[Transcranial direct current stimulation for depressive disorders]. / Transkranielle Gleichstromstimulation bei depressiven Störungen.

Major depressive disorders are one of the most prevalent psychiatric disorders worldwide but approximately 20-30 % of patients do not respond to standard guideline conform treatment. Recent neuroimaging studies in depressive patients revealed altered activation patterns in prefrontal brain areas and that successful cognitive behavioral therapy and psychopharmacological interventions are associated with a reversal of these neural alterations. Therefore, a direct modulation of prefrontal brain activation by non-invasive brain stimulation techniques, such as transcranial direct current stimulation (tDCS) seems to be a promising and innovative approach for the treatment of depressive disorders. In addition, recent neuropsychological findings indicated an augmentation of positive tDCS effects by simultaneous external activation of the stimulated brain area, for example by cognitive training tasks. Based on these findings, the possibility to augment cognitive-emotional learning processes during cognitive behavioral therapy by simultaneous tDCS to increase antidepressive therapeutic effects is discussed in this article.

Association of gamma-glutamyltransferase with severity of disease at diagnosis and prognosis of ovarian cancer.

BACKGROUND: Gamma-glutamyltransferase (GGT) - a membrane-bound enzyme crucially involved in the cell's detoxification pathway and apoptotic balance - is involved in tumour development, progression and chemotherapy resistance. Elevated GGT serum levels are associated with increased cancer risk in women and worse prognosis in gynaecologic cancers. The present study investigated the prognostic role of GGT in ovarian cancer patients. METHODS: In this multicenter study, pre-therapeutic GGT levels were ascertained in 634 consecutive patients with epithelial ovarian cancer (EOC, n=567) and borderline tumour of the ovary (BTO, n=67). Gamma-glutamyltransferase serum levels were associated with clinicopathological parameters and uni- and multivariate survival analyses were performed. Immunohistochemistry of GGT was performed in ovarian cancer tissue and correlated with GGT serum levels. RESULTS: Pre-therapeutic GGT serum levels were higher in patients with EOC (28.56 (38.24) U l(-1)) than in patients with BTO (20.01 (12.78) U l(-1), P=0.01). High GGT serum levels were associated with advanced FIGO stage (P<0.001) and with worse overall survival in univariate (P<0.001) and multivariable analysis (P=0.02, HR 1.2 (1.1-1.5)). We further investigated the association between systemic GGT serum levels and local GGT expression in EOC tumour tissue and observed an association between these two parameters (P=0.03). CONCLUSION: High pre-therapeutic GGT serum levels are associated with advanced tumour stage and serve as an independent prognostic marker for worse overall survival in patients with EOC. Gamma-glutamyltransferase expression in ovarian cancer tissue is reflected in GGT serum levels.

Ketamine specifically reduces cognitive symptoms in depressed patients: An investigation of associated neural activation patterns.

Major depressive disorder (MDD) is characterized by heterogeneous cognitive, affective and somatic symptoms. Hence, the investigation of differential treatment effects on these symptoms as well as the identification of symptom specific biomarkers might crucially contribute to the development of individualized treatment strategies. We here aimed to examine symptom specific responses to treatment with ketamine, which repeatedly demonstrated rapid antidepressant effects in severe MDD. Additionally, we investigated working memory (WM) related brain activity associated with changes in distinct symptoms in order to identify specific response predictors. In a sample of 47 MDD patients receiving a single sub-anesthetic dose of ketamine, we applied a three-factor solution of the Beck Depression Inventory (BDI) to detect symptom specific changes 24 h post-infusion. A subsample of 16 patients underwent additional fMRI scanning during an emotional working memory task prior to ketamine treatment. Since functional aberrations in the default mode network (DMN) as well as in the dorsolateral prefrontal cortex (DLPFC) have been associated with impaired cognitive and emotional processing in MDD, we investigated neural activity in these regions. Our results showed that ketamine differentially affects MDD symptoms, with the largest symptom reduction in the cognitive domain. WM related neuroimaging results indicated that a more pronounced effect of ketamine on cognitive symptoms is predicted by lower DMN deactivation and higher DLPFC activation. Findings thereby not only indicate that ketamine's antidepressant efficacy is driven by a pro-cognitive mechanism, but also suggest that this might be mediated by increased potential for adaptive adjustment in the circumscribed brain regions.

