Interrelationship between demethylation of p-nitroanisole and conjugation of p-nitrophenol in rat liver

Heterologous expression and purification of the infectious salmon anemia virus hemagglutinin esterase

This study presents the heterologous production and purification of a soluble and functional form of the hemagglutinin esterase (HE) of the infectious salmon anemia virus (ISAV) isolate 4 (Glesvaer/2/90). The HE possesses receptor binding and receptor destroying enzyme (RDE) activity and is probably involved in the infection process. The recombinant HE protein (recHE 4) was expressed in insect cells (Sf9) using the baculovirus expression vector system. Both the transmembrane region and the cytoplasmic tail were deleted, and a C-terminal His(6)-tag was attached to facilitate identification and purification of the recHE 4 protein. As determined by Western analysis the recHE 4 was secreted at 20 degrees C and not at 28 degrees C. By testing three HE constructs differing in their promoter and secretion signal sequences it was clear that the HE's own secretion signal sequence is more important than the promoter with respect to the amount of secreted recHE 4 obtained under the conditions used. A one-step purification by nickel-affinity chromatography resulted in a highly purified recHE 4, identified by matrix assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) analysis. Also, the recHE 4 is glycosylated and contains disulfide bridges within the molecule. Functional studies including the verification of the receptor destroying enzyme (RDE) activity as well as the binding to Atlantic salmon erythrocytes (hemagglutination) indicate that the recHE 4 has similar functions as its native counterpart. In conclusion, insect cells secrete a functional form of the ISAV 4 HE. This is suitable for further analyses on its function and immunogenicity.

Para-nitrophenol glucuronidation and sulfation in rat and human liver slices

Para-nitrophenol (PNP) is a well-known substrate for both phase I (hydroxylation at cytochrome P450) and phase II reactions (glucuronidation and sulfation). HPLC separation of PNP conjugates has already been described, but not for respective studies with liver slices, which nowadays have proven to be a suitable model for metabolic studies. Therefore we adapted an HPLC method for the simultaneous measurement of PNP glucuronidation (PNP-G) and sulfation (PNP-S) in this in vitro system. Both activities are substantially maintained over an incubation period of 24 h. PNP-G activity, however, seems to be better preserved, as indicated by stable values for PNP-G but reduced PNP-S values after 48 h liver slice preincubation. 24 h exposure of the slices to beta-naphthoflavone or phenobarbital does not change PNP-G or PNP-S activities.

High-performance liquid chromatographic analysis of p-nitrophenol and its conjugates in biological samples

p-Nitrophenol (pNP) and its conjugated metabolites, generated in a perfused rat liver preparation, are readily separated and quantitated in serum perfusate and bile samples using a reverse-phase high-performance liquid chromatographic method. Serum perfusate samples can be analyzed following protein precipitation with acetonitrile: following protein precipitation with 1.5 M perchloric acid (1 part to 2 parts serum) there was degradation of pNP sulfate to pNP when samples were stored at room temperature. pNP can also be analyzed in blood perfusate samples following extraction with a number of organic solvents including ethyl acetate or isobutanol-methylene chloride (4:1, v/v). Rat liver perfusions at a constant input concentration of 40 microM demonstrated a high hepatic extraction ratio of pNP (mean of 0.90) due to the formation of the sulfate and glucuronide conjugates; no pNP glucoside was detected in perfusate or bile samples.

Influence of substrate concentration on the phase I and phase II metabolism of 4-methoxybiphenyl by rat isolated hepatocytes

1. The metabolism of 4-methoxybiphenyl to 4-hydroxybiphenyl and its sulphate and glucuronic acid conjugates has been studied in rat isolated hepatocytes at various concentrations of 4-methoxybiphenyl. 2. The proportions of metabolites produced remained constant at concentrations of 4-methoxybiphenyl less than 25-50 microM, but at higher concentrations the pattern of metabolism changed, there being proportionally more unconjugated 4-hydroxybiphenyl and its glucuronide, and less 4-hydroxybiphenyl sulphate. 3. The amount of 4-hydroxybiphenyl sulphate produced approached a plateau at the highest level of overall metabolism, whereas the amount of 4-hydroxybiphenyl glucuronide produced was linearly related to the amount of unconjugated 4-hydroxybiphenyl detected. 4. These data are consistent with the view that sulphation acts as a high-affinity, low-capacity pathway, whereas glucuronidation acts as a low-affinity, high-capacity pathway.

