Specific labelling of microsomal proteins by reactive intermediates generated from 2-acetylaminofluorene in vitro

The effect of schistosomiasis on the covalent binding of 2-acetylaminofluorene to mouse liver macromolecules in vivo and in vitro

The covalent binding of [14C]acetylaminofluorene (AAF) to macromolecules in vivo and in vitro was measured in Schistosoma mansoni-infected and in non-infected mice. Liver microsomes from infected mice demonstrated a 42% decreased capacity to mediate covalent binding of AAF to DNA. In addition, the extent of binding of AAF to liver macromolecules in vivo was generally less in infected than non-infected mice.

Activation of microsomal glutathione transferase activity by reactive intermediates formed during the metabolism of phenol

The activity of microsomal glutathione transferase was increased 1.7-fold in rat liver microsomes which carried out NADPH dependent metabolism of phenol. Known phenol metabolites were therefore tested for their ability to activate the microsomal glutathione transferase. The phenol metabolites benzoquinone and 1,2,4-benzenetriol both activated the glutathione transferase in microsomes 2-fold independently of added NADPH. However, NADPH was required to activate the enzyme in the presence of hydroquinone. Catechol did not activate the enzyme in microsomes. The purified enzyme was activated 6-fold and 8-fold by 5 mM benzenetriol and benzoquinone respectively. Phenol, catechol or hydroquinone had no effect on the purified enzyme. When microsomal proteins that had metabolized [14C]phenol were examined by SDS polyacrylamide gel electrophoresis and fluorography it was found that metabolites had bound covalently to a protein which comigrated with the microsomal glutathione transferase enzyme. We therefore suggest that reactive metabolites of phenol activate the enzyme by covalent modification. It is discussed whether the binding and activation has general implications in the regulation of microsomal glutathione transferase and, since some reactive metabolites might be substrates for the enzyme, their elimination through conjugation.

Covalent binding of benzo[a]pyrene to cytochrome P-450 beta NF-B2 and other proteins in reconstituted mixed-function oxidase systems

A reconstituted mixed-function oxidase system, containing the major beta-naphthoflavone-induced isozyme of rat liver cytochrome P-450 bound benzo[a]pyrene covalently in the presence of NADPH. NADPH-cytochrome P-450 reductase was required for binding and a maximum rate of adduct formation was obtained at 8 units of reductase per nmol cytochrome P-450. Phosphatidylcholine inhibited this reaction. Benzo[a]pyrene was bound to the cytochrome, but not to the reductase, as shown by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Approximately 6 molecules of benzo[a]pyrene bound to each molecule cytochrome P-450 during prolonged incubations. No binding occurred when the beta-naphthoflavone-induced isozyme of cytochrome P-450 was replaced by the major isozyme induced by phenobarbital, but both cytochromes incorporated benzo[a]pyrene to approximately the same extent when they were incubated together in the presence of the reductase and NADPH. Metabolically activated benzo[a]pyrene also bound covalently to purified epoxide hydrolase, when this enzyme was added to the reconstituted mixed-function oxidase system.

Two-dimensional electrophoresis of membrane proteins. Factors affecting resolution of rat-liver microsomal proteins

A comparison has been made of published techniques for the resolution of rat liver microsomal proteins by two-dimensional electrophoresis. The method of Kaderbhai and Freedman (Biochim. Biophys. Acta 601 (1980) 21-20) gives good resolution of acidic proteins but excludes hydrophobic integral membrane proteins of pI greater than 7, including cytochrome P-450 apoproteins. The method of Vlasuk and Walz (Anal. Biochem. 105 (1980) 112-120) gives good resolution of proteins of pI 5-8, including cytochromes P-450, but fails to resolve a major acidic protein of pI less than 5. Isoelectric focusing of microsomal proteins is improved by the use of high concentrations of urea and low concentrations of sample proteins. Zwitterionic detergents of the general formula R . N+(CH3)2 . CH2CH2CH2SO3- are effective in solubilizing microsomal proteins, either alone or in presence of non-ionic detergent; compounds with a long alkyl chain (C14 or C16) are most effective. Isoelectric focusing of microsomal proteins solubilized by zwitterionic detergents did not give good resolution, probably because of incomplete dissociation and denaturation of the proteins. These detergents could not be used in the presence of high concentrations of urea. Although no single method of two-dimensional electrophoresis gives complete resolution of the whole range of microsomal proteins, conditions can be optimized for specific sets of proteins of interest. The technique can be used to monitor differences in microsomal composition between rat strains, or following induction, and for a variety of other studies.

