Antioxidant-dependent inhibition of diquat-induced toxicity in vivo

Effect of haemoperfusion on plasma paraquat concentration in vitro and in vivo

This study was to observe the paraquat (PQ) reduction rate after haemoperfusion (HP) on the groups of a relatively large number: 50 survivors out of 105 patients with acute PQ poisoning. We started off by measuring the clearance of haemodialysis (HD) and HP for the PQ in vitro. At the blood flow of 250 mL/min, the PQ clearance was greater in HP than in HD during the first 90 minutes: 215 versus 175 mL/min at 30 minutes, 213 versus 201 mL/min at 60 minutes, and 199 versus 179 mL/min at 90 minutes. The clearance in HP decreased rapidly after two hours. By the end of the dialysis, however, the final concentration in container decreased to 5.7 mg/mL in HD and 1.5 mg/mL in HP, which implied that although HD was cleared more effectively during the later stages, the overall elimination was greater in HP. Following this preliminary investigation, we performed HP on all the patients in order to assess the extracorporeal elimination.

One hundred and five patients who had swallowed one to three mouthfuls of PQ (24.5% w/v) solution were subjected to the in vivo study. The reduction rate of PQ was checked out by measuring the PQ concentration in plasma before and after four hours of HP. Seeing the reduction rate was significantly higher in the survivors group than in the nonsurvivors group (80.39 ± 19.9 versus 67.29 ± 19.2%, P <0.01), we concluded that adequate HP appears to be an indispensable treatment for patients with acute PQ poisoning.

Therapeutic effects of smecta or smectite powder on rats with paraquat toxication

BACKGROUND:

The plasma concentration of paraquat is closely related to the prognosis of patients with paraquat toxication, and the most common cause of death from paraquat poisoning is multiple organ failure (MOF). This study aimed to evaluate therapeutic effect of smecta on the plasma concentrations of paraquat and multi-organ injury induced by paraquat intoxication in rats.

METHODS:

A total of 76 healthy adult SD rats were randomly divided into group A (control group, n=6), group B (poisoned group, n=30) and group C (smecta-treated group, n=30). Rats in groups B and C were treated intragastrically with PQ at 50 mg/kg, and rats in group A was treated intragastrically with saline (1 mL). Rats in group C were given intragastrically smecta at 400 mg/kg 10 minutes after administration of PQ, while rats in other two groups were treated intragastrically with 1 mL saline at the same time. Live rats in groups B and C were sacrificed at 2, 6, 24, 48, 72 hours after administration of PQ for the determination of paraquat plasma concentrations and for HE staining of the lung, stomach and jejunum. The rats were executed at the end of trial by the same way in group A.

RESULTS:

The plasma concentration of paraquat (ng/mL) ranged from 440.314±49.776 to 4320.6150±413.947. Distinctive pathological changes were seen in the lung, stomach and jejunum in group B. Lung injuries deteriorated gradually, edema, leukocyte infiltration, pneumorrhagia, incrassated septa and lung consolidation were observed. Abruption of mucosa, hyperemic gastric mucosa and leukocyte infiltration were obvious in the stomach. The hemorrhage of jejunum mucosa, the abruption of villus, the gland damage with the addition of inflammatory cell infiltration were found. Compared to group B, the plasma concentration of paraquat reduced (P<0.01) and the pathological changes mentioned above were obviously alleviated in group C (P<0.05, P<0.01).

CONCLUSION:

Smecta reduced the plasma concentration of paraquat and alleviated pathologic injury of rats with PQ poisoning.

Effect of diquat-induced oxidative stress on iron metabolism in male Fischer-344 rats

