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G J Mulder

Publications and source records attributed to G J Mulder.

At least 127 records · Page 7Linked to original sources
PubMed1988

The role of N-sulfation in the N-hydroxy-2-acetylaminofluorene-mediated outgrowth of diethylnitrosamine-initiated hepatocytes to gamma-glutamyltranspeptidase-positive foci in male rat liver.

The ability of N-hydroxy-2-acetylaminofluorene (N-OH-AAF) to promote the appearance of gamma-glutamyltranspeptidase-positive (GGT+) foci initiated by diethylnitrosamine (DEN) was studied in a slightly modified Solt and Farber protocol. This protocol consisted of the following treatments: initiation with a single dose of DEN followed by selection/promotion with several non-necrogenic doses of N-OH-AAF and partial hepatectomy. Treatment with N-OH-AAF resulted in a 25-fold increase of the liver volume occupied by GGT+ cells as compared to controls. The role of N-sulfation of N-OH-AAF in the GGT+ foci-selecting activity of N-OH-AAF was studied using the sulfotransferase inhibitor pentachlorophenol (PCP). Inhibition of the N-sulfation pathway with PCP during selection with N-OH-AAF resulted in a greater than 80% decrease in the volume occupied by GGT+ cells, without effects on the number of GGT+ foci generated with this protocol. Also, PCP reduced the number of so called oval and bile duct cells generated by the N-OH-AAF/partial hepatectomy treatment. It is concluded that N-sulfation of N-OH-AAF is responsible for the N-OH-AAF-mediated outgrowth of DEN-initiated hepatocytes to preneoplastic GGT+ foci.

2-Acetylaminofluorene↗
PubMed1988

Sulphation and glucuronidation of xamoterol in the dog: dose dependence and site of sulphation.

1. Xamoterol has been administered both intravenously and orally over a 100-fold dose range to male beagle dogs. 2. Over the dose range examined, sulphation was not saturable, with the proportion of the dose excreted as the sulphate conjugate remaining constant. 3. Extensive first-pass sulphation of an oral dose of xamoterol occurred in the intestine with approximately 50% of sulphation occurring during absorption. 4. The intestine is not a major site of sulphation for circulating xamoterol. 5. The liver is not believed to play an important role in the first-pass sulphation of xamoterol.

Administration, Oral↗
PubMed1987

The covalent binding of cyclosporin A to rat liver macromolecules in vivo and in vitro: the role of cytochrome P-450.

Incubation of rat liver S9 with [3H]cyclosporin A ([3H]CSA) resulted in covalent binding of CSA to macromolecules. Binding was dependent on the presence of NADPH and could be inhibited by SKF-525A. Incubation of isolated rat liver parenchymal cells with CSA resulted in a concentration-dependent binding which increased with incubation time for at least 2 h. Addition of SKF-525 A (0.5 mM) decreased the binding by 83% while viability of the cells was unchanged. Pretreatment of the cells with diethylmaleate doubled the binding of CSA, indicating that glutathione can prevent binding of CSA. When [3H]CSA was injected i.v. radioactivity was covalently bound to liver and kidney macromolecules. Induction of cytochrome P-450 by phenobarbital resulted in enhanced in vivo covalent binding in liver and kidney. Whether this covalent binding of CSA is related to its cytotoxicity is yet unclear.

Animals↗
PubMed1987

Sex differences in sulfation and glucuronidation of phenol, 4-nitrophenol and N-hydroxy-2-acetylaminofluorene in the rat in vivo.