Kinetics of the formation and secretion of the aniline metabolite 4-aminophenol and its conjugates by isolated rat hepatocytes

The rates of secretion of 4-aminophenol and its sulphate and glucuronide conjugate were determined in cultures of rat hepatocytes with aniline and 4-aminophenol as substrates. When 4-aminophenol (300 microM) was used as substrate, 4-aminophenol disappeared from the medium within 30 min. Secretion of conjugates continued for more than 60 min, when 70% of the 4-aminophenol had been secreted as conjugates. At a low concn. of 4-aminophenol, the sulphate ester was the main metabolite, while secretion of the glucuronide showed a more than proportional rise ('lag phase') with increasing substrate concn. At higher concn. (greater than 300 microM) about equal amounts of both conjugates were formed. Without inorganic sulphate, sulphation of 4-aminophenol was greatly diminished and the lag phase in glucuronide secretion was not found. With 1 mM aniline as substrate up to 300 microM of conjugated 4-aminophenol was secreted with a linear time-dependence for at least two hours. With aniline as substrate the sulphate ester was the most important conjugate and lag phases in the secretion of both conjugates were minimal. Phenobarbitone pretreatment in vivo stimulated the secretion of conjugated products after incubation with aniline. No dramatic changes in the profile of the lag phases were seen. The differences in the conjugation profiles of both substrates can be explained by taking into consideration the differences in the expected intracellular concentrations.

Drug metabolism activities of isolated rat hepatocytes in monolayer culture

The levels of cytochrome P-450 in hepatocytes cultured as monolayers for 22 hrs in Dulbecco's modified Eagle medium supplemented with serum and insulin was reduced to approximately 40% of initial values of freshly isolated hepatocytes. In correspondence with this the activities of the cytochrome P-450 monooxygenases aryl hydrocarbon (benzo(a)pyrene) hydroxylase (AHH) and ethylmorphine (EM) N-demethylase were reduced to 40 and 22% of their initial activities, respectively. Modifying the culture medium through omission of cysteine and cystine, and adding dexamethazone and delta-amino levulinic acid, increased the content of cytochrome P-450 to 59% and EM N-demethylase to 46% of initial values, but was without effect on AHH activity. However, further modifications by adding high concentrations of asparagine and leucine increased AHH activity to 62% of initial values, but did not further enhance the total content of cytochrome P-450 or the EM N-demethylase activity. The activities of cytochrome P-450 reductase, flavin containing monooxygenase, epoxide hydrolase and glutathione S-transferase decreased less (to about 70-80% of initial values) than cytochrome P-450 associated monooxygenase activities, whereas UDP-glucuronyl transferase decreased to about 50% of initial values. In contrast to what was observed regarding cytochrome P-450 and associated monooxygenase activities, modification of the incubation conditions did not affect the non-cytochrome P-450 enzymatic activities.

Induction of liver microsomal cytochrome P-450 and associated monooxygenases by octachlorostyrene in inbred strains of mice. Lack of correlation with the murine Ah locus

Single intraperitoneal injections of octachlorostyrene (OCS) and hexachlorobenzene in genetically polycyclic aromatic hydrocarbon "responsive' C57/BL/6 (B6) mice led to a time- and dose-dependent increase in the levels of liver microsomal cytochromes P-450 and b5 as well as in the activities of NADPH cytochrome P-450 (cytochrome c) reductase, ethylmorphine (EM) N-demethylase, 4-nitroanisole (PNA) O-demethylase and acetanilide 4-hydroxylase (AcA hydroxylase). No, or only a very moderate, increase in the activity of aryl hydrocarbon hydroxylase was seen after OCS and HCB, respectively. Pretreatments with phenobarbital (PB) or 3-methylcholanthrene (MC) both increased AcA hydroxylase activity to a similar degree, whereas pretreatment with polychlorinated biphenyls (Aroclor 1254) had an effect equal to the sum of PB and MC. Judged from sodium dodecylsulfate polyacrylamide gel electrophoresis studies, OCS and HCB predominantly increased a microsomal polypeptide of apparent mol. wt 52,000, similar to PB. A reduced response was seen after OCS or HCB treatment of aromatic hydrocarbon "non-responsive' DBA/2 (D2) mice compared to B6 mice, both with respect to AcA hydroxylase as well as EM demethylase and PNA demethylase activities. OCS treatment of B6D2F1 mice resulted in a doubling of AcA hydroxylase activity, but in mice of the (B6D2)D2 backcross no distinct subgrouping of individual AcA hydroxylase activities were apparent. These results demonstrate that OCS is an inducer of the PB-type in mice and that induction of AcA hydroxylase by OCS is not regulated by the Ah locus.

