Two specific azodye-carcinogen-binding proteins of the rat liver. The identity of amino acid residues which bind the azodye

Liver function from Y to Z: The guidance of William Jakoby

In the 1960s, my lab was interested in understanding how bilirubin and other organic anions are transferred from the plasma through the liver cell and into the bile. We performed gel filtration of liver supernatants and identified two protein fractions, designated Y and Z, which bound organic anions including bilirubin, and thus we proposed that they were involved in hepatic uptake of organic anions from plasma. Subsequently, the Y and Z proteins responsible for this binding activity were purified, cloned, and sequenced. With Bill Jakoby, we identified Y protein as a member of the glutathione S-transferase (GST) protein family. In separate studies, Z was found to be a member of the fatty acid-binding protein (FABP) family. These proteins have since been shown to have additional surprising roles, but understanding of their full role in physiology and disease has not yet been achieved. In the 1960s, bilirubin metabolism was a "hot" topic. Along with other groups, my lab was studying various forms of inheritable jaundice in an effort to dissect the mechanism of bilirubin's transfer from plasma into the hepatocyte and its role in intracellular metabolism and biliary secretion. These processes were eventually identified and found to be related to the basic mechanisms whereby the liver handles many anionic drugs, metabolites, and hormones. Because the mechanism of hepatic uptake of bilirubin was unknown, A.J. Levi, Z. Gatmaitan, and I took advantage of advances in gel permeation chromatography to study this process. In 1969, we described two hepatic cytoplasmic protein fractions, designated Y and Z, that bound bilirubin and various organic anionic dyes in vivo and in vitro and, based on tissue distribution, abundance, and effects of genetic and pharmacologic models, were proposed to participate in organic anion uptake (Levi et al., 1969) [1]. In the decades since then, the Y and Z proteins have been identified as members of large protein families that were cloned and sequenced. Several surprising functions emerged, whereas others are proposed based on binding properties. Many challenges remain in understanding the full role of these proteins in physiology and disease.

Liver function from Y to Z

In the 1960s, my lab was interested in understanding how bilirubin and other organic anions are transferred from the plasma through the liver cell and into the bile. We performed gel filtration of liver supernatants and identified two protein fractions, designated Y and Z, which bound organic anions including bilirubin, and thus we proposed that they were involved in hepatic uptake of organic anions from plasma. Subsequently, the Y and Z proteins responsible for this binding activity were purified, cloned, and sequenced. Y was identified as a member of the glutathione S-transferase (GST) protein family and Z found to be a member of the fatty acid–binding protein (FABP) family. These proteins have since been shown to have additional surprising roles, but understanding of their full role in physiology and disease has not yet been achieved.

Site-directed inactivation of human lung acidic glutathione S-transferase by 1-chloro-2,4-dinitrobenzene in the absence of glutathione

Human lung acidic glutathione S-transferase is irreversibly inhibited by 1-chloro-2,4-dinitrobenzene (CDNB) in the absence of the co-substrate glutathione (GSH). The time-dependent inactivation is pseudo-first-order and demonstrates saturation kinetics, suggesting that inactivation occurs from an EI complex. The Ki was 0.14 mM; and kobs was 0.32 min-1 at 0.6 mM CDNB. The enzyme was protected against CDNB inactivation by GSH. The other two classes of glutathione S-transferase, the basic and near-neutral, are not significantly inactivated by CDNB. Incubation with [14C]CDNB indicated covalent binding to all three classes of transferase. One peptide fraction was found to be radiolabelled in both the basic and acidic transferases when these were incubated with [14C]CDNB and GSH, cleaved with cyanogen bromide, and chromatographed by HPLC. Incubation in the absence of GSH yielded one and two additional labelled peptide fractions for the basic and acidic transferases, respectively. Our results suggest that while CDNB arylates all three classes of human transferases, only the acidic transferase possesses a specific GSH-sensitive CDNB binding site, binding to which leads to time-dependent inactivation.