Oxidation of persistent environmental pollutants by a white rot fungus.

The white rot fungus Phanerochaete chrysosporium degraded DDT [1,1,-bis(4-chlorophenyl)-2,2,2-trichloroethane], 3,4,3',4'-tetrachlorobiphenyl, 2,4,5,2',-4',5'-hexachlorobiphenyl, 2,3,7,8-tetrachlorodibenzo-p-dioxin, lindane (1,2,3,4,5,6-hexachlorocylohexane), and benzo[a]pyrene to carbon dioxide. Model studies, based on the use of DDT, suggest that the ability of Phanerochaete chrysosporium to metabolize these compounds is dependent on the extracellular lignin-degrading enzyme system of this fungus.

The mechanism of liver microsomal lipid peroxidation.

In the presence of Fe-3+ and complexing anions, the peroxidation of unsaturated liver microsomal lipid in both intact microsomes and in a model system containing extracted microsomal lipid can be promoted by either NADPH and NADPH : cytochrome c reductase or by xanthine and xanthine oxidase. Erythrocuprein effectively inhibits the activity promoted by xanthine and xanthine oxidase but produces much less inhibition of NADPH-dependent peroxidation. The singlet-oxygen trapping agent, 1, 3-diphenylisobenzofuran, had no effect on NADPH-dependent peroxidation but strongly inhibited the peroxidation promoted by xanthine and xanthine oxidase. NADPH-dependent lipid peroxidation was also shown to be unaffected by hydroxyl radical scavengers.. The addition of catalase had no effect on NADPH-dependent lipid peroxidation, but it significantly increased the rate of malondialdehyde formation in the reaction promoted by xanthine and xanthine oxidase. The results demonstrate that NADPH-dependent lipid peroxidation is promoted by a reaction mechanism which does not involve either superoxide, singlet oxygen, HOOH, or the hydroxyl radical. It is concluded that NADPH-dependent lipid peroxidation is initiated by the reduction of Fe-3+ followed by the decomposition of hydroperoxides to generate alkoxyl radicals. The initiation reaction may involve some form of the perferryl ion or other metal ion species generated during oxidation of Fe-2+ by oxygen.

Comparative studies of rat liver and lung NADPH-cytochrome c reductase.

NADPH-cytochrome c reductase, the flavoprotein component of the liver microsomal mixed-function oxidases, has been compared to the corresponding rat lung microsomal enzyme. Both enzymes were purified by the same methods and have identical ionic strength optima towards the reduction of cytochrome c. Antibody directed against the liver reductase identically inhibited the reduction of cytochrome c and ferricyanide by both enzymes. Double diffusion immunoprecipitation on Ouchterlony plates of deoxycholate-solubilized liver and lung microsomes resulted in converging precipitin lines indicating similar antigenic sites. The apparent molecular weights of the detergent-solubilized and bromelain-solubilized lung enzymes were determined by sodium dodecylsulfate-polyacrylamide gel electrophoresis to be 79 000 and 71 000, respectively. From the above criteria we conclude that the enzymes in these two tissues are very similar or identical proteins.

Rabbit liver microsomal lipid peroxidation. The effect of lipid on the rate of peroxidation.