Activation of 3,4,3',4'-tetrachlorobiphenyl to protein-bound metabolites by rat liver microsomal cytochrome P-448-containing monooxygenase system

The specificity of two major types of cytochrome P-450 of rat liver microsomes induced by phenobarbital (PB) and 3-methylcholanthrene (MC) toward activation of three 14C-labeled tetrachlorobiphenyl (TCB) isomers to protein-bound metabolites was examined by intact microsomal and reconstituted monooxygenase systems. The monooxygenase system containing PB-inducible form(s) of cytochrome P-450 was much more active than those inducible by MC in activating two TCB isomers, i.e., 2,4,2',5'-[14C]TCB, for the binding reaction. However, the binding of 3,4,3',4'-[14C]TCB was catalyzed most actively by the system containing a MC-inducible (P-448 type) cytochrome P-450 from rat liver microsomes. Thus two forms of cytochrome P-450 had different substrate specificities for the metabolism of TCB. In another series of experiments, the site of protein molecule bound to TCB metabolites was examined by the cytochrome P-448-containing reconstituted system with 3,4,3',4'-[14C]TCB as substrate. The following lines of evidence supported the hypothesis that the active metabolites of TCB bind covalently to cysteine residues of protein molecules a) TCB-binding activity was strongly inhibited by thiol compounds such as cysteine and glutathione and b) of the various kinds of proteins tested as target for TCB metabolites, only those which contained free sulfhydryl groups on the protein molecule were radiolabeled. Moreover, we found that during the metabolism of 3,4,3',4'-TCB, the heme content of the reconstituted system was lowered, probably reflecting the alkylation of the sulfhydryl groups in the active sites of cytochrome P-448.

Covalent binding of benzo[a]pyrene to rat liver cytosolic proteins and its effect on the binding to microsomal proteins

Benzo[a]pyrene will bind covalently to rat liver cytosolic proteins when incubated with microsomes and NADPH. The binding is most extensive when microsomes from 3-methylcholanthrene-treated rather than phenobarbital-treated or control rats are used. The binding to cytosolic proteins increases when incubations are performed with increasing concentrations of cytosol. At the same time the covalent binding of benzo[a]pyrene to microsomal proteins decreases. Two cytosolic polypeptides are the main targets for benzo[a]pyrene. These have the same mobility in polyacrylamide gels as the subunits of purified glutathione S-transferase B. These subunits also react covalently with benzo[a]pyrene when the transferase is incubated with microsomes and NADPH.

Metabolic activation of carbon tetrachloride: induction of cytochrome P-450 with phenobarbital or 3-methylcholanthrene and its effect on covalent binding

Anaerobic incubation of [14C]carbon tetrachloride with normal rat liver microsomes and microsomes from rats treated with the inducers phenobarbital (PB) and 3-methylcholanthrene (MC) reveals distinct differences in metabolic activation. While the increase in CO-binding pigment is comparable for both inducers, metabolism of CC14 is enhanced only by PB-induction; MC-induced microsomes are equivalent to microsomes from untreated animals. Sodium dodecyl sulphate (SDS)-electrophoresis of microsomal proteins confirms the expected increase at 52 000 daltons (cytochrome P-450 PB) on PB-induction, at 56 000 daltons (cytochrome P-450MC) on MC-induction; after anaerobic incubation with [14C]CC14 the electrophoretic pattern is largely unchange. The highly reactive radical intermediates of CC14-metabolism should attack the closest possible partner. Most of protein-bound radioactivity is located in the mass range between 47 000 and 54 000 daltons, indicating that cytochrome P-450PB is the isoenzyme mainly responsible for CC14-activation; cytochrome P-450MC plays virtually no role in metabolic activation. The direct participation of NADPH-cytochrome P-450 reductase appears unlikely, since the specific binding to proteins in the corresponding mass range is not elevated. A significant percentage of label is attached to proteins at 120 000 daltons and above, presumably oligomers of cytochrome P-450 apoprotein.

Alterations in the enzyme activity and polypeptide composition of rat hepatic endoplasmic reticulum during acute exposure to 2-acetylaminofluorene

Studies have been made of the morphology, enzyme activity and protein composition of liver endoplasmic reticulum in rats exposed to acute doses of the carcinogen, 2-acetylaminofluorene (2-AAF). Electron microscopic examination revealed numerous ultrastructural changes in the hepatocyte; most consistent alterations were the disorganisation of endoplasmic reticulum system with apparent increase of smooth endoplasmic reticulum. Administration of 2-AAF to rats immediately depressed microsomal glucose-6-phosphatase activity and eventually induced epoxide hydratase activity 6--7-fold over control activity. The induction was time-dependent and maximal rates of induction were observed at dosages greater than 40 mg/kg body wt. The treatment also induced cytochrome b5 content, NADH and NADPH cytochrome c reductase activities (1.0--1.5-fold). Only very small changes in the total content of cytochrome P-45- were noted. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) of microsomal proteins from 2-AAF pretreated animals showed time-dependent induction of two polypeptides which differed slightly in migration, in the region of Mr = 48000; the fast-migrating induced polypeptide has been identified as epoxide hydratase. Two-dimensional PAGE analysis of microsomal proteins from 2-AAF exposed rats showed a reproducible deletion of a protein with molecular weight in the region of 67000. The basis for the alterations in the protein composition of endoplasmic reticulum in response to 2-AAF treatment is discussed.