Induction of xanthine oxidase and depression of cytochrome P-450 by interferon inducers: genetic difference in the responses of mice

Interleukin-6 selectively induces drug metabolism to potentiate the genotoxicity of dietary carcinogens in mammary cells

Breast cancer is the most commonly diagnosed malignancy in females, the etiology being multifactorial and includes the role of lifestyle exposure to DNA-damaging chemicals such as dietary carcinogens benzo (a) pyrene (BaP) and 2-amino-1-methyl-6-phenylimidazo [4, 5-b] pyridine (PhIP). Both compounds require cytochrome P450 (CYP)-mediated metabolic activation to DNA-damaging species, and both induce transcriptional responses through the nuclear receptors Aryl hydrocarbon receptor (AhR) and estrogen receptor α (ERα). BaP and PhIP are mammary carcinogens in rodents. Clinically, circulating IL-6 expression is linked with poor prognosis of cancer and 35% of the deaths in breast cancer are linked with inflammation. The objective of this work was to investigate the molecular toxicology and local activation of BaP and PhIP in the presence of IL-6. Our laboratory has previously reported that miR27b can regulate CYP1B1 expression in colorectal cells, here we have investigated if this mechanism is working in mammary cell models, MCF-7 and MDA-MB-231 cells. Treatment (24 h) of cells with BaP (10 nM-10 µM) and PhIP (100 nM-100 µM) significantly induced genetic damage (micronuclei formation) in a dose-dependent manner in both cell lines. This effect was potentiated in the presence of human IL-6 at concentrations reported to be expressed in clinical breast cancer. On its own, IL-6 treatment failed to induce micronuclei frequency above the control levels in these cells. Compared to BaP or PhIP treatment alone, IL-6 plus BaP or PhIP selectively induced CYP1B1 significantly in both cell lines. Additionally, miR27b expression was downregulated by IL-6 treatments and transfection with miR27b inhibitor confirmed that miR27b is a regulator of CYP1B1 in both cell lines. These data show that BaP- and PhIP-induced DNA damage in mammary cells is potentiated by the inflammatory cytokine IL-6 and that inflammation-induced CYP expression, specifically CYP1B1 via miR27b, is responsible for this effect.

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

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

Effect of interferon-gamma on purine catabolic and salvage enzyme activities in rats

To determine whether interferon-gamma affects rat purine catabolic and salvage enzyme activities, rats were injected with interferon-gamma (600000 U/kg, i.p.) and, similarly to a vehicle-injected control group, killed before or after injection at 6, 12, and 24 h. Organ homogenates were prepared and enzymatic reactions with substrates were carried out, after which the products were measured either chromatographically or spectrophotometrically. Western and Northern blotting also were performed. In contrast to the vehicle-injected rats, interferon-gamma-injected rats showed a significant rise in xanthine oxidoreductase activity in the liver, while enzyme activity was unchanged in the spleen, kidney, and lung. Western analysis of hepatic xanthine oxidoreductase showed an increased concentration of this protein 12 and 24 h after interferon-gamma injection. Northern analysis disclosed an enhanced mRNA expression coding for this enzyme, peaking 12 h after injection. Contrastingly, the activities of adenosine deaminase, purine nucleoside phosphorylase, hypoxanthine guanine phosphoribosyltransferase, and adenine phosphoribosyltransferase were not affected by interferon-gamma in any organ tested. While interferon-gamma causes an increased hepatic biosynthesis of xanthine oxidoreductase, the physiologic role of this enzyme induction remains undetermined.