Sulfation and glucuronidation of phenol, 4-nitrophenol (4NP) and N-hydroxy-2-acetylaminofluorene (N-OH-AAF) were studied in adult (60 days) male and female rats. Within 3 hours almost 50% of a dose of phenol was excreted in urine as phenyl sulfate; male rats excreted slightly more phenyl sulfate than females. This probably was due to a slower excretion of phenyl sulfate by the females. No sex difference in glucuronidation of phenol was found. Over a period of 24 hours male and female rats excreted almost 35% of a dose of 4NP as 4NP-sulfate in urine and almost 40% as 4NP-glucuronide. No differences in the excretion of 4NP-conjugates were found between sexes. However, almost twice as much of a dose of N-OH-AAF was excreted after 4 hours as the N-O-glucuronide in bile and urine in female than in males. On the other hand, females excreted less of the AAF-glutathione conjugates that are derived from the reaction of AAF-N-sulfate with glutathione in vivo [Meerman et al., Chem.-Biol. Interactions, 39, 149, 1982] in bile, than males. This indicates that sulfation of N-OH-AAF is less active in females than in males. Most likely, sulfation of the phenols is catalyzed by a different sulfotransferase than that of N-OH-AAF.

2-Acetylaminofluorene↗
PubMed1987

Prevention by thioethers of the hepatotoxicity and covalent binding to macromolecules of N-hydroxy-2-acetylaminofluorene and its sulfate ester in rat liver in vivo and in vitro.

In order to find potentially effective compounds that could prevent the covalent binding of the carcinogen N-hydroxy-2-acetylaminofluorene (N-OH-AAF) to rat liver macromolecules in vivo, the prevention of the covalent binding to RNA of the sulfate ester of the carcinogen N-OH-AAF by a series of thioethers was investigated in vitro. The most effective thioethers, which inhibited the covalent binding by 70% or more, were studied for their protection against acute hepatotoxicity of N-OH-AAF in the rat in vivo. Three of these thioethers, thiazolidine, methyl 4-(methylthio)benzoate, and 2-(methylthio)benzimidazole significantly decreased the hepatoxicity of N-OH-AAF, by 45, 71 and 83%, respectively. The effects of these thioethers on the covalent binding of N-OH-AAF to cellular macromolecules in vivo were also studied. Methyl 4-(methylthio)benzoate and 2-(methylthio)benzimidazole decreased the adduct formation of N-OH-AAF to DNA by 54 and 44%, respectively, but had no effect on protein adduct formation. Only 2-(methylthio)benzimidazole caused a slight decrease (23%) in the AAF-- protein adduct formation. 2-Acetylaminofluorene (AAF) and methyl 4-(methyl-sulfinyl)benzoate were the main products in the incubation of methyl 4-(methylthio)benzoate with AAF-N-sulfate in vitro. This suggests that the thioether attacks the nitrenium ion which is formed by spontaneous breakdown of AAF-N-sulfate; the formation of a sulfonium--AAF conjugate is postulated which decomposes into AAF and a sulfinyl compound.

2-Acetylaminofluorene↗
PubMed1987

Autoradiographic studies on in vivo covalent binding of N-hydroxy-2-acetylaminofluorene in rat liver containing gamma-glutamyltranspeptidase-positive foci. The effect of the sulfation-inhibitor pentachlorophenol.

The role of sulfation in the covalent binding of [ring-3H]-N-hydroxy-2-acetylaminofluorene (N-OH-AAF) in rat liver containing gamma-glutamyltranspeptidase-positive (GGT+) foci was studied in vivo by autoradiography. GGT+ foci were induced by initiation with diethylnitrosamine, followed by a selection protocol consisting of a 2-week exposure to 2-aminofluorene in the drinking water and a single administration of CCl4. Both surrounding ('normal') liver tissue and, to a lesser extent, GGT+ foci covalently bound N-OH-AAF, administered 10 days after selection. The sulfation inhibitor, pentachlorophenol (PCP), reduced the covalent binding of N-OH-AAF in cells surrounding the foci. However, PCP had no effect on binding of radiolabel in GGT+ foci. Thus, reduced sulfotransferase activity may contribute to the resistance of GGT+ preneoplastic lesions to carcinogen cytotoxicity. The results suggest also, that cells in GGT+ foci are able to bind N-OH-AAF covalently by a sulfotransferase-independent pathway.

2-Acetylaminofluorene↗
PubMed1987

Pharmacokinetics of xamoterol glucuronidation in the rat in vivo and in liver perfusion.