Diquat toxicity causes iron-mediated oxidative stress; however, it remains unclear how diquat affects iron metabolism. Here, we examined the effect of diquat-induced oxidative stress on iron metabolism in male Fischer-344 rats, with particular focus on gene expression. Hepatic nonheme iron content was unchanged until 20 h after diquat treatment. Hepatic free iron levels increased markedly in the early stages following treatment and remained elevated for at least 6 h, resulting in severe hepatotoxicity, until returning to control levels at 20 h. The level of hepatic ferritin, especially the H-subunit, increased 20 h after diquat treatment due to elevated hepatic ferritin-H mRNA expression. These results indicate that early elevated levels of free iron in the liver of diquat-treated rats cause hepatotoxicity, and that this free iron is subsequently sequestered by ferritin synthesized under conditions of oxidative stress, thus limiting the pro-oxidant challenge of iron. The plasma iron concentration decreased at 6 and 20 h after diquat treatment, whereas the level of plasma interleukin-6 increased markedly at 3 h and remained high until 20 h. In the liver of diquat-treated rats, expression of hepcidin mRNA was markedly upregulated at 3 and 6 h, whereas ferroportin mRNA expression was downregulated slightly at 20 h. Transferrin receptor 1 mRNA expression was significantly upregulated at 3, 6, and 20 h. These results indicate that inhibition of iron release from iron-storage tissues, through stimulation of the interleukin-6-hepcidin-ferroportin axis, and enhanced iron uptake into hepatocytes, mediated by transferrin receptor 1, cause hypoferremia.

Paraquat intoxication in subjects who attempt suicide: why they chose paraquat

BACKGROUND/AIMS

Paraquat (PQ) has been used in suicide attempts; an estimated 2,000 toxic ingestions occur annually, with 60-70% mortality. We sought to determine why PQ is such a common agent for suicide attempts in Korea.

METHODS

We analyzed 250 cases (143 males, 107 females) of attempted suicide by PQ ingestion from January to December 2007. The procurement of the PQ was divided into two categories: purchased and preexisting.

RESULTS

Men were more likely to have purchased PQ than women (66% vs. 22%, p=0.042). Additionally, men were more likely to be unmarried (n=34, 23.9% vs. n=10, 9.3%) or divorced or separated (n=16, 11.3% vs. n=5, 4.6%) than the women (p<0.001). The group who intentionally selected PQ (38.4%) consisted of 96 cases (54 males, 42 females) and the group who did not intentionally select PQ (61.6%) included 154 cases (89 males, 65 females). The incidence of PQ purchase was higher in the intentional selection PQ group (46.9% vs. 18.2%, p<0.01).

CONCLUSIONS

Only 38% of patients who attempted suicide with PQ intentionally selected PQ. Thus, greater control of PQ availability is needed, especially in patients at risk.

The area of ground glass opacities of the lungs as a predictive factor in acute paraquat intoxication

Even though plasma paraquat (PQ) levels have known to be an informative predictor, many patients succumb at low PQ levels in acute PQ intoxication. This study was designed to see whether the high resolution computerized tomography (HRCT) of the lungs would be a predictive measure in acute PQ intoxication. HRCT of the lungs was obtained from 119 patients with acute PQ intoxication on 7 days after PQ ingestion. The areas with ground glass opacities (GGOs) were evaluated at five levels with the area measurement tool of the picture archiving and communication systems. Among 119 patients, 102 survived and 17 died. The plasma PQ levels were significantly higher in the non-survivors than in the survivors (2.6+/-4.0 microg/mL vs. 0.2+/-0.4 microg/mL, P=0.02). The area with GGOs was 2.0+/-6.4% in the survivors and 73.0+/- 29.9% in the non-survivors (P<0.001). No patients survived when the area with GGOs was more than 40% but all of the patients survived when the area affected by GGOs was less than 20%. In conclusion, the area of GGOs is a useful predictor of survival in acute PQ intoxication, especially in patients with low plasma PQ levels.

Protective effects of wheat bran against diquat toxicity in male Fischer-344 rats

After injection with 0.1 mmol diquat/kg body weight, survival time was markedly shorter in Fischer-344 rats fed a purified diet than in rats fed a regular diet, and much more severe hepatotoxicity and nephrotoxicity were observed in the former than in the latter. The longer the feeding period on the purified diet, the shorter the survival time after diquat administration. These results indicate that the purified diet lacked components present in the regular diet that had protective effects against diquat toxicity. These two diets had nearly the same composition and content of vitamins and minerals. We tested the ingredients of the regular diet to determine which ones reduce diquat toxicity. We found that wheat bran had a protective effect, but that rice bran and bean-curd refuse (okara) did not.