Regulation of cytochromes P450 during inflammation and infection

Hepatic P450 activities are profoundly affected by various infectious and inflammatory stimuli, and this has clinical and toxicological consequences. Whereas the expression of most P450s in the liver is suppressed, some are induced. Many of the effects observed in vivo can be mimicked by pro-inflammatory cytokines and IFNs, and P450s are differentially regulated by these agents. Therefore, different cytokine profiles and concentrations in the vicinity of the hepatocyte in different models of inflammation may result in qualitatively and quantitatively different effects on populations of P450s. In addition to cytokines, glucocorticoids may have an important role in P450 regulation in stress conditions, including that caused by inflammatory stimuli. Although in many cases the decreases in activity are due primarily to a down-regulation of P450 gene transcription, it is likely that modulation of RNA and protein turnover, as well as enzyme inhibition, contributes to some of the observed effects. The mechanisms whereby these effects are produced may also vary with both the P450 under study and the time course of the effect. The complexity of the P450 response to inflammation and infection means that all of the above factors must be considered when trying to predict the effect of a given infectious or inflammatory condition on the clinical or toxic response of humans or animals to an administered drug or toxin. The question of whether the down-regulation of the hepatic P450 system to inflammation or infection is a homeostatic or pathological response cannot be answered at present. It is difficult to discern the physiological benefit of reducing hepatic P450 activities, unless it is to prevent the generation of reactive oxygen species generated by uncoupled catalytic turnover of the enzymes. On the other hand, as we proposed some years ago [64], the suppression of P450 may be due to the liver's need to utilize its transcriptional machinery and energy for the synthesis of APPs involved in the inflammatory response. In that case, one could ask why the organism has gone to the trouble of employing differential mechanisms for suppression of P450. One answer could be that the response evolved after the divergence of many of the P450 genes, necessitating the evolution of multiple redundant mechanisms for P450 suppression. In contrast to the down-regulation of P450s in the liver, the induction of several forms in this and other tissues suggests a more specific homeostatic role of these effects, e.g., in generation or catabolism of bioactive metabolites.

Down-regulation of the hepatic cytochrome P450 by an acute inflammatory reaction: implication of mediators in human and animal serum and in the liver

1. Infection and inflammation trigger a cascade of mediators that eventually will down-regulate the hepatic cytochrome P450 (P450). The present study aimed to characterize the mediators contained in the serum of rabbits with an acute inflammatory reaction (AIR) induced by the s.c. injection of turpentine (5 ml), and in the serum of humans with an acute upper respiratory tract viral infection. 2. Hepatocytes from control (H(CONT)) rabbits and rabbits with an AIR (H(INFLA)) were isolated and cultured. Compared with H(CONT) in H(INFLA) the production of theophylline metabolites, 3-methylxanthine (3MX), 1-methyluric acid (1MU), and 1,3-dimethyluric acid (1,3DMU) was reduced as was the amount of total P450, while lipid peroxidation was increased. Incubation of H(INFLA) with serum of rabbits with an AIR (RS(INFLA)) for 4 h further reduced the formation of the metabolites of theophylline as well as the amount of P450, and enhanced the lipid peroxidation. RS(INFLA) obtained 6, 12 and 24 h after the injection of turpentine showed the same ability to down-regulate hepatic P450 as the serum obtained at 48 h. 3. The efficacy (Emax) of RS(INFLA) to inhibit the formation of theophylline metabolites differed, i.e. 1,3DMU > 1MU > 3MX, and the potency of serum mediators (IC50) was similar for 3MX and 1MU, but lower for 1,3DMU. 4. Incubation of serum of human volunteers (HS(INFLA)) with a viral infection with H(CONT) or H(INFLA) reduced the production of theophylline metabolites, as well as the amount of P450, and increased the lipid peroxidation. HS(INFLA) depressed 1,3DMU more efficiently than 3MX and 1MU. HS(INFLA) reduced 3MX with greater efficacy than did RS(INFLA). Potency was very variable but not different from rabbits. 5. It is concluded that the serum of rabbits with an AIR or of humans with a viral infection contain several mediators that inhibit noncompetitively various isoenzymes of the hepatic P450. The decrease in P450 induced by HS(INFLA) or RS(INFLA) is closely associated with the increase in lipid peroxidation (r2= 0.8870) suggesting that lipid peroxidation could directly or indirectly be involved in the P450 down-regulation.