Xamoterol has a phenolic hydroxyl group at which both sulphation and glucuronidation may occur. In the rat in vivo it is almost exclusively glucuronidated. In bile the glucuronide conjugate is the only metabolite of xamoterol. In urine equal amounts of the glucuronide conjugate and unchanged xamoterol are present, together with a small amount of the sulphate conjugate (approx. 1% of the dose). The t1/2 of excretion of the glucuronide in bile was the same as the t1/2 for excretion of unchanged xamoterol in urine. The t1/2 for urinary excretion of the glucuronide conjugate was longer than that for its biliary excretion. In the isolated perfused rat liver glucuronidation was also the almost exclusive metabolic pathway. The extraction ratio of xamoterol by the liver was approx. 0.15. Since xamoterol is a very water-soluble compound, and is almost exclusively glucuronidated in the rat, it is an excellent model substrate for investigations on the pharmacokinetics of glucuronidation in the rat in vivo and in isolated perfused organs.

Animals↗
PubMed1987

Stereoselective glutathione conjugation and amidase-catalyzed hydrolysis of alpha-bromoisovalerylurea enantiomers in isolated rat hepatocytes.

alpha-Bromoisovalerylurea (BIU) is used as model substrate for studies on the pharmacokinetics of glutathione conjugation in vivo. Its metabolism in isolated rat hepatocytes is presently studied. A major part of the substrate was conjugated with glutathione, but also amidase-catalyzed hydrolysis occurred, resulting in the products urea and alpha-bromoisovaleric acid (BI). The amidase activity was located in the microsomal fraction of the rat liver. The product of hydrolysis, BI, also was conjugated efficiently with glutathione. In glutathione-depleted hepatocytes, no glutathione conjugates but only urea and BI were formed. A pronounced stereoselectivity in the metabolism of the BIU enantiomers was observed: (R)-BIU was conjugated with glutathione much faster than (S)-BIU. (S)-BIU was hydrolyzed substantially in the cells and the glutathione conjugate of the hydrolytic product, (S)-BI, could be detected. At high BIU concentrations (500 microM of the racemate) intracellular glutathione was seriously depleted; then, the cosubstrate availability most likely was the rate-limiting factor in the conjugation of BIU with glutathione. More urea was formed from (racemic) BIU in isolated rat hepatocytes in the present study than in the perfused liver and the intact rat in previous studies. This in vivo-in vitro difference is tentatively assigned to differences in glutathione availability in these systems. The results suggest that BI may also be a useful model substrate to study the kinetics of glutathione conjugation in vivo and in vitro.

Amidohydrolases↗
PubMed1986

The binding of reactive metabolites of the carcinogen N-hydroxy-2-acetylaminofluorene to DNA and protein in isolated rat liver nuclei: effects of glutathione and methionine.

The covalent binding of reactive metabolites of the carcinogen N-hydroxy-2-acetylaminofluorene to DNA and protein in isolated, intact rat liver nuclei was studied. The chemically synthesized 2-acetylaminofluorene-N-sulfate became covalently bound to DNA and protein to form adducts, 50% to 60% of which retained the N-acetyl group. Glutathione decreased the covalent binding of acetylated adducts to DNA by 18% and to protein by 50%. Methionine was more effective; it decreased DNA binding by 52% and protein binding by 79%. N-Hydroxy-2-acetylaminofluorene was deacetylated by the nuclear preparation. Almost exclusively, deacetylated 2-aminofluorene adducts to DNA and protein were formed. Glutathione decreased the covalent binding of deacetylated adducts to DNA by only 14%. Protein binding, however, was decreased by 57%. Methionine had no effect on the formation of these adducts to DNA and protein. Formation of 2-aminofluorene-glutathione conjugates was reduced by ascorbic acid by 65%. Covalent binding of deacetylated adducts to DNA and protein, however, was not decreased by ascorbic acid. These data suggest that "harder" nucleophiles like methionine can be used to protect macromolecules in vivo from damage by "hard" electrophiles such as those generated from the reactive 2-acetylaminofluorene-N-sulfate. However, such nucleophiles seem not to be effective with N-hydroxylamines, such as N-hydroxy-2-aminofluorene, formed by deacetylation of N-hydroxy-2-acetylaminofluorene.

