ferritin

The ability of ferritin to catalyze rat liver microsomal chemiluminescence was determined in the absence and presence of the redox cycling agent paraquat, and with either NADPH or NADH as reductant. Microsomal chemiluminescence was used as a index of lipid peroxidation. In the absence of added ferritin, NADPH-dependent microsomal light emission was 4-fold greater than the NADH-dependent reaction, and was not sensitive to superoxide dismutase, catalase or DMSO. Ferritin stimulated NADPH-, but not NADH-dependent chemiluminescence in a time- and concentration-dependent manner. The stimulation by ferritin was completely sensitive to superoxide dismutase, but not to catalase or DMSO, suggesting the requirement for superoxide to mobilize iron from ferritin. An iron ligand was not required for the stimulation by ferritin; the addition of certain ligands such as EDTA, DETAPAC or desferrioxamine resulted in inhibition of the stimulation by ferritin. Paraquat potentiated the effect of ferritin on microsomal chemiluminescence with NADPH as cofactor and was weakly stimulatory with NADH. The potentiation by paraquat plus ferritin was prevented by superoxide dismutase and was further elevated by ligands such as ATP. Chemiluminescence proved to be a more sensitive parameter than production of thiobarbituric acid-reactive components to evaluate the stimulation of oxygen radical production by iron released from ferritin, in the absence or in the presence of paraquat.

Stimulation of microsomal production of reactive oxygen intermediates by rifamycin SV: effect of ferric complexes and comparisons between NADPH and NADH

Rifamycins are antibacterial antibiotics which are especially useful for the treatment of tuberculosis. Reactive oxygen intermediates are produced in the presence of rifamycin SV and metals such as copper or manganese. Experiments were carried out to evaluate the interaction of rifamycin SV with rat liver microsomes to catalyze the production of reactive oxygen species. At a concentration of 1 mM, rifamycin SV increased microsomal production of superoxide with NADPH as cofactor 3-fold, and with NADH as reductant by more than 5-fold. Rifamycin SV increased rates of H2O2 production by the microsomes twofold with NADPH, and 4- to 8-fold with NADH. In the presence of various iron complexes, microsomes generated hydroxyl radical-like (.OH) species. Rifamycin SV had no effect on NADPH-dependent microsomal .OH production, irrespective of the iron chelate. A striking stimulation of .OH production was found with NADH as the reductant, ranging from 2- to 4-fold with catalyst such as ferric-EDTA and ferric-DTPA to more than 10-fold with ferric-ATP, -citrate, or -histidine. Catalase and competitive .OH scavengers lowered rates of .OH production (chemical scavenger oxidation) and prevented the stimulation by rifamycin. Superoxide dismutase had no effect on the NADH-dependent rifamycin stimulation of .OH production with ferric-EDTA or -DTPA, but was inhibitory with the other ferric complexes. In contrast to the stimulatory effects on production of O2-., H2O2, and .OH, rifamycin SV was a potent inhibitor of microsomal lipid peroxidation. These results show that rifamycin SV stimulates microsomal production of reactive oxygen intermediates, and in contrast to results with other redox cycling agents, is especially effective with NADH as the microsomal reductant. These interactions may contribute to the hepatotoxicity associated with use of rifamycin, and, since alcohol metabolism increases NADH availability, play a role in the elevated toxic actions of rifamycin plus alcohol.

The role of an endogenous nonheme iron in microsomal redox reactions

Microsomal membranes contain a nonheme iron which serves in vitro for the peroxidation of unsaturated lipids or the oxidation of several other chemicals. These redox reactions are reviewed in light of a recent identification of two or more iron-binding proteins in the microsomal milieu. Indirect evidence that the microsomal iron might serve in vivo for the synthesis of heme iron is also presented and discussed. Consistent with this, the newly identified iron proteins not only participate in redox reactions but also release their bound iron upon incubation with certain intermediates of heme synthesis.