Effects of phenobarbital and interleukin-6 on cytochrome P4502B1 and 2B2 in cultured rat hepatocytes

The objectives of this study were to characterize further the effects of phenobarbital (PB) on cytochrome P4502B1 and 2B2 (P4502B1/2) enzyme activity and immunoreactivity in rat hepatocytes and to investigate the mechanism(s) mediating the ability of interleukin-6 (IL-6) to inhibit this induction. PB caused a concentration-dependent increase in benzyloxyresorufin O-deethylase (BROD) activity with maximal effects (a 25-fold increase) at concentrations of 0.3 to 1 mM. The induction of BROD activity was linear over 24 hr of exposure. Immunoblot profiles of P4502B1/2 agreed with measurements of enzyme activity. In addition to inducing P4502B1/2, PB (0.75 mM) also increased the levels of P450 reductase by approximately 2-fold following a 24-hr exposure to PB. When IL-6 was added concomitantly with or up to 12 hr after the addition of PB, the PB induction of BROD activity and immunoreactivity was inhibited significantly. When 18 hr elapsed between the time of addition of PB and IL-6, the inhibitory effects of IL-6 were no longer apparent, suggesting that the actions of IL-6 were mediated by early events in the induction process. IL-6 did not affect the PB induction of P450 reductase. To determine whether IL-6 altered the degradation of P4502B1/2, hepatocytes were exposed to PB for 24 hr, then washed, and the loss of BROD activity and immunoreactivity following incubation with a protein synthesis inhibitor was measured. IL-6 did not alter the rate of loss of either enzyme activity or immunoreactivity, indicating that the effects of IL-6 could not be attributed to the enhanced degradation of P4502B1/2. Results suggest that the inhibition of PB-induced BROD activity by IL-6 is due to an action on early cellular and molecular events in the induction process.

The oxidative inactivation of cytochrome P450 in monooxygenase reactions

Possible mechanisms of cytochrome P450 self-inactivation during catalytic turnover have been considered. Two ways of hemoprotein inactivation are so far known. The first, studied extensively by many authors, is the formation of active substrate intermediates, capable of modifying heme and apoenzyme. The second way, revealed quite recently and resulting from uncoupled cytochrome P450-catalyzed monooxygenase reactions, is yet to be clarified. Briefly, it involves formation of hydrogen peroxide in the hemoprotein active center, which interacts with the enzyme associated Fe2+, thereby generating hydroxyl radicals that bleach the heme and modify the apoenzyme. This mechanism operates with substrates and cytochrome P450 forms with partially coupled monooxygenase reactions, thus causing the formation of hydrogen peroxide as a byproduct.

Dissociation of xanthine oxidase induction and cytochrome P450 depression during interferon induction in the rat

Interferon (IFN) and IFN inducers down-regulate hepatic cytochrome P450 (P450) through a pretranslational mechanism involving depression of P450 mRNA levels and a subsequent decrease in P450 synthesis. Current evidence suggests that interferon induces the synthesis of a protein which subsequently mediates the down-regulation of P450. Xanthine oxidase (XO) activity is induced by interferons in rodents, and the XO inhibitor allopurinol (AP) inhibits the down-regulation of P450 by interferons in the mouse and hamster so it has been proposed as the putative intermediate protein. In studies undertaken in rats to further characterize the molecular basis of the protective effect of AP, we observed that AP (20 and 50 mg/kg) did not protect against down-regulation of P450 by the interferon inducer polyinosinic-polycytidylic acid (10 mg/kg). In fact, at 50 mg/kg AP had an additive effect on the depression of CYP2E1. Total XO induction in the rat was only 30-50% compared with 100-500% in mice and hamsters, and this induction was inhibited completely by AP. Therefore, XO does not mediate the down-regulation of hepatic cytochrome P450 by interferons in the rat.

Cloning of the cDNA encoding human xanthine dehydrogenase (oxidase): structural analysis of the protein and chromosomal location of the gene

The primary structure of human xanthine dehydrogenase (hXDH) was determined by cloning and sequence analysis of the cDNAs encoding the enzyme. The nucleotide (nt) sequence has an open reading frame of 3999 nt encoding a protein of 1333 amino acids (aa) with a calculated M(r) of 146,604. The deduced aa sequence of hXDH is homologous to the previously reported rat XDH (rXDH) and Drosophila melanogaster XDH sequences with identities of 90.2 and 52.0%, respectively. The aa residues involved in both the reversible and the irreversible conversion from the dehydrogenase type to the oxidase type of rXDH are completely conserved between the rat and the human enzymes. This implies that the molecular mechanisms of the conversion of hXDH from dehydrogenase to oxidase are common to those of the well-characterized rXDH. Five sequence variations were detected in the isolated cDNA clones. Spot blot hybridization using flow-sorted human chromosome revealed that the hXDH-encoding gene (hXDH) was located on chromosome 2.