Inhibition of paranitroanisole and antipyrine monooxygenation in isolated rat hepatocytes by compounds interacting with mitochondrially related carbohydrate metabolism

The monooxygenation of paranitroanisole (PNA) and antipyrine (AP) were measured in isolated rat hepatocytes incubated with compounds interacting with mitochondrially related carbohydrate metabolism. Phenylpyruvate, an inhibitor of pyruvate carboxylase, reduced the rate of PNA and AP metabolism to about 60 and 20%, respectively, in hepatocytes both from fasted and fed rats. Inhibition of amino acid transaminase with aminooxyacetate, decreased the metabolism of both PNA and AP to 60-70% of control values in hepatocytes from fasted rats, whereas this effect was not seen in fed rats. n-Butylmalonate, an inhibitor or mitochondrial malate/phosphate exchange, had only minimal effects on PNA and AP monooxygenation in both the nutritional states. The simultaneous presence of glyoxylate and pyruvate, known to inhibit the NADPH specific isocitrate dehydrogenase, reduced the metabolism of both PNA and AP in hepatocytes from fasted rats to about 60 and 35% of control values respectively, while the effect was not so marked in hepatocytes from fed rats. The metabolism both of PNA and of AP in hepatocytes from fasted rats was reduced to 50-60% of control values with the addition of NH4Cl. This effect could be blocked either by incubating the hepatocytes with pyruvate or by using hepatocytes isolated from fed rats. The addition of various carbon intermediates generally reduced the effect of the inhibitors used. Phenobarbital-treatment did not change the effects observed with cells from uninduced animals. The inhibitors did not alter PNA or AP metabolism in microsomal incubations, and therefore most likely reduced the monooxygenation in intact cells by affecting NADPH generation pathways.

Pharmacokinetic evidence for the occurrence of extrahepatic conjugative metabolism of p-nitrophenol in rats

p-Nitrophenol (PNP), as a model compound for the study of conjugative metabolism, was administered intravenously to rats. PNP and its conjugated metabolites, i.e. PNP-glucuronide (PNP-Glu) and PNP-sulfate (PNP-Sul), were determined in body fluids by reversed-phase high-performance liquid chromatography using ion-pair systems. Linear pharmacokinetics was applicable in the dose range of 1.6 to 8 mg/kg. The metabolic clearance which was obtained from the area under the PNP blood concentration curve (AUCiv) and from the excretion ratio of the total conjugates as PNP-Glu and PNP-Sul was so close to the hepatic blood flow that the PNP conjugation reactions seemed to be limited by the hepatic blood flow, that is the hepatic extraction ratio (EH) was expected to be 1. However, AUCpv, following portal vein administration of PNP (4 mg/kg), was not zero but was significantly different from AUCiv after the same dosing (P less than 0.05). Consequently, comparison between the AUC values from both dosing routes and the excretion ratio of PNP-Glu and PNP-Sul gave and EH of 0.43. Such a difference in EH obtained by the two methods suggested a contribution by extrahepatic conjugative metabolism. It was shown that the intrinsic hepatic clearance obtained, assuming exclusively hepatic conjugative metabolism, was certainly overestimated. Furthermore, the results of the conjugation reaction in tissue homogenates suggested a contribution by extrahepatic glucuronidation.