2-Acetylaminofluorene↗
PubMed1986

Kinetics of sulfation in the rat in vivo and in the perfused rat liver.

Sulfation of phenols and similar low-molecular-weight substrates in the rat in vivo is a rather complex process. Besides enzyme kinetic parameters, cosubstrate availability (indirectly measured by serum sulfate concentration) and competition with glucuronidation also play a role. For some substrates extensive extrahepatic sulfation occurs, accounting for more than 50% of the total-body sulfation capacity. However, the hepatic contribution may be under-estimated when drugs are administered into the hepatic portal vein, because saturation of hepatic metabolism may occur under those conditions. Inside the liver, sulfation is located primarily in zone 1, the periportal area. This can be shown in the single-pass perfused rat liver by perfusion in either the normal or retrograde flow direction. In the rat sulfate conjugates are eliminated preferentially in urine, whereas glucuronides are excreted to a high extent in bile. Therefore, it is important to collect both bile and urine in the characterization of pharmacokinetics of conjugation in vivo. Selective inhibition of sulfation by pentachlorophenol and 2,6-dichloro-4-nitrophenol facilitates studies of the role of sulfation in elimination of its substrates, and the competition between sulfation and glucuronidation for the same substrate.

Alkaloids↗
PubMed1986

alpha-Bromoisovalerylurea as model substrate for studies on pharmacokinetics of glutathione conjugation in the rat. I. (Bio-) synthesis, analysis and identification of diastereomeric glutathione conjugates and mercapturates.

In order to find a model substrate for kinetic characterization of glutathione conjugation in vivo alpha-bromoisovalerylurea (BIU) was studied. After administration of racemic [14C]urea BIU to rats, two radioactive metabolites were found in bile by high-performance liquid chromatography. The identity of these metabolites was established by various methods. Based on the hydrolytic activity of gamma-glutamyltranspeptidase (presence of the gamma-glutamyl moiety), high resolution nuclear magnetic resonance (isovaleryl and glutathionyl moieties) and fast atom bombardment mass spectrometry (molecular weight and fragmentation pattern), they were identified as glutathione conjugates of BIU. Because both conjugates in bile had these characteristics in common they must be diastereomers. Incubation of BIU with glutathione in the presence of rat liver cytosol resulted in formation of the same diastereomeric glutathione conjugates. Chemical synthesis of the diastereomers confirmed their identity. The major urinary excretion products of [14C]urea BIU in the rat were identified as diastereomeric mercapturates. A convenient chromatographic separation of the diastereomeric glutathione conjugates and the mercapturic acids is described. Electrochemical detection was used to determine the presence of the thioethers in both urine and bile. Pharmacokinetic results on BIU conjugation are described in the accompanying paper.

Acetylcysteine↗
PubMed1986

alpha-Bromoisovalerylurea as model substrate for studies on pharmacokinetics of glutathione conjugation in the rat. II. Pharmacokinetics and stereoselectivity of metabolism and excretion in vivo and in the perfused liver.