The binding of an aminoazo dye carcinogen to a specific methionine residue in rat liver alcohol dehydrogenase in vivo

On the administration of 3'-methyl-N,N-dimethyl-4-aminoazobenzene to rats pure aminoazo dye-bound alcohol dehydrogenase accounting for 45% of the total soluble protein bound aminoazo dye is isolated from the liver soluble supernatant. Tryptic digestion of that purified aminoazo dye-bound enzyme yields an aminoazo dye-bound nonapeptide which has a sequence identical to amino acids 301-309 in the known sequence of alcohol dehydrogenase (H. Jornvall and O. Markovic, Eur. J. Biochem., 29 (1972) 167-174) with the exception of methionine 306 which is replaced by an aminoazo dye modified amino acid. The nature of the aminoazo dye adduct was determined by studying the structure of the related tetrapeptide obtained by Pronase B digestion and shown by proton NMR spectroscopy and fast atom bombardment mass spectroscopy to have the structure 3-(Val. Asn. Pro. Homocystein-S-yl)-4-methylamino-3'-methylazobenzene. This carcinogen-protein adduct is assumed to arise from attack of the ultimate carcinogenic metabolite, N-sulphonyloxy-4-methylamino-3'-methylazobenzene (FF. Kadlubar, J.A. Miller and E.C. Miller, Cancer Res., 36 (1976) 2350-2359) at the sulphur of methionine 306 followed by spontaneous S-demethylation. This highly specific reaction of carcinogen with alcohol dehydrogenase lowers its Vmax and increases its Km with cyclohexanone thereby reducing its catalytic efficiency for this substrate. This highly specific reaction of the carcinogen with alcohol dehydrogenase may be regarded as a major detoxication reaction.

Purification and characterization of fatty acid binding protein in mammalian lung

A fatty acid-binding protein (FABP) has been isolated and characterized from rat lung tissue. Rat lung FABP has a slightly higher molecular weight than liver FABP, but immunologically, lung FABP is similar to that of liver FABP. Long chain acyl CoA synthetase, a key enzyme in fatty acid metabolism is stimulated by partially purified lung FABP, suggesting a physiologic role of the protein in the activation of long chain fatty acids in pulmonary tissue.

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.

The role of glutathione in detoxication

Glutathione (GSH) is a strong nucleophile which reacts well with soft electrophiles, but poorly with both weak and strong electrophiles. Weak electrophiles have low reactivity with all nucleophiles while strong electrophiles react well with weak nucleophiles including superabundant H(2)O. There are enzymes, the GSH transferases, which catalyze GSH conjugation with all the types of electrophiles described above. In order to deal with the wide variety of potential substrates, a multiplicity of GSH transferases exists-each tissue having its own collection and each enzyme having a different substrate specificity. These enzymes are often very abundant, e.g., in the rat liver cytosol, their concentration is 0.2 mM. THE FOLLOWING SUBSTRATES ARE CONSIDERED IN SOME DETAIL: 1-chloro-2,4-dinitrobenzene, the electrophile derived metabolically from paracetamol N-acetyliminoquinone?), benzo(a)pyrene-4-5-oxide, cholesterol-5alpha,6alpha-oxide, benzo(a)pyrene-7,8-diol-9,10-oxide and the electrophiles derived metabolically from aflatoxin B(1) (the 2,3-oxide?). According to the substrate, optimal enzyme rates vary over seven orders of magnitude from 10(-5) to 10(-12) mole/min/mg. Despite the wide embrace of the GSH transferases, not all metabolically produced electrophiles are substrates. We know of the following examples: N-methylol-4-aminoazobenzene and its 4'-hydroxy derivative (these are soft electrophiles and react well with GSH noncatalytically), N-sulfonyloxy-N-methyl-4-aminoazobenzene, N-sulfonyloxy-N-acetyl-2-aminofluorene (these are strong electrophiles which do not react selectively with GSH) and N-hydroxy-2-aminofluorene which appears to react only slowly with GSH. It is of interest in the present context that all these compounds are derived from either arylamine or arylamide carcinogens. Whether the reaction be enzymic or nonenzymic, conjugation with GSH is a very important means of detoxication accounting in some cases for up to 60% of the biliary metabolites. As seen in the example of aflatoxin B(1), very low enzymic rates observed in vitro are sufficient to account for apparently high rates of biliary excretion of GSH conjugates.GSH transferases have evolved other functions apart from the catalysis of GSH conjugation. GSH transferase B participates in the hepatic uptake of bilirubin and the intracellular distribution of the heme prosthetic group. It also has GSH peroxidase activity which suggests that it might participate in the detoxication of by-products of oxygen utilization including those produced by the action of cytochrome P-450. It is shown that GSH transferase B inhibits lipid peroxidation in vitro.