Rat and rabbit liver microsomes catalyze an NADPH-cytochrome P-450 reductase-dependent peroxidation of endogenous lipid in the presence of the chelate, ADP-Fe3+. Although liver microsomes from both species contain comparable levels of NADPH-cytochrome P-450 reductase and cytochrome P-450, the rate of lipid peroxidation (assayed by malondialdehyde and lipid hydroperoxide formation) catalyzed by rabbit liver microsomes is only about 40% of that catalyzed by rat liver microsomes. Microsomal lipid peroxidation was reconstituted with liposomes made from extracted microsomal lipid and purified protease-solubilized NADPH-cytochrome P-450 reductase from both rat and rabbit liver microsomes. The results demonstrated that the lower rates of lipid peroxidation catalyzed by rabbit liver microsomes could not be attributed to the specific activity of the reductase. Microsomal lipid from rabbit liver was found to be much less susceptible to lipid peroxidation. This was due to the lower polyunsaturated fatty acid content rather than the presence of antioxidants in rabbit liver microsomal lipid. Gas-liquid chromatographic analysis of fatty acids lost during microsomal lipid peroxidation revealed that the degree of fatty acid unsaturation correlated well with rates of lipid peroxidation.

Lactoperoxidase-catalyzed lipid peroxidation of microsomal and artificial membranes.

Lactoperoxidase, in the presence of H2O2, I-, and rat liver microsomes, will peroxidize membrane lipids, as evidenced by malondialdehyde formation. Fe3+ assists in the formation of malondialdehyde. Fe3+ can be added at the end of the reaction period as well as at the beginning with equal effectiveness, suggesting that it only acts to assist in the conversion of lipid peroxides, previously formed by lactoperoxidase, to malondialdehyde. The addition of EDTA to the microsomal reaction mixture results in a 40% decrease in malondialdehyde formation. The antioxidant butylated hydroxytoluene will completely block the formation of malondialdehyde. Malondialdehyde formation is not dependent upon the production of superoxide, singlet oxygen, or hydroxyl radicals. Peroxidation of membrane lipids by this system is equally effective in both intact microsomes and in liposomes, indicating that iodination of microsomal protein is not required for lipid peroxidation to occur.

Reconstituted microsomal lipid peroxidation: ADP-Fe3+-dependent peroxidation of phospholipid vesicles containing NADPH-cytochrome P450 reductase and cytochrome P450.

A reconstituted lipid peroxidation system consisting of rat liver microsomal NADPH-cytochrome P450 reductase and cytochrome P450 incorporated into phospholipid vesicles was developed and characterized. Peroxidation of the vesicles required NADPH and ADP-Fe3+, just as in the NADPH-dependent peroxidation of microsomes. The peroxidation of the vesicles was inhibited 30-50% by superoxide dismutase, depending upon their cytochrome P450 content: those with higher cytochrome P450 contents exhibited greater rates of malondialdehyde formation which were less sensitive to inhibition by superoxide dismutase. When cytochrome P450 was incorporated into vesicles, EDTA-Fe3+ was not required for lipid peroxidation, distinguishing this system from the one previously described by Pederson and Aust [Biochem. Biophys. Res. Comm. 48, 789; 1972]. Since at least 50% of the malondialdehyde formation in the vesicular system was not inhibited by superoxide dismutase, alternative means of iron reduction (O2-.-independent) were examined. It was found that rat liver microsomes or a reconstituted mixed function oxidase system consisting of NADPH-cytochrome P450 reductase and cytochrome P450 in dilauroylphosphatidylcholine micelles reduced ADP-Fe3+ under anaerobic conditions.

An investigation into the mechanism of citrate-Fe2+-dependent lipid peroxidation.