The hypnotic drug alpha-bromoisovalerylurea (BIU) has been studied in the rat with respect to its potential use as model substrate to investigate the pharmacokinetics of glutathione conjugation in vivo. The major metabolites of racemic BIU are the diastereomeric glutathione conjugates (bile) and mercapturates (urine). BIU was metabolized mainly by glutathione conjugation: after i.v. administration of [14C]BIU to freely moving rats, 89% of the dose was recovered in urine within 24 hr, mostly as mercapturates. The rate-limiting step in the clearance of BIU from blood most likely is glutathione conjugation as it was shown that rate-limitation is not due to flow-limited clearance in the liver (the initial extraction ratio of BIU in the perfused liver preparation was low: hepatic extraction ratio = 0.23), protein binding (60% was unbound in plasma) or enzyme saturation (linear pharmacokinetics in the dose range studied: 22-270 mumol/kg). Water solubility of BIU was sufficient to allow its i.v. administration, whereas the absence of toxic effects enables animal as well as human studies. Thus, BIU is a promising model substrate for studies of glutathione conjugation in vivo. In pentobarbital-anesthetized rats with a bile duct catheter, equal amounts of metabolites were excreted in bile (almost exclusively as the two diastereomeric BIU glutathione conjugates) and urine (mostly as the two diastereomeric mercapturates). Based on similar experiments with bile duct-ligated rats, it was concluded that the appearance of the mercapturates in urine could also occur without biliary excretion and subsequent gut metabolism of the BIU glutathione conjugates. The ability of the liver to metabolize BIU was studied in a hemoglobin-free, recirculating liver perfusion system. Of the recovered radioactivity 40% was excreted in bile within 2 hr, almost exclusively in the form of the two BIU glutathione conjugates. Also, glutathione conjugates were found in the perfusate (16% of the radioactivity present in the perfusate after 2 hr). A distinct stereoselectivity was observed in the metabolite excretion rates. The excretion half-lives of the two diastereomeric glutathione conjugates in bile differed 2- to 3-fold, both in anesthetized rats and in the perfused liver preparation. A similar difference in excretion half-lives was found for the urinary excretion of the diastereomeric mercapturates. Thus, BIU can be used to investigate in vivo the stereoselectivity of glutathione conjugation.

Animals↗
PubMed1985

Secretion of the organic anion harmol sulfate from liver into blood. Evidence for a carrier-mediated mechanism.

In the liver, drugs with phenolic groups can be converted to sulfate or glucuronide conjugates and are then transported into bile or back into the bloodstream. The mechanism for transport of drugs and drug conjugates from the hepatocytes into the blood at the sinusoidal side of the cell are not well defined. In the case of carrier-mediated transport of these strongly polar conjugates, saturability of transport and mutual competition between structurally related compounds would be anticipated. This competitive aspect was investigated by using two organic anions, dibromosulfophthalein (DBSP) and harmol sulfate. The latter compound was generated by the hepatocytes from harmol, which was continuously infused into the rat in vivo and in isolated perfused rat livers. In addition we loaded the perfused rat livers with preformed harmol sulfate and studied its efflux rate to the perfusate as influenced by DBSP. In steady state, about 80% of harmol was sulfated and 20% was glucuronidated. Harmol sulfate was mainly excreted in the urine, the glucuronide was equally excreted in urine and bile. DBSP lowered the urinary excretion of harmol sulfate by about 30% which was due to a decrease in plasma concentration. However, renal clearance of harmol sulfate (3.2 +/- 0.2 ml/min) was unchanged. At the same time DBSP doubled the biliary clearance of harmol sulfate (1.0 +/- 0.1 and 2.2 +/- 0.2 ml/min in controls and DBSP studies respectively). DBSP decreased glucuronide excretion both in urine and bile. The increase in biliary output and decrease in urinary excretion of harmol sulfate is explained by competitive interaction between the organic anion DBSP and harmol sulfate at the sinusoidal level. Efflux experiments in single pass perfused isolated livers showed a clear decrease of harmol sulfate transport from liver into plasma by DBSP and provided evidence for such an inhibitory phenomenon (t 1/2 of efflux was 3.58 +/- 0.21 compared with 2.46 +/- 0.07 min for controls). These results indicate that organic anion transport from the hepatocyte into the blood stream is very likely carrier-mediated. A decrease in renal output of drug conjugates therefore may not only be due to a decrease in the conjugation process but also to a lower liver to blood transport rate which at the same time may produce a higher biliary output.

Alkaloids↗
PubMed1985

Research in the Division of Toxicology. The importance of reactive intermediates of drug biotransformation in drug toxicity.

Within the Center for Bio-Pharmaceutical Sciences research on toxicology has its main object in toxification and detoxification of xenobiotics. The pharmacokinetics of sulfation, glucuronidation and other conjugation reactions is studied. Conjugation may prevent tissue damage that is caused by covalent binding of reactive intermediates. Systems for testing the toxicity of such intermediates are being developed, and a similar screening programme is pursued for elucidating the involvement of reactive intermediates in carcinogenesis.