Evidence that the Ya and Yc subunits of glutathione transferase B (ligandin) are the products of separate genes

A study of the structure of glutathione transferase B (ligandin) has been made with a view to understanding the relationship between the structures of the subunits of which it is composed. It consists of a mixture of a homodimer (YaYa) and a heterodimer (YaYc) in which the monomers are defined by their apparent molecular weights, that of Ya being 22000 and Yc 25000. Soluble tryptic peptides from the native homodimer YaYa have been compared with those from an artificial homodimer YcYc produced by rehybridization of native YaYc. Approximately 10 peptides specific to YaYa, 12 specific to YcYc and 21 common to both have been detected. Some of the above peptides are derived from variants of the monomers themselves. YaYa and YcYc have two C termini which are the same in both dimers, namely phenylalanine and lysine. Also there are four cysteinyl peptides, of which three are common to YaYa and YcYc and one specific to each. These results suggest that Ya and Yc are derived from at least two different but related genes.

Interaction of chemical carcinogens with macromolecules

The nature of certain critical cellular reactions is discussed in terms of both mutagenic and carcinogenic effects. Emphasis is placed on the ability of the ultimate carcinogen, normally formed in vivo by metabolism, to react with nucleic acids and, in particular, with nuclear DNA. The actions of N-nitroso compounds is examined in some detail and a possible correlation of the carcinogenic action of these compounds with their ability to react with oxygen-atoms in nucleic acids in considered. The formation of a specific lesion, O6-alkylguanine, in DNA and the capacity for its repair in different tissues is discussed with respect to tissue susceptibility to tumor induction. This discussion is extended to compare differences between species in the (tissue) specificity of action of particular N-nitroso compounds.

Glutathione adducts of N-methyl-4-aminoazobenzene formed in vivo and by reaction of N-benzoyloxy-N-methyl-4-aminoazobenzene with glutathione

N-Benzoyloxy-N-methyl-4-aminoazobenzene (N-BzO-MAB) is believed to be an analogue of the ultimate carcinogenic form of N,N-dimethyl-4-aminoazobenzene (DAB). The reaction of N-BzO-MAB with glutathione in vitro yielded one major and two minor aminoazo dye-glutathione adducts. After purification by ion exchange chromatography and high pressure liquid chromatography, analysis of chemical properties, and the measurement of ultraviolet, visible, proton magnetic resonance, and mass spectra, the major and one minor adduct were identified as 3-(glutathion-S-yl)-N-methyl-4-aminoazobenzene (3-GS-MAB) and 2'-(glutathion-S-yl)-N-methyl-4-aminoazobenzene (2'-GS-MAB) respectively. The other minor adduct was tentatively identified as 4'-(glutathion-S-yl)-N-methyl-4-aminoazobenzene (4'-GS-MAB). Fractionation and analyses of biliary metabolites from rats given DAB revealed the presence of two aminoazo dye-glutathione adducts. One of these was identical to 3-GS-MAB in its chromatographic and chemical properties and its visible and ultraviolet spectra. The other adduct was partially characterized and judged to be a 4-aminoazobenzene-glutathione adduct. The role of glutathione in the detoxification of carcinogenic aminoazo dyes is discussed.