Induction of liver microsomal cytochrome P-450 and associated monooxygenases by octachlorostyrene in the rat

The inducing effects of octachlorostyrene (OCS) and hexachlorobenzene (HCB), chlorinated hydrocarbon contaminants found in fish, on the microsomal cytochrome p-450 system were studied in rats. Administration of single doses of OCS and HCB at 50mg/kg intraperitoneally and higher led to increases in microsomal protein an cytochrome p-450 content, cytochrome p-450 reductase, ethylmorphine N-demethylase, 4-nitroanisole O-demethylase, acetanilide 4-hydroxylase, but not in benzo(a)pyrene hydroxylase activities. Similar patterns of induction were seen after 14 days repeated intraperitoneal or oral dosing. Siodium dodecyl sulfate polyacrylamide gel electrophoresis of microsomes from OCS and HCB treated rats showed increases in proteins similar to those found after phenobarbital (PB) induction. Under the present experimental conditions OCS and HCB were found to be PB-type inducers.

Carcinogen metabolism and carcinogen-induced strand breaks in DNA of isolated liver cells

Viable rat hepatocytes were isolated and incubated for various times in suspension or monolayer. RNA and DNA synthesis and the contents of cytochromes P450 and b5, as well as microsomal functions, were studied throughout the incubation. Incubation with dimethylnitrosamine (DMN) resulted in protein and RNA alkylation that were linear with time over a period of 4 hours. Correspondingly the appearance of single strand breaks in DNA could be demonstrated following in vivo or in vitro administration of the carcinogen.

THe formation of cytochrome P-450-metabolic intermediate complexes from amines, in the isolated perfused rat liver

1. Cytochrome P-450-metabolic intermediate (MI) complexes were formed from SKF 525-A, propoxyphene, acetylmethadol, noracetylmethadol, norbenzphetamine, and N-hydroxyamphetamine, but not methadone, in the isolated perfused rat liver. 2. The amount of MI complex from SKF 525-A (8% of the cytochrome P-450) after 1 h exceeded that for all other compounds (1--2%). 3. Both MI complex and residual substrate contributed to the inhibition of mixed-function oxidase activity observed. No substrate altered the total cytochrome P-450 concentration of NADPH-cytochrome c reductase activity. 4. The formation of MI complexes in isolated perfused liver corresponds to that seen in whole animals, and contrasts with that seen in microsomal preparations.

Differential inhibition of biphenyl hydroxylation in perfused rat liver

A differential inhibition of biphenyl hydroxylation by alpha-naphthoflavone and metyrapone was observed in isolated perfused rat liver. alpha-Naphthoflavone inhibited 2- and 4-hydroxylation in livers from beta-naphthoflavone-pretreated animals but had no effect on both reactions in livers from phenobarbital-pretreated animals. Metyrapone inhibited 2- and 4-hydroxylation in phenobarbital-stimulated livers, but only insignificant inhibition of 2-hydroxylation and a slight enhancement of 4-hydroxylation by metyrapone was observed in beta-naphthoflavone-stimulated livers. Conjugation of 2-hydroxybiphenyl and 4-hydroxybiphenyl by isolated perfused livers was also studied. 4-Hydroxybiphenyl preferentially formed sulphates in livers from untreated animals but after induction glucuronidation was as effective as sulphation or even exceeded sulphation. Only glucuronic acid conjugates of 2-hydroxybiphenyl were detected.

Reductive drug metabolism in isolated perfused rat liver under restricted oxygen supply

1. Hepatic azo and nitro reductase activities were studied in the perfused rat liver under normal and restricted oxygen supply. 2. Formation of sulphanilamide or p-aminobenzoic acid from neoprontosil or p-nitrobenzoic acid under aerobic conditions of liver perfusion was negligible, even at a reduced oxygen saturation of a pO2 of 300 mm Hg in the haemoglobinfree perfusion system. At a pO2 of 200 mm Hg reductase activities were almost maximal. 3. Conjugation of sulphanilamide (0-08 mM) was similar under aerobic and anaerobic conditions. Hepatic elimination of p-aminobenzoic acid (0-08 mM) showed an oxygen-dependent increase for 15 min after addition of substrate. 4. p-Nitroanisole demethylation was inhibited 80% under hypoxic perfusion at 200 mm Hg pO2 and was completely inhibited after gassing with anoxic mixtures. 5. Restitution of aerobic conditions after 30 min anaerobic perfusion restored hepatic respiration, lactate pyruvate ratio, and pH value to levels found under aerobic conditions, but bile flow remained 50% reduced.