Microsomal iron-dependent NADPH oxidation: evidence for the involvement of membrane-bound nonheme iron in NADPH oxidation by rat heart microsomes

Rat heart microsomes were found to contain nonheme iron and two lines of evidence suggested that this iron was involved in NADPH oxidation. As first evidence, pretreatment of rats with iron gluconate increased microsomal iron content and NADPH oxidation. As second evidence, treatment of microsomes with nonionic detergent Triton N-101 decreased membrane iron content and NADPH oxidation. Triton N-101-solubilized nonheme iron was nondialyzable and ammonium sulfate-precipitable, indicative of association with protein(s). This protein-bound iron per se did not oxidize NADPH but its addition to detergent-treated microsomes restored very high rates of NADPH oxidation, that were abolished by inhibiting NADPH-cytochrome P450 reductase with p-hydroxymercuribenzoate. Since heart microsomes did not contain cytochrome P450, these results suggested that stimulation of NADPH oxidation was mediated by direct electron transfer from reductase to iron. Purified rat heart ferritin and hemosiderin did not stimulate NADPH oxidation and the stimulation observed with detergent-solubilized microsomal iron was much higher than that observed with EDTA-Fe3+, a very effective electron acceptor for the reductase. This suggested that (i) microsomal iron was different from other intracellular iron-storage proteins, and (ii) microsomal iron was unusually permissive to one-electron transfer from reductase.

Iron release and erythrocyte damage in allyl alcohol intoxication in mice

Allyl alcohol administration in a toxic dose (1.5 mmol/kg) to starved mice causes the development of hemolysis in nearly 50% of the animals. Malonic dialdehyde (MDA) appears in plasma of the animals showing hemolysis. The treatment of mice with desferrioxamine after allyl alcohol intoxication completely prevents lipid peroxidation and hemolysis, suggesting the involvement of iron in the allyl alcohol-induced erythrocyte damage. Erythrocytes obtained from intoxicated mice before the development of hemolysis show, upon incubation, release of iron, lipid peroxidation and lysis. Studies carried out with reconstituted systems of erythrocyte lysates, containing ghosts and different fractions of erythrocyte cytosol and incubated in the presence of acrolein (the major metabolite of allyl alcohol), strongly suggest that iron is released from hemoglobin. This iron appears to promote lipid peroxidation which is accompanied by erythrocyte lysis. Thus, the allyl alcohol-induced hemolysis appears to be a model for iron delocalization from iron stores.

NADPH- and adriamycin-dependent microsomal release of iron and lipid peroxidation

In a previous study (Minotti, G., 1989, Arch. Biochem. Biophys. 268, 398-403) NADPH-supplemented microsomes were found to reduce adriamycin (ADR) to semiquinone free radical (ADR-.), which in turn autoxidized at the expense of oxygen to regenerate ADR and form O2-. Redox cycling of ADR was paralleled by reductive release of membrane-bound nonheme iron, as evidenced by mobilization of bathophenanthroline-chelatable Fe2+. In the present study, iron release was found to increase with concentration of ADR in a superoxide dismutase- and catalase-insensitive manner. This suggested that membrane-bound iron was reduced by ADR-. with negligible contribution by O2-. or interference by its dismutation product H2O2. Following release from microsomes, Fe2+ was reconverted to Fe3+ via two distinct mechanisms: (i) catalase-inhibitable oxidation by H2O2 and (ii) catalase-insensitive autoxidation at the expense of oxygen, which occurred upon chelation by ADR and increased with the ADR:Fe2+ molar ratio. Malondialdehyde formation, indicative of membrane lipid peroxidation, was observed when approximately 50% of Fe2+ was converted to Fe3+. This occurred in presence of catalase and low concentrations of ADR, which prevented Fe2+ oxidation and favored only partial Fe2+ autoxidation, respectively. Lipid peroxidation was inhibited by superoxide dismutase via increased formation of H2O2 from O2-. and excessive Fe2+ oxidation. Lipid peroxidation was also inhibited by high concentrations of ADR, which favored maximum Fe2+ release but also caused excessive Fe2+ autoxidation via formation of very high ADR:Fe2+ molar ratios. These results highlighted multiple and diverging effects of ADR, O2-., and H2O2 on iron release, iron (auto-)oxidation and lipid peroxidation. Stimulation of malondialdehyde formation by catalase suggested that lipid peroxidation was not promoted by reaction of Fe2+ with H2O2 and formation of hydroxyl radical. The requirement for both Fe2+ and Fe3+ was indicative of initiation by some type of Fe2+/Fe3+ complex.

tert-butyl hydroperoxide-dependent microsomal release of iron and lipid peroxidation. I. Evidence for the reductive release of nonheme, nonferritin iron