A low-molecular-weight protein from rat liver that resembles ligandin in its binding properties

A protein of S20,W 1.6S and mol.wt. 14000, which binds covalently a metabolite of the aminoazodye carcinogen NN-dimethyl-4-amino-3'-methylazobenzene, was isolated from rat liver cytosol from both carcinogen-treated and normal rats. The protein binds non-covalently palmitoyl-CoA, fatty acids, bilirubin, sex steroids and their sulphates, bile acids and salts, bromosulphophthalein, diethylstilboestrol and 20-methylcholanthrene with a wide range of affinities. The protein is isolated as three components with isoelectric points of 5.0, 5.9 and 7.6 by a method involving isoelectric focusing. All three components have closely similar amino acid analyses, tryptic-peptide 'maps' and u.v. spectra. Each single component redistributes into all three on further electrophoresis. However, the three forms differ in their binding characteristics, the form of pI 7.6 having much the highest affinity for compounds bound non-covalently. The protein was identified immunologically in rat liver, small intestine, adipose tissue, skeletal muscle, myocardium and testis. The protein was compared with other hepatic binding-protein preparations of similar molecular weight.

Carcinogenic mechanisms: a critical review and a suggestion that oncogenesis may be adaptive ontogenesis

The precise mechanism(s) whereby normal cells become malignant are not known with any degree of certainty. However, many mechanisms have been proposed on the basis of available experimental evidence as interpreted by the proposer. These fall into two main groups and are based upon changes in genetic structure (somatic mutation hypotheses) or in genetic expression (epigenetic hypotheses). Yet a third group embodies elements of the first two. The more important of all these proposals are critically reviewed and yet another hypothesis is ventured. In this hypothesis, the induction of neoplasia is envisaged as embodying (a) initiation of preferentially partly-differentiated and resting stem cells and (b) promotion of the initiated cells, through mitosis and further differentiation and by adaptations of normal ontogenic mechanisms, into a variety of novel phenotypes which are malignant or potentially so. Cancer-specifying genes, altered chromosomes, de-differentiations and interrupted re-differentiations are not considered to be causally involved, although the last three of these can be present as epiphenomena. Evidence cited in support of this proposal appears to show a general absence from cancer cells of any single property, including an abnormality in genetic constitution or in cellular expression, which is specific to malignancy. Malignancy is thus envisaged as abnormal expressions of the genetic potential of the zygote. Some practical and theoretical implications of this concept are discussed.

Immunological studies of Y protein. A major cytoplasmic organic anion-binding protein in rat liver

An antibody produced against rat Y protein, the major cytoplasmic organic anion-binding protein in liver, was characterized. The antibody precipitated Y protein from liver supernatant fractions and specifically removed the organic anion-binding capacity from this fraction.Y protein was detected by immunodiffusion with this antibody in the supernates of rat liver, kidney, and small intestinal mucosa and was not detected in supernates of 16 other tissues including bile and serum. Precipitation with anti-Y was not detected with supernates of liver from 20 other species, including man. Quantitative radial immunodiffusion revealed Y protein to constitute 4.5% of supernatant protein in rat liver and approximately 2% of supernatant protein in rat kidney and small intestinal mucosa. Phenobarbital administration increased the concentration of Y protein in rat liver by 280%, but not in kidney or small intestinal mucosa, and was associated with increased plasma disappearance of sulfobromphthalein sodium, indocyanine green, and bilirubin, and increased hepatic, but not kidney or small intestinal mucosal, content of these organic anions. These observations provide further evidence indicating that the concentration of Y protein is a major determinant of organic anion flux across the plasma membrane of the liver cell.Immunodiffusion and immunoelectrophoresis revealed serological identity between Y protein, cortisol metabolite-binding protein I. and the major azocarcinogen-binding protein.