Diquat induces renal proximal tubule injury in glutathione reductase-deficient mice

Reactive oxygen species (ROS) have been associated with many human diseases, and glutathione (GSH)-dependent processes are pivotal in limiting tissue damage. To test the hypothesis that Gr1(a1Neu) (Neu) mice, which do not express glutathione reductase (GR), would be more susceptible than are wild-type mice to ROS-mediated injury, we studied the effects of diquat, a redox cycling toxicant. Neu mice exhibited modest, dose- and time-dependent elevations in plasma alanine aminotransferase (ALT) activities, 126+/-36 U/l at 2 h after 5 micromol/kg of diquat, but no ALT elevations were observed in diquat-treated C3H/HeN mice for up to 6 h after 50 micromol/kg of diquat. Histology indicated little or no hepatic necrosis in diquat-treated mice of either strain, but substantial renal injury was observed in diquat-treated Neu mice, characterized by brush border sloughing in the proximal tubules by 1 h and tubular necrosis by 2 h after doses of 7.5 micromol/kg. Decreases in renal GSH levels were observed in the Neu mice by 2 h post dose (3.4+/-0.4 vs 0.2+/-0.0 micromol/g tissue at 0 and 50 micromol/kg, respectively), and increases in renal GSSG levels were observed in the Neu mice as early as 0.5 h after 7.5 micromol/kg (105.5+/-44.1 vs 27.9+/-4.8 nmol/g tissue). Blood urea nitrogen levels were elevated by 2 h in Neu mice after doses of 7.5 micromol/kg (Neu vs C3H, 32.8+/-4.1 vs 17.9+/-0.3 mg/dl). Diquat-induced renal injury in the GR-deficient Neu mice offers a useful model for studies of ROS-induced renal necrosis and of the contributions of GR in defense against oxidant-mediated injuries in vivo.

Relationship between body iron stores and diquat toxicity in male Fischer-344 rats

The effects of body iron stores on diquat (DQ)-induced toxicity were examined in male Fischer-344 rats, which are sensitive to this herbicide. The rats (5 weeks old) were fed diets containing 40 (lower iron storage [LIS] group) or 320 ppm iron (higher iron storage [HIS] group) for 5 weeks. The concentrations of nonheme iron and ferritin in the liver and kidney were significantly higher in the HIS group than in the LIS group (P<0.0001), although there was no significant differences between the HIS and LIS groups in hematological parameters, including red blood cell count, hemoglobin concentration, and mean corpuscular volume. Three hours after administration of 0.1 mmol DQ/kg, serum alanine aminotransferase and urea nitrogen were significantly higher than in controls (saline injection) for both the LIS and HIS groups (P<0.01), and, after DQ injection, these parameters were significantly higher in the HIS group than in the LIS group (P<0.01). When the rats were injected with 0.075 or 0.1 mmol DQ/kg, the survival time was significantly shorter in the HIS group than in the LIS group (P<0.05). These findings suggest that higher body iron stores result in more severe DQ toxicity in Fischer-344 rats.

Urinary excretion of biomarkers for radical-induced damage in rats treated with NDMA or diquat and the effects of calcium carbimide co-administration

The urinary excretion of seven aldehydes, acetone, coproporphyrin III and 8-hydroxy-2'-deoxyguanosine (8-OH-dG) as non-invasive biomarkers of oxidative damage was measured in rats treated with diquat or N-nitrosodimethylamine (NDMA), two compounds causing hepatic damage by different mechanisms. Furthermore, the effect of co-administration of the aldehyde dehydrogenase inhibitor, calcium carbimide (CC) on the urinary excretion of the aldehydes was determined. Slight hepatotoxicity was found at the end of the experiment after treatment with NDMA (0.5, 4 and 8 mg/kg at t = 0, 48 and 96 h, respectively) or diquat (6.8 and 13.6 mg/kg at t = 0 and 48 h, respectively). In diquat treated rats slight nephrotoxicity was also found. Urinary excretion of aldehydes, acetone and coproporphyrin III remained largely unchanged in rats treated with NDMA. In the rats treated with diquat, the urinary excretion of several aldehydes was several-fold increased. An increase was also found in the urinary excretion of 8-OH-dG after the second dose of diquat. Treatment of rats with CC did not significantly influence the urinary excretion of aldehydes in control and NDMA rats. However, in rats treated with diquat, CC caused a potentiating effect on the excretion of acetaldehyde, hexanal and malondialdehyde (MDA), indicating that oxidation of aldehydes to carbonylic acids by aldehyde dehydrogenases (ALDHs) might be an important route of metabolism of aldehydes. In conclusion, increased urinary excretion of various aldehydes, acetone, coproporphyrin III and 8-OH-dG was observed after administration of diquat, probably reflecting oxidative damage induced by this compound. No such increases were found after NDMA administration, which is consistent with a different toxicity mechanism for NDMA. Therefore, excretion of aldehydes, acetone, coproporphyrin III and 8-OH-dG might be used as easily accessible urinary biomarkers of free radical damage.