Rat liver microsomes were found to contain a small pool of nonheme, nonferritin iron. In the presence of ADP, low concentrations of tert-butyl hydroperoxide promoted the reductive release of nonheme, nonferritin iron, as evidenced by mobilization of bathophenanthroline-chelatable Fe2+. Iron release was inhibited by SKF 525-A and metyrapone, which are known to interfere with cytochrome P450-catalyzed reactions. Iron release was also inhibited by high concentrations of t-BOOH, which caused rapid and extensive destruction of cytochrome P450. These observations suggested that iron release was catalyzed by cytochrome P450. Treatment of rats with phenobarbital (PB) caused simultaneous increase of cytochrome P450 and decrease of nonheme, nonferritin iron. The effects of PB were minimized by simultaneous administration of hematin, an inhibitor of heme synthesis, indicating that the nonheme iron was utilized for the synthesis of the heme iron of inducible cytochrome P450 isozymes. Consistently, microsomes from the liver of PB-treated rats were found to release low amounts of Fe2+, unless rats had also been treated with hematin to prevent utilization of nonheme, nonferritin iron for the synthesis of heme iron.

Adriamycin-dependent release of iron from microsomal membranes

Microsomes incubated with NADPH and the cardiotoxic anticancer drug adriamycin reductively release their bound nonheme iron, which is accounted for by ferritin and an as yet uncharacterized nonferritin pool. The reaction is mediated by one-electron reduction of adriamycin to semiquinone radical and subsequent reoxidation of this radical at the expense of membrane iron to regenerate adriamycin and promote Fe2+ release. The semiquinone radical of adriamycin can also reoxidize at the expense of molecular oxygen to form superoxide. However, superoxide dismutase does not inhibit Fe2+ release, indicating either that superoxide is not involved in iron reduction or that superoxide reacts at sites which are sterically inaccessible to the enzyme. It is proposed that the reductive mobilization of membrane-bound iron may mediate the therapeutic or toxic effects of adriamycin, irrespective of the superoxide dismutase content of the target cells.

Reactions of adriamycin with microsomal iron and lipids

Iron plays a central role in oxidative injury, reportedly because it catalyzes superoxide- and hydrogen peroxide-dependent reactions yielding a powerful oxidant such as the hydroxyl radical. Iron is also thought to mediate the cardiotoxic and antitumour effects of adriamycin and related compounds. NADPH-supplemented microsomes reduce adriamycin to a semiquinone radical, which in turn re-oxidizes in the presence of oxygen to form superoxide and hence hydrogen peroxide. During this redox cycling membrane-bound nonheme iron undergoes superoxide dismutase- and catalase-insensitive reductive release. Membrane iron mobilization triggers lipid peroxidation, which is markedly enhanced by simultaneous addition of superoxide dismutase and catalase. The results indicate that: i) lipid peroxidation is mediated by the release of iron, yet the two reactions are governed by different mechanisms; and ii) oxygen radicals are not involved in or may actually inhibit adriamycin-induced lipid peroxidation. Microsomal iron delocalization and lipid peroxidation might represent oxyradical-independent mechanisms of adriamycin toxicity.

Iron chelation as a possible mechanism for aspirin-induced malondialdehyde production by mouse liver microsomes and mitochondria

To investigate the possibility that lipid peroxidation is the mechanism responsible for aspirin-induced liver damage, pure neutralized acetylsalicylic acid (ASA), 0.6-90.9 mM, was added to calcium-aggregated mouse liver microsomes followed by incubation in NADPH buffer at 37 degrees C for 60 min and subsequent measurement of malondialdehyde (MDA). MDA production at ASA concentrations from 1.2 to 4.6 mM was greater than control (P less than 0.004). Peak MDA values were observed with 4.6 mM ASA, 39.58 +/- 6.73 nmol MDA/mg protein vs. 16.16 +/- 2.85 (P less than 0.004). Higher concentrations of ASA were inhibitory compared with the value at 4.6 mM (P less than 0.001). Aspirin had similar effects on MDA production by mouse liver mitochondria. MDA production with either ASA or buffer was completely suppressed by the potent iron-chelating agents desferrioxamine and alpha,alpha' dipyridyl when these were added to the microsomal preparations. Since MDA production in this system is known to be affected by iron-chelating agents (enhanced at low concentration, inhibited at higher concentration), the iron-chelating properties of ASA were investigated. Conductivity titration curves of Fe(OH)3 added to water or ASA suggested that the ASA was complexing with iron. The presence of an iron-ASA complex was established by high pressure liquid chromatographic analysis of the solution from this study. We conclude that aspirin enhances MDA production by hepatic microsomes and mitochondria via an aspirin-iron chelate and that this represents at least one mechanism by which aspirin may produce liver damage.