Molecular cloning of a cDNA coding for mouse liver xanthine dehydrogenase. Regulation of its transcript by interferons in vivo

The cDNA coding for xanthine dehydrogenase (XD) is isolated from mouse liver mRNA by cross-hybridization with a DNA fragment of the Drosophila melanogaster homologue. Two lambda bacteriophage overlapping clones represent the copy of a 4538-nucleotide-residue-long transcript with an open reading frame of 4005 nucleotide residues, coding for a putative polypeptide of 1335 amino acid residues. Comparison of the deduced amino acid sequence of the mouse XD with those of the Drosophila and the rat homologues shows a high conservation of this protein (55% identity between mouse and Drosophila, and 94% identity between mouse and rat). RNA blotting analysis demonstrates that interferon-alpha (IFN-alpha) and its inducers, i.e. poly(I).poly(C), bacterial lipopolysaccharide (LPS) and tilorone (2,7-bis-[2-(diethylamino)ethoxy]fluoren-9-one), increase the expression of XD mRNA in liver. Poly(I).poly(C) also induces XD mRNA in several other tissues in vivo. Protein synthesis de novo is not required for the elevation of XD mRNA after IFN-alpha treatment, since cycloheximide does not block the induction. The elevation of XD mRNA concentration is relatively fast and precedes the induction of both XD and xanthine oxidase (XO) enzymic activities.

Oxidative imbalance in HIV infected patients

We present an outline of the complex interplay of oxidants and antioxidants in infectious diseases in general, and in particular with reference to the HIV infection, and subsequent opportunistic infections. Viral and opportunistic infections may directly or indirectly cause an imbalance in prooxidant/antioxidant mechanisms and result in generation of increased steady state concentrations of reactive oxidants. In HIV patients a prooxidant state could lead to a self-perpetuation of infection via stimulated expression of genes carrying the virus genome, and subsequently to immunosuppression, and promotion of initiated cells to neoplastic growth.

Factors involved in the depression of hepatic mixed function oxidase during infections with Listeria monocytogenes

A number of infections are capable of depressing the capacity of the liver to metabolize drugs. We have studied a number of factors which could be involved in the depression of cytochrome P-450 and related drug biotransformation enzymes during infections with Listeria monocytogenes. During the course of the infection, drug metabolism and heme content of hepatic microsomes were depressed but heme oxygenase was elevated. A free radical scavenger alpha-tocopherol did not prevent the loss and xanthine oxidase activities did not correlate with the time course of the loss. Infections in susceptible (balb/c) mice produced a larger loss in drug metabolism than in resistant (C57BL/6) mice, and an avirulent strain of the bacteria was without effect. A preparation of hemolysin isolated from Listeria monocytogenes produced a dose-dependent loss of cytochrome P-450 in isolated hepatocytes. These experiments indicate that the loss of drug metabolism during Listeria infections is most likely due to hemolysin released by the bacteria.

Inhibition of carcinogenesis by minor anutrient constituents of the diet

A continuing study of chemopreventive agents has focused on several categories of naturally occurring compounds that inhibit carcinogen activation and are effective in preventing carcinogen-induced neoplasia when administered at short time-intervals before carcinogen challenge. The inhibitory compounds are: aromatic isothiocyanates found in cruciferous vegetables, monoterpenes present in citrus fruits and caraway-seed oil, and organosulphur compounds occurring in Allium species. Preliminary work indicates that glucobrassicin and indoles existing in cruciferous vegetables also have these attributes. Almost all carcinogens that are consumed in food require metabolic activation. Thus, inhibition of carcinogen activation reactions could be effective against this type of exposure. In addition, three naturally occurring compounds, i.e. phenethyl isothiocyanate, D-limonene and dipropyl sulphide inhibit activation of the tobacco-specific carcinogen NNK, and accordingly may have the capacity to diminish carcinogenic response to exposures to tobacco. The property of cruciferous vegetables, orange oil, benzyl isothiocyanate, and D-limonene, to act as both blocking and suppressing agents has been discussed. Two possible mechanisms for this multi-phase activity were presented. The first is that these inhibitory substances activate a complex integrated defence mechanism against toxic compounds which entails both blocking and suppressing components. The blocking component is the initial line of defence, and the suppressing component constitutes a 'fail-safe' backup to assure that if any of the toxic material attacks cellular constituents, its effects will be nullified. The second possible mechanism considered is that the inhibitors, because of high reactivity, have multiple biological effects that are separate and not part of a single, coordinated response. Inhibitors that have both blocking and suppressing effects could be particularly useful as chemopreventive agents. A simple interim classification of foods in terms of their potential impact on the occurrences of cancer has been proposed.