Diquat-dependent protein carbonyl formation. Identification of lipid-dependent and lipid-independent pathways

In a previous report on diquat-dependent oxidative damage in rat hepatic microsomes, protein oxidation, as measured by protein carbonyl (PC) formation, was observed in addition to lipid peroxidation (LP). Both phenomena were antioxidant sensitive. Inhibition of PC formation was somewhat surprising given the proposed mechanism of metal-catalyzed protein oxidation. Studies reported here examined diquat-dependent PC formation in greater detail. In rat hepatic microsomes, diquat-dependent thiobarbituric acid-reactive substances (TBARS) and PC formation were time and concentration dependent. In this system, LP was inhibited completely by U-74006F or U-78517G, whereas PC formation was inhibited only partially by these antioxidants. In an essentially lipid-free system consisting of purified rat hepatic cytochrome P450 reductase, BSA and an NADPH-generating system, PC formation was also observed, but was not antioxidant-sensitive. Under these conditions, minimal diquat-dependent TBARS formation was observed. The observation of relative antioxidant insensitivity is consistent with H2O2 (generated during the diquat redox cycle) catalyzing protein oxidation via a site-specific, metal-catalyzed mechanism. Thus, different pathways would appear to be involved in diquat-dependent PC formation in lipid-containing and lipid-free systems. Carbon tetrachloride induces LP following reductive activation to the trichloromethyl free radical, a pathway not directly involving H2O2 generation. In the microsomal system, CCl4 induced TBARS and PC formation, both of which were completely inhibitable by antioxidants. Taken together, these data suggest that diquat induces PC formation by lipid-dependent (antioxidant-sensitive) and lipid-independent (antioxidant-insensitive) pathways. In microsomes, both pathways contribute to diquat-dependent PC formation. Data for the lipid-independent pathway are consistent with the mechanism of metal-catalyzed protein oxidation proposed by Stadtman and colleagues (reviewed in Free Radic Biol Med 9: 315-325, 1990), while the lipid-dependent pathway is likely secondary to LP itself--via a Michael-type addition reaction between hydroxyalkenals and protein sulfhydryl groups, amino groups or other protein nucleophiles. The latter pathway is also responsible for carbon tetrachloride-dependent PC formation. Additional studies are in progress to further characterize the lipid-independent mechanism.

S-(1,2-dichlorovinyl)-L-cysteine-induced nephrotoxicity in the New Zealand white rabbit: characterization of proteinuria and examination of the potential role of oxidative injury

S-(1,2-dichlorovinyl)-L-cysteine (DCVC)-induced nephrotoxicity in vivo was investigated in New Zealand White rabbits. A primary emphasis in these studies was further characterization of DCVC-induced nephrotoxicity using a variety of serum and urinary analytes, including sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Additionally, the role of oxidative injury was assessed to address the dichotomy between reports indicating that such a mechanism is important in vivo and those indicating that such mechanisms do not contribute substantially to the mechanism of effects observed in vitro. Urine was collected prior to and at 8 and 24 hr after iv administration of DCVC. Serum was collected 15 min prior to and 24 hr after DCVC administration. Rabbits were euthanized 24 hr post-DCVC administration, and kidneys were fixed in formalin and further processed for light microscopic examination. DCVC (10 mg/kg, iv) induced a 45-50-fold increase in total urinary protein excretion, a 10-15-fold increase in urinary N-acetyl-beta-D-glucosaminidase concentration, plus a marked glucosuria by 24 hr postadministration. Additionally, DCVC increased serum creatinine levels by about 2-fold, with a trend toward increased blood urea nitrogen. SDS-PAGE analysis of rabbit urine confirmed the clinical finding of marked proteinuria in DCVC-treated animals, which in contrast to previously reported data was due to the presence of both low and high molecular weight proteins. Antioxidants had no significant effect on DCVC-dependent renal injury, nor was there evidence for DCVC-induced lipid peroxidation, as measured by either thiobarbituric acid-reactive substances or a commercial assay for malondialdehyde and hydroxalkenals.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid decreases in indigenous covalent DNA modifications (I-compounds) of male Fischer-344 rat liver DNA by diquat treatment