Rat liver microsomal NADPH-dependent release of iron from ferritin and lipid peroxidation

Microsomes prepared by the usual method of differential centrifugation were found to contain ferritin, superoxide dismutase (SOD), and catalase which could be removed by chromatography on Sepharose CL-2B. Addition of purified rat liver ferritin to chromatographed microsomes resulted in a significant stimulation of NADPH-dependent lipid peroxidation which was inhibited by exogenously added SOD. Iron release from ferritin by these microsomes was also inhibited by SOD. Ferritin did not promote NADPH-dependent microsomal lipid peroxidation when added to microsomes isolated in the usual manner, presumably due to the endogenous SOD present in the microsomes. Accordingly, only very low rates of iron release from ferritin were observed with these microsomes. Paraquat (PQ), which generates superoxide O2-. via redox cycling, greatly stimulated iron release from ferritin and lipid peroxidation in chromatographed microsomes. Paraquat had no effect on iron release from ferritin or lipid peroxidation in microsomes. which were not chromatographed unless they were first treated with CN- to inhibit endogenous SOD. These studies indicate that the majority of microsomal iron is contained within ferritin and that following release by O2-. this iron serves to promote the peroxidation of microsomal lipids.

Effect of radio-detoxified endotoxin on the liver microsomal drug metabolizing enzyme system in rats

E. coli endotoxin (LPS) depresses the hepatic microsomal mono-oxygenase activity. Radio-detoxified LPS (TOLERIN: 60 Co irradiated endotoxin preparation) decreases this biotransforming activity to a smaller extent. Phenobarbital, an inducer of this mono-oxygenase system, failed to induce in LPS-treated animals. In radio-detoxified LPS-treated rats, phenobarbital induced the mono-oxygenase and almost fully restored the biotransformation.

The effect of muscular exercise on the microsomal enzyme system of the rat liver

Regular muscular work--in the form of swimming or running--was studied for its effect on hexobarbital sleeping time and on some components of the hepatic monooxygenase system. Sleeping times became shorter after both kinds of exercise. Thus, the causative factor in this type of enzyme induction is regular physical exercise. Antipyrine elimination was also faster in rats exercised by swimming. Regular exertion elicited a rise in the concentration or activity of some components of the hepatic microsomal monooxygenase system (cytochrome P450, cytochrome b5 and NADPH cytochrome reducatase EC 1.6.2.3.). The inducing effect of muscular work appears to be most similar to the phenobarbital type of induction. In the development of this exercise-induced rise of enzymatic activity some as yet unidentified humoral and metabolic changes associated with daily physical training are thought to play a role.

Electron paramagnetic resonance studies of cytochrome P-450 in plant microsomes

The technique of electron paramagnetic resonnance spectrometry has been applied to the study of plant microsomal electron-transport components. Only tulip-bulb microsomes were found to give strong enough signals to allow detailed study. At 77 K in the oxidised state, signals were observed at g values of 2.40, 2.25 and 1.93, characteristic of cytochrome P-450 in the low-spin state, and also at g = 4.27, attributable to ferric iron in a rhombic environment. The signals at g = 2.40, 2.25 and 1.93 disappeared upon reduction with sodium dithionite. At 10 K in the oxidised state, signals at g = 8.3 and 3.3 appeared, and these were attributed to high-spin cytochrome P-450. At this temperature a further signal at g = 6, due to cytochrome P-420, was seen in aged tulip-bulb microsomes. Redox titration of both high-spin and low-spin cytochrome P-450 gave the same apparent midpoint potential of -315 +/- mV at pH 6.8 and 25 degrees C. The significance of this value is discussed. Addition of "type I" or "type II" ligands to oxidized cytochrome P-450 caused an increase and a decrease, respectively, in the ratio of the high-spin to the low-spin form. A second effect of aniline, a type II ligand of cytochrome P-450, was to remove the g = 6 signal, suggesting that it also interacts with cytochrome P-420. No iron-sulphur proteins similar to those found in some other cytochrome P-450 electron-transport chains could be detected in any of the microsomes analysed.