Chelation by citrate was found to promote the autoxidation of Fe2+, measured as the disappearance of 1,10-phenanthroline-chelatable Fe2+. The autoxidation of citrate-Fe2+ could in turn promote the peroxidation of microsomal phospholipid liposomes, as judged by malondialdehyde formation. At low citrate-Fe2+ ratios the autoxidation of Fe2+ was slow and the formation of malondialdehyde was preceded by a lag phase. The lag phase was eliminated by increasing the citrate-Fe2+ ratio, which also increased the rate of Fe2+ autoxidation. The Fe2+ autoxidation product required for the initiation of lipid peroxidation was characterized as being Fe3+. As direct evidence of this, linear initial rates of lipid peroxidation were obtained via the combination of citrate-Fe2+ and citrate-Fe3+, optimum activity occurring at a Fe3+-Fe2+ ratio of 1:1. Evidence is also presented to suggest that the superoxide and the hydrogen peroxide that are formed during the autoxidation of citrate-Fe2+ can either stimulate or inhibit lipid peroxidation by affecting the yield of citrate-Fe3+ from citrate-Fe2+. No evidence was obtained for the participation of the hydroxyl radical in the initiation of lipid peroxidation by citrate-Fe2+.

The consequences of hydroxyl radical formation on the stoichiometry and kinetics of ferrous iron oxidation by human apoferritin.

Despite previous detection of hydroxyl radical formation during iron deposition into ferritin, no reports exist in the literature concerning how it might affect ferritin function. In the present study, hydroxyl radical formation during Fe(II) oxidation by apoferritin was found to be contingent on the "ferroxidase" activity (i.e., H subunit composition) exhibited by apoferritin. Hydroxyl radical formation was found to affect both the stoichiometry and kinetics of Fe(II) oxidation by apoferritin. The stoichiometry of Fe(II) oxidation by apoferritin in an unbuffered solution of 50 mM NaCl, pH 7.0, was approximately 3.1 Fe(II)/O(2) at all iron-to-protein ratios tested. The addition of HEPES as an alternate reactant for the hydroxyl radical resulted in a stoichiometry of about 2 Fe(II)/O(2) at all iron-to-protein ratios. HEPES functioned to protect apoferritin from oxidative modification, for its omission from reaction mixtures containing Fe(II) and apoferritin resulted in alterations to the ferritin consistent with oxidative damage. The kinetic parameters for the reaction of recombinant human H apoferritin with Fe(II) in HEPES buffer (100 mM) were: K(m) = 60 microM, k(cat) = 10 s(-1), and k(cat)/K(m) = 1.7 x 10(5) M(-1) x (-1). Collectively, these results contradict the "crystal growth model" for iron deposition into ferritin and, while our data would seem to imply that the ferroxidase activity of ferritin is adequate in facilitating Fe(II) oxidation at all stages of iron deposition into ferritin, it is important to note that these data were obtained in vitro using nonphysiologic conditions. The possibility that these findings may have physiological significance is discussed.

Mechanism(s) involved in meat mutagen formation and inhibition.

The Maillard reaction, which involves Amadori rearrangement as a key step, also results in sugar fragmentation and free radical formation. The imidazoquinoline meat mutagens (2-amino-3-methylimidazo[4,5-f]-quinoline, or IQ, and 2-amino-3,4-dimethylimidazo[4,5-f]quinoline, or MeIQ) are formed from a reaction mixture containing alkylpyridine free radicals and creatinine. The imidazoquinoxaline meat mutagens (2-amino-3,4-dimethylimidazo[4,5-f]-quinoxaline, or MeIQx, and 2-amino-3,4,8-trimethylimidazo[4,5-f]-quinoxaline, or 4,8-DiMeIQx) may be produced by reacting a mixture containing dialkylpyrazine free radicals and creatinine. Two different pathways for free radical formation are proposed. One involves bimolecular ring formation from the enaminol form of the glycoaldehyde alkylimine and is followed by oxidative formation of the free radical. The other pathway involves formation of N,N1-dialkylpyrazinium ions from glyoxal monoalkylimine followed by reduction to produce the free radicals. The respective intermediates (glycoaldehyde alkylimine and glyoxal monoalkylamine) are formed by reacting glycoaldehyde and glyoxal with amino compounds. The glycoaldehyde system reacts faster and produces more free radicals than the glyoxal system. The reactions help to explain the formation of imidazoquinoxaline meat mutagens and their predominance in fried fish and why these mutagens are present in larger quantities in fried ground beef than the imidazoquinoline-type meat mutagens. These two pathways may not be the only mechanisms involved in formation of meat mutagens, but other free radical reactions may also contribute to meat mutagenicity and are mentioned briefly.(ABSTRACT TRUNCATED AT 250 WORDS)

Transition metals as catalysts of "autoxidation" reactions.