Inhibition of carcinogenesis by naturally-occurring and synthetic compounds

A continuing study of chemopreventive agents has focused on three categories of naturally-occurring compounds that inhibit carcinogen activation and are effective in preventing carcinogen-induced neoplasia when administered at short time intervals prior to carcinogen challenge. The three are: aromatic isothiocyanates found in cruciferous vegetables, monoterpenes from citrus fruits and caraway seed oils, and organosulfur compounds occurring in Allium species. The short time-interval effects could be significant in terms of their impact on responses of humans to carcinogen exposures. The capacity of sodium cyanate, cruciferous vegetables, orange oil, benzyl isothiocyanate, and D-limonene to act as both blocking and suppressing agents has been discussed. Two possible mechanisms for this multiphase activity were presented. The first is that these inhibitory substances activate a complex integrated defense mechanism against toxic compounds which entails both blocking and suppressing components. The blocking component is the initial line of defense, and the suppressing component constitutes a "fail-safe" backup to assure that if any of the toxic material attacks cellular constituents, its effects will be nullified. The second possible mechanism considered is that the inhibitors, because of high reactivity, have multiple biological effects that are separate and not part of a single, coordinated response. Inhibitors that have both blocking and suppressing effects could be particularly useful as chemopreventive agents.

Induction and depression of cytochrome P-450-dependent mixed-function oxidase by a cloned consensus alpha-interferon (IFN-alpha CON1) in the hamster

A novel analogue of human alpha-interferon (IFN-alpha CON1) was tested for its ability to modify the hepatic cytochrome P-450-dependent mixed-function oxidase system in the hamster. This cloned interferon was derived by selecting the most frequently observed amino acid sequences at each position in the known human alpha-interferon subtypes. IFN-alpha CON1 had a biphasic effect on cytochrome P-450 and related drug biotransformation in the hamster causing an initial increase followed by a significant depression. IFN-alpha CON1 also had a biphasic effect on cytochrome P-450 in the lung, adrenal and spleen but only a depressant effect in the kidney. This effect was not due to morphological damage and followed the species specificity for this type of interferon. Both the increase and the decrease in cytochrome P-450 could be prevented by the administration of the protein synthesis inhibitor puromycin. Various isozymes of cytochrome P-450 induced by phenobarbital, beta-napthaflavone and clofibrate were also depressed by this interferon. The results presented in this report suggest that IFN-alpha CON1 interferon will likely depress drug biotransformation in humans because the antiviral effects and the "anti-cytochrome P-450" effect of interferons cannot be separated, and this interferon has antiviral properties in both hamster and human cells. Clinically relevant drug interactions may be common during the concomitant use of this interferon and other drugs that are metabolized by cytochrome P-450.

Synergistic antiproliferative effect of interferon-beta in combination with bleomycin or neocarzinostatin on HeLa cells in culture: additive effect when combined with adriamycin or mitomycin C

Human fibroblast interferon (IFN)-beta was administered in combination with the free radical-generating antiproliferative agents bleomycin (BLM), neocarzinostatin (NCS), adriamycin (ADM), and mitomycin C (MMC) to HeLa cells in culture. IFN showed a true synergistic antiproliferative activity in the presence of BLM or NCS. These effects were observed regardless of the ratio of IFN to BLM or NCS concentrations. However, the effect of IFN in the presence of ADM or MMC was additive. The possibility that IFN-beta potentiates the antiproliferative effects of these free radical-generating agents in a different manner is also discussed.