Academies and Institutes↗
PubMed1985

Glucuronidation and sulfation in the rat in vivo. The role of the liver and the intestine in the in vivo clearance of 4-methylumbelliferone.

The role of the liver in the conjugation of 4-methylumbelliferone (4MU), mainly glucuronidation, was investigated in the rat in vivo. The liver extracted 4MU almost completely (97%) during steady-state infusion, as measured by the difference between 4MU concentration in portal and hepatic venous blood. Previously, it was shown that the intestinal region extracts 40% of the 4MU of the incoming arterial blood. The liver and the gastrointestinal region are so efficient that their conjugation activity can account for total body clearance of 4MU (50-60 ml/min per kg). These results and other evidence on extrahepatic conjugation of phenolic substrates suggest that glucuronidation may be limited to the liver, (the kidney) and the gastrointestinal region, while sulfation may occur more widespread throughout the body. Protein binding studies showed the sulfate conjugate to be even more protein-bound than unconjugated 4MU, while 4MU glucuronide was much less bound to rat plasma proteins.

Animals↗
PubMed1985

Reaction of 2-acetylaminofluorene-N-sulfate with RNA and glutathione: evidence for the generation of two reactive intermediates with different reactivities towards RNA and glutathione.

The sulfate ester of N-hydroxy-2-acetylaminofluorene (AAF-N-sulfate) is one of the reactive intermediates of this carcinogen. This ester breaks down spontaneously to a very reactive nitrenium ion, which reacts with nucleophilic groups in protein, DNA, RNA and glutathione (GSH). Reactions involving the nitrenium ion with several nucleophiles under various conditions were studied. The adduct formation to RNA was much higher in Tris-HCI buffer than in phosphate buffer (at pH 7.4), while adduct formation to deoxy-guanosine monomers was the same in both buffers. The presence of 150 mM KCI had the same decreasing effect in both cases. Ionic strength effects may be involved in these phenomena. GSH decreased RNA adduct formation by 20-45%, while other thiols were much more effective. On the other hand, RNA did not decrease the formation of GSH conjugates from AAF-N-sulfate. The decrease in RNA adduct formation by thiols corresponded with an increase in the formation of 2-acetylaminofluorene (AAF) from AAF-N-sulfate, while no N-hydroxy-AAF was formed. These results suggest that two independent reactive intermediates are formed from AAF-N-sulfate, with different reactivities towards RNA and glutathione. Possibly these intermediates are the 'hard' triplet state nitrenium ion and the 'soft' singlet state nitrenium ion. Cysteine, cysteamine and penicillamine were most effective in the inhibition of RNA adduct formation; the extent of inhibition correlated with the extent of AAF formation. The mechanisms involved are discussed.

2-Acetylaminofluorene↗
PubMed1985

Alteration of transit time and direction of flow to probe the heterogeneous distribution of conjugating activities for harmol in the perfused rat liver preparation.

Previous studies in the once-through perfused rat liver preparation have shown that the techniques of normal and retrograde delivery of substrate and computer simulation of enzyme distributions along the sinusoidal flow path in liver were useful in delineating the relative distributions of sulfation and glucuronidation activities for harmol metabolism (Pang et al., J. Pharmacol. Exp. Ther. 224: 647-653, 1983). The observed steady-state hepatic extraction ratios of harmol and the steady-state formation rates of harmol sulfate and harmol glucuronide were consistent with three enzyme-distribution models which described a proximal localization of sulfation activity and a more distal distribution of glucuronidation activities. The present study was a further refinement on the definition of the distribution of these activities. The experimental approach used included the perturbation of delivery of harmol (10 microM) at 8, 12 and 16 ml/min by normal flow, and the alternation of normal and retrograde directional flows for delivery of harmol (10 microM) at constant hepatic blood flows (8, 12 or 16 ml/min) in the single-pass perfused rat liver preparation. The observed steady-state sulfation and glucuronidation rates were compared against predicted values afforded by models previously shown to be adequate and additional enzyme-distributed models. The observed and predicted data point to a periportal distribution of sulfation activity and an even distribution of glucuronidation activity for the metabolism of harmol.

Alkaloids↗