Superoxide (O2-), hydrogen peroxide (H2O2), and hydroxyl radical (.OH) produced from the "autoxidation" of biomolecules, such as ascorbate, catecholamines, or thiols, have been implicated in numerous toxicities. However, the direct reaction of dioxygen with the vast majority of biomolecules, including those listed above, is spin forbidden, a condition which imposes a severe kinetic limitation on this reaction pathway. Therefore, an alternate mechanism must be invoked to explain the "autoxidations" reactions frequently reported. Transition metals are efficient catalysts of redox reactions and their reactions with dioxygen are not spin restricted. Therefore it is likely that the "autoxidation" observed for many biomolecules is, in fact, metal catalyzed. In this paper we discuss: 1) the quantum mechanic, thermodynamic, and kinetic aspects of the reactions of dioxygen with biomolecules; 2) the involvement of transition metals in biomolecule oxidation; and 3) the biological implications of metal catalyzed oxidations. We hypothesize that true autoxidation of biomolecules does not occur in biological systems, instead the "autoxidation" of biomolecules is the result of transition metals bound by the biomolecules.

Modification of ferritin during iron loading.

Recombinant human ferritin loaded with iron via its own ferroxidase activity did not sediment through a sucrose-density gradient as a function of iron content. Analysis of the recombinant ferritin by native PAGE demonstrated an increase in altered migration pattern of the ferritins with increasing sedimentation, indicating an alteration of the overall charge of ferritin. Additionally, analysis of the ferritin by SDS-PAGE under nonreducing conditions demonstrated that the ferritin had formed large aggregates, which suggests disulfide bonds are involved in the aggregation. The hydroxyl radical was detected by electron spin resonance spectroscopy during iron loading into recombinant ferritin by its own ferroxidase activity. However, recombinant human ferritin loaded with iron in the presence of ceruloplasmin sedimented through a sucrose-density gradient similar to native ferritin. This ferritin was shown to sediment as a function of iron content. The addition of ceruloplasmin to the iron loading assay eliminated the detection of the DMPO-*OH adduct observed during loading using the ferroxidase activity of ferritin. The elimination of the DMPO-*OH adduct was determined to be due to the ability of ceruloplasmin to completely reduce oxygen to water during the oxidation of the ferrous iron. The implications of these data for the present models for iron uptake into ferritin are discussed.

Pulmonary ferritin: differential effects of hyperoxic lung injury on subunit mRNA levels.

Ferritin is an iron storage protein that is regulated at the transcriptional and transcriptional levels, resulting in a complex mixture of tissue- and condition-specific isoforms. The protein shell of ferritin is composed of 24 subunits of two types (heavy or light), which are encoded by two distinct and independently regulated genes. In the present studies, the isoform profile for lung ferritin differed from other tissues (liver, spleen, and heart) as determined by isoelectric focusing (IEF) and polyacrylamide gel electrophoresis (PAGE). Lung ferritin was composed of equal amounts of heavy and light subunits. Differences in isoform profiles were the result of tissue-specific differential expression of the ferritin subunit genes as demonstrated by Northern blot analyses. Like heart ferritin, lung ferritin exhibited a low iron content that did not increase extensively in response to iron challenge, which contrasts with ferritins isolated from liver or spleen. When animals were exposed to hyperoxic conditions (95% oxygen for up to 60 h), ferritin heavy subunit mRNA levels did not markedly change at any of the investigated time points. In contrast, ferritin light subunit mRNA increased severalfold in response to hyperoxic exposure. Investigation of the cytoplasmic distribution of ferritin mRNA showed that a substantial portion was associated with the ribonucleoprotein (RNP) fraction of the cytosol, suggesting that a pool of untranslated ferritin mRNA exists in the lung. Upon hyperoxic insult, all ferritin light subunit mRNA pools (RNP, monosomal, polysomal) were elevated, although a specific shift from RNP to polysomal pools was not evident. Therefore, the increase in translatable ferritin mRNA in response to hyperoxia resulted from transcriptional rather than specific translational activation. The observed pattern of light chain-specific transcriptional induction of ferritin is consistent with the hypothesis that hyperoxic lung injury is at least partially iron mediated.