Effect of induction of T-cell-dependent antibody with sheep red blood cells on P-450-dependent and -independent xenobiotic metabolizing enzymes

The effect of an antigenic challenge with sheep red blood cells (SRBC) on the activities of cytochrome P-450-dependent and -independent xenobiotic metabolizing enzymes and on lipid peroxidation in the liver was investigated. The studies were carried out using three mouse strains of C57B1/10 and three strains of C3H backgrounds which are cogenic, differing genetically at the H-2 complex. The basal levels of aryl hydrocarbon hydroxylase (AHH) and 7-ethoxycoumarin O-deethylase (7-Ec) were different among congenic strains. The activity of 7-Ec was lower in C3H background mice than in B10 background mice. Similarly, the difference due to the strain and the H-2 locus was detected in the activities of P-450-independent enzymes such as malathion and diethyl succinate carboxylesterases, glutathione S-transferase, and epoxide hydrolases in microsomal and cytosolic fractions. The degree of immune responsiveness in these mice was determined by a plaque forming cell assay. Within the same background, the H-2b mouse strain was a high responder and the H-2k a low responder to SRBC. However, treatment with SRBC had no significant depressive effect on P-450-dependent enzyme activities except in C3H/He. Activity of AHH was suppressed in C3H/He mice. Treatment with SRBC had no effect on P-450-independent enzyme activities except for malathion carboxylesterase: the activity was increased in C3H/He and C3H.JK, whereas it was decreased in B10. The basal level of lipid peroxidation was lower in C3H/He and C3H.JK. The treatment produced a significant enhancement in lipid peroxidation in C3H/He, B10 and B10.BR (P less than 0.05) with a concomitant increase in xanthine oxidase activity (P less than 0.05). Thus, the present study revealed that a specific antigenic challenge, unlike non-specific immunostimulants (e.g. poly IC, endotoxin), does not necessarily inhibit P-450-dependent xenobiotic metabolizing enzymes even though antigen challenge increased XO activity and lipid peroxidation. The possible roles of an increase in lipid peroxidation and xanthine oxidase activity in immune response to SRBC and xenobiotic metabolizing enzymes are discussed.

Influence of H-2 haplotypes on poly IC induction of xanthine oxidase and poly IC induced decreases in P-450 mediated enzyme activities

Polyriboinosinic-polyribocytidylic acid (poly IC), a potent interferon inducer, induced xanthine oxidase 24 hours after treatment with 5 mg/kg ip to different degrees among four H-2 congenic mice (P less than 0.05): B10 (H-2b: 236 +/- 27% of the control value) greater than B10.RIII (H-2r: 171 +/- 29%) = B10.F (H-2n: 161 +/- 12%) greater than B10.BR (H-2k: 136 +/- 15%). Aryl hydrocarbon hydroxylase (AHH) activity showed an inverse correlation with inducibility of xanthine oxidase (r = -0.71, P less than 0.01). However, there were no significant changes in activities of heme pool associated enzymes, such as catalase, tryptophan pyrrolase and d-aminolevulinic acid synthase in these mice. H-2 haplotype seems to have an influence on poly IC induction of xanthine oxidase thereby causing a decrease in AHH.

Induction of xanthine oxidase and heme oxygenase and depression of liver drug metabolism by interferon: a study with different recombinant interferons

Induction of xanthine oxidase in mouse liver by interferon (IFN) was studied with three different recombinant human leukocyte IFN molecules: IFLrA, IFLrD and the hybrid IFLrA/D(Bgl II). The ability of different IFN species to induce xanthine oxidase correlated with their ability to depress liver cytochrome P-450-dependent drug metabolism, supporting the hypothesis that reactive oxygen metabolites generated by xanthine oxidase might be responsible for this impairment of liver function by IFN. The antioxidant N-acetylcysteine protected in vivo against the depression of liver drug metabolism by IFLrA/D. IFLrA/D was also found to induce liver microsomal heme oxygenase, an effect that was probably secondary to the observed depression of cytochrome P-450.