I-compounds are indigenously appearing covalent DNA modifications that can be detected by 32P-postlabeling assay in tissues of normal animals without known exposure to any carcinogens or toxins. Although these compounds have not been structurally identified, indirect evidence from earlier work suggested the possibility of involvement of molecular fragments derived from lipid peroxides. Diquat is a herbicide that stimulates lipid peroxidation and massive intrahepatic oxidant stress through redox cycling-mediated generation of reactive oxygen species. In the present study, we examined the effects of diquat on hepatic I-compounds of male Fischer-344 rats. Two groups of rats, approximately 14 weeks and 8 weeks old, were given a hepatotoxic dose (0.1 mmol/kg) of diquat or equal volumes of saline, i.p. Two and 6 h later plasma alanine aminotransferase (ALT) activities were measured and hepatic DNA I-compound levels were examined by nuclease P1-enhanced 32P-postlabeling. Elevated ALT activities were observed in some animals in both groups, at both time points, but considerable inter-animal variation was seen. A total of 15-16 I-compound fractions were measured in control and in diquat-treated animals, but no extra spots indicative of treatment-induced adducts were detected. Despite the qualitative similarities, the quantities of individual I-compounds were markedly decreased at 2 h in diquat-treated animals of both age groups. In 14 week old rats the hepatic I-compound contents were decreased at 2 h by 22-59%, which was statistically significant (ANOVA, P < 0.05) for all of the 9 polar I-compound fractions and none of the non-polar fractions. Eleven I-spots from this group showed significant negative linear correlations (P < 0.05) with ALT values. In 8 week old rats treated with diquat a 22-43% depletion in I-compound contents was statistically significant for 4 of the 7 nonpolar and 2 of the 8 polar adduct fractions, but there was no significant correlation of I-compound contents with ALT values at the 2 h time point. By 6 h most of the I-spot levels had returned to normal or above normal values in both groups of animals. While most I-spots from 14 week old rats did not correlate with ALT levels at 6 h, two I-spots displayed positive correlations in the 8 week group. Overall, the susceptibility to diquat-associated DNA alterations appeared to differ with age.(ABSTRACT TRUNCATED AT 400 WORDS)

Diquat- and acetaminophen-induced alterations of biliary efflux of iron in rats

The effects of diquat on the biliary efflux of nonheme iron in rats were studied as a means of examining the possible effects of diquat metabolism on hepatocellular iron metabolism and the association of altered iron metabolism with the initiation of acute hepatic necrosis. Administration of hepatotoxic doses (0.1 mmol/kg) of diquat to male Fischer-344 rats increased biliary iron concentrations from 6 microM to more than 15 microM. However, increases in biliary efflux of iron were not observed during the first 60 min following exposure to diquat, despite the rapid increases in biliary glutathione disulfide concentrations, which increased maximally within 40 min. Biliary efflux of iron was not altered by diquat in Sprague-Dawley rats, which are resistant to hepatic necrosis in response to diquat, despite the marked oxidant stress responses observed in these animals. Conversely, hepatotoxic doses of acetaminophen (1500 mg/kg) caused significant decreases in biliary iron efflux. The rapid decreases in biliary iron caused by acetaminophen and the delay in diquat-induced iron efflux suggested the possibility that some fraction of the biliary iron was being excreted as reversibly formed GS-Fe2+ chelates, with inhibition of export by glutathione disulfide (GSSG) in the case of diquat, or by 3-(glutathion-S-yl)-acetaminophen (GS-AAP) in the case of the acetaminophen-treated animals. However, 50-200 mg/kg doses of acetaminophen showed little effect on biliary iron excretion despite producing biliary GS-AAP conjugate concentrations almost 1000 times the 6 microM concentrations of iron, which would not appear to support the hypothesis of excretion of GS-Fe2+ chelates. The data demonstrate a significant effect of diquat on hepatic iron metabolism in Fischer-344 rats, and the possible importance of this iron redistribution to reactive oxygen-mediated cell damage in vivo is indicated by the absence of similar responses in diquat-treated Sprague-Dawley rats.