Effect of hydrogen peroxide on the initiation of microsomal lipid peroxidation.

Hydrogen peroxide reacts with reduced transition metals to generate the highly reactive hydroxyl radical (X OH), most often proposed as the predominant species for initiating microsomal lipid peroxidation. To assess the potential involvement of X OH, generated from hydrogen peroxide, in microsomal lipid peroxidation, we have altered the concentration of microsomal hydrogen peroxide and measured the resulting rates of malondialdehyde production. Hydrogen peroxide concentration in microsomes was changed by adding exogenous catalase, by washing to reduce both endogenous catalase activity and hydrogen peroxide-dependent glutathione oxidase activity, and by inhibiting endogenous catalase activity with azide in either the presence or absence of exogenous hydrogen peroxide. In only one instance was the rate of lipid peroxidation affected; exogenous hydrogen peroxide added to microsomes, previously incubated with azide, inhibited lipid peroxidation, the opposite effect from that predicted if X OH, generated from hydrogen peroxide, is actually the major initiating species. Neither these results, nor the inability of known X OH traps to inhibit microsomal lipid peroxidation, support the role of free hydrogen peroxide in the initiation of microsomal lipid peroxidation.

Mechanisms of degradation by white rot fungi.

White rot fungi use a variety of mechanisms to accomplish the complete degradation of lignin and a wide variety of environmental pollutants. Both oxidative and reductive reactions are required for the metabolism of both lignin and environmental pollutants. The fungi secrete a family of peroxidases to catalyze both direct and indirect oxidation of chemicals. The peroxidases can also catalyze reductions using electron donors to generate reductive radicals. A cell-surface membrane potential can also be used to reduce chemicals such as TNT.

Induction of drug metabolizing enzymes in polybrominated biphenyl-fed lactating rats and their pups.

Polybrominated biphenyls (PBBs) cause a mixed-type (phenobarbital- plus 3-methylcholanthrene-like) induction of liver microsomal drug metabolizing enzymes in rats. However, 2,2',4,4',5,5'-hexabromobiphenyl and 2,2',3,4,4',5,5'-heptabromobiphenyl, which together comprise less than 80% of PBBs (FireMaster), were shown to be strictly phenobarbital-type inducers. Other components (unidentified) must therefore cause the 3-methylcholanthrene-like effects. The potential for PBBs to exert effects on neonates through milk was examined. Lactating rats were fed 0, 0.1, 1.0, or 10 ppm FireMaster for the 18 days following delivery, at which time mothers and most pups were sacrificed. Pups nursing from mothers fed 10 ppm PBBs showed significant increases in liver weights and microsomal protein, and both mothers and pups had increased cytochrome P-450, aminopyrine demethylation, benzo[a]pyrene hydroxylation, and UDP-glucuronyltransferase. Pups nursing from rats fed 1.0 ppm had increases in microsomal protein, cytochrome P-450, aminopyrine demethylation, and benzo[a]pyrene hydroxylation, while their mothers were unaffected. Several pups from the 0, 0.1, and 1.0 ppm groups were maintained on their mother's diets, raised, and allowed to mate. Their pups showed much the same responses to PBBs as did the original group of pups. The effects on both generations of adult female rats were also comparable. PBBs cause a mixed-type induction in both lactating rats and their nursing pups; PBB components responsible for both aspects of this induction must be transmitted through milk. Nursing rats are approximately tenfold more sensitive to the effects of PBBs in their mother's diets than are the dams. The approximate no-effect level for microsomal induction in nursing rats is 0.1 ppm PBBs in the diet of the adult.