Inhibition of S-(1,2-dichlorovinyl)-L-cysteine-induced lipid peroxidation by antioxidants in rabbit renal cortical slices: dissociation of lipid peroxidation and toxicity

Precision-cut, rabbit renal slices were used to examine the effects of three novel antioxidants (U-74006, U-74500, and U-78517) on S-(1,2-dichlorovinyl)-L-cysteine (DCVC)-induced lipid peroxidation and toxicity. Slices exposed to DCVC showed a dose- and time-dependent increase in lipid peroxidation (TBARS) and a decrease in cellular viability, as evidenced by the loss of intracellular potassium, during the course of a 3 hour incubation. Subsequent studies employed DCVC concentrations of 100 microM. Microemulsion formulations of U-78517, U-74500, and U-74006 (100 microM) inhibited DCVC-induced lipid peroxidation by 100 +/-, 50 +/-, and < 5% (not significant), respectively. However, none of these antioxidants had a significant effect on DCVC-dependent cytotoxicity, as indicated by intracellular potassium release. The effects of U-78517, the most potent of the three antioxidants, were similar to those observed with two model antioxidants, diphenyl-p-phenylenediamine (DPPD) and the iron chelator, deferoxamine. Aminooxyacetic (AOAA), an inhibitor of renal cysteine conjugate beta-lyase, had only a minimal effect on DCVC-induced lipid peroxidation, and no effect on toxicity. These data represent the first report of DCVC-induced lipid peroxidation in rabbit renal cortical slices, a system which has been widely used to investigate mechanisms of nephrotoxicity, including that induced by DCVC. Our results demonstrate that DCVC-induced lipid peroxidation in renal slices can be inhibited by a variety of antioxidant compounds operating by different mechanisms. Because inhibition of lipid peroxidation had minimal effect on DCVC-dependent cytotoxicity, the data suggest that DCVC-induced lipid peroxidation is not a major mechanism in the cytotoxicity induced by this compound.

Inhibition of in vitro lipid peroxidation by 21-aminosteroids. Evidence for differential mechanisms

In a previous report (Ryan and Petry, Arch Biochem Biophys 300: 699-704, 1993), the effects of two 21-aminosteroids (U-74500A and U-74006F) on the oxidation and reduction of iron in a buffer/organic solvent system were investigated. In those studies, U-74500A was found to be an efficient iron reductant and potential iron chelator, whereas U-74006F had little effect on iron redox chemistry. As an extension of those studies, we now report the effects of U-74006F and U-74500A on lipid peroxidation in systems that are dependent upon iron oxidation/reduction. In liposomes, U-74500A inhibited ADP:Fe(II)-dependent lipid peroxidation in a concentration-dependent manner, whereas U-74006F was minimally effective in this system. The mechanism of U-74500A-dependent inhibition probably involved interactions with iron, as iron oxidation was inhibited in the presence of this compound. No effects on iron oxidation were observed in the presence of U-74006F. Addition of Ferrozine to liposomal incubation mixtures indicated that at least two iron pools were present in samples containing U-74500A, one immediately bound by Ferrozine, and another that was bound more slowly. Furthermore, ADP:Fe(III)/ascorbate-dependent lipid peroxidation was blocked completely by U-74500A, presumably by formation of a redox inert complex upon reduction of the iron. U-74500A partially protected ADP:Fe(II) from oxidation by H2O2 and lipid hydroperoxides, indicating that the U-74500A:iron complex was stable in the presence of biologically relevant oxidants. U-74006F did not markedly affect iron oxidation or reduction when incorporated into phospholipid liposomes. In microsomal lipid peroxidation systems containing ADP:Fe(III) and NADPH, both U-74500A and U-74006F inhibited lipid peroxidation. U-74006F-dependent inhibition of microsomal lipid peroxidation was dependent on both NADPH and Fe(III). Further, it was enhanced when U-74006F was allowed to preincubate in this system prior to iron addition. Preincubation of U-74006F with microsomes, NADPH, and ADP:Fe(III) produced several metabolites detectable by HPLC. These results suggest that U-74500A inhibits lipid peroxidation by directly affecting iron redox chemistry, whereas U-74006F-mediated inhibition is enhanced by preincubation with a metabolically competent microsomal system.