Paraquat toxicity: proposed mechanism of action involving lipid peroxidation.

The purpose of this study was to investigate the hypothesis that paraquat pulmonary toxicity results from cyclic reduction-oxidation of paraquat with sequential generation of superoxide radicals and singlet oxygen and initiation of lipid peroxidation. In vitro mouse lung microsomes catalyzed an NADPH-dependent, single-electron reduction of paraquat. Incubation of paraquat with NADPH, NADPH-cytochrome c reductase, and purified microsomal lipid increased malondialdehyde production is a concentration dependent manner. Addition of either superoxide dismutase or a single oxygen trapping agent 1,3-dipheylisobenzo furan inhibited paraquat stimulated lipid peroxidation. In vivo, pretreatment of mice with phenobarbital decreased paraquat toxicity, possibly by competing for electrons which might otherwise reduce paraquat. In contrast, paraquat toxicity in mice was increased by exposure to 100% oxygen and by deficiencies of the antioxidants selenium, vitamin E, or reduced glutahione (GSH). Paraquat, given IP to mice, at 30 mg/kg, decreased concentrations of the water-soluble antioxidant GSH in liver and lipid soluble antioxidants in lung. Oxygen-tolerant rats, which hae increased activities of pulmonary enzymes which combat lipid peroxidation, were also tolerant to lethal doses of paraquat as indicated by an increased paraquat LT50. Furthermore, rats chronically exposed to 100 ppm paraquat in the water had elevated pulmonary activities of glucose-6-phosphate dehydrogenase and GSH reductase. These results were consistent with the hypothesis that lipid peroxidation is involved in the toxicity of paraquat.

Studies on the microsomal metabolism and binding of polybrominated biphenyls (PBBs).

The metabolism of polybrominated biphenyls (PBBs) was studied in vitro by using rat liver microsomes in the presence of NADPH and atmospheric O2. Quantitative recoveries of all PBBs were obtained after incubations with control or 3-methylcholanthrene (MC) induced microsomes. Of the twelve major components, losses of only peaks 1 (2,4,5,2',5'-pentabromobiphenyl) and 3 (a hexabromobiphenyl) were observed following incubations with microsomes from phenobarbital (PB)- or PBBS- pretreated rats. Of seven structurally identified PBB components, only peak 1 has a bromine-free para position. Peaks 1, 2, and 5 all have two adjacent unsubstituted carbons, yet only peak 1 is metabolized. Of two dibromobiphenyl model compounds studied, the 2,2'-congener was very rapidly metabolized by PB-induced microsomes whereas its 4,4'-isomer was not. These results suggest that the presence of a free para position is required for the metabolism of brominated biphenyls. Of lesser importance appears to be the number of bromines or the availability of two adjacent unsubstituted carbons. In vivo evidence for the metabolism of peaks 1 and 3 was also provided by their drastically diminished levels in liver and milk extracts. When a 14C-PBB mixture consisting almost exclusively of peaks 4 (2,4,5,2',4',5'-hexabromobiphenyl) and 8 (2,3,4,5,2',4',5'-heptabromobiphenyl) was incubated with PB- or PBBs- induced microsomes and NADPH, only traces of radioactivity remained with the microsomes after extensive extraction. However, less radioactivity was bound to microsomes from MC pretreated or especially control rats. No radioactivity was bound to exogenous DNA included in similar microsomal incubations, regardless of the type of microsomes used. Under the same conditions, [3H]-benzo[a]pyrene metabolites were bound to DNA, and PBB-induced microsomes enhanced this binding more than six-fold.