Identification of rat liver carboxylesterase isozymes (EC 3.1.1.1) using polyacrylamide gel electrophoresis and isoelectric focusing

Differential metabolism of the pyrrolizidine alkaloid, senecionine, in fischer 344 and sprague-dawley Rats

The pyrrolizidine alkaloids (PAs), contained in a number of traditional remedies in Africa and Asia, show wide variations in metabolism between animal species but little work has been done to investigate differences between animal strains. The metabolism of the PA senecionine (SN) in Fischer 344 (F344) rats has been studied in order to compare to that found in the previously investigated Sprague-Dawley (SD) rats (Drug Metab. Dispos. 17: 387, 1989). There was no difference in the formation of (+/-) 6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP, bioactivation) by hepatic microsomes from either sex of SD and F344 rats. However, hepatic microsomes from male and female F344 rats had greater activity in the N-oxidation (detoxication) of SN by 88% and 180%, respectively, when compared to that of male and female SD rats. Experiments conducted at various pH showed an optimum pH of 8.5, the optimal pH for flavin-containing monooxygenase (FMO), for SN N-oxidation by hepatic microsomes from F344 females. In F344 males, however, a bimodal pattern was obtained with activity peaks at pH 7.6 and 8.5 reflecting the possible involvement of both cytochrome P450 (CYP) and FMO. Use of specific inhibitors (SKF525A, 1-benzylimidazole and methimazole) showed that the N-oxide of SN was primarily produced by FMO in both sexes of F344 rats. In contrast, SN N-oxide formation is known to be catalyzed mainly by CYP2C11 rather than FMO in SD rats. This study, therefore, demonstrated that there were substantial differences in the formation of SN N-oxide by hepatic microsomes from F344 and SD rats and that this detoxification is catalyzed primarily by two different enzymes in the two rat strains. These findings suggest that significant variations in PA biotransformation can exist between different animal strains.

Identification of microsomal rat liver carboxylesterases and their activity with retinyl palmitate

Retinyl esters are a major endogenous storage source of vitamin A in vertebrates and their hydrolysis to retinol is a key step in the regulation of the supply of retinoids to all tissues. Some members of nonspecific carboxylesterase family (EC 3.1.1.1) have been shown to hydrolyze retinyl esters. However, the number of different isoenzymes that are expressed in the liver and their retinyl palmitate hydrolase activity is not known. Six different carboxylesterases were identified and purified from rat liver microsomal extracts. Each isoenzyme was identified by mass spectrometry of its tryptic peptides. In addition to previously characterized rat liver carboxylesterases ES10, ES4, ES3, the protein products for two cloned genes, AB010635 and D50580 (GenBank accession numbers), were also identified. The sixth isoenzyme was a novel carboxylesterase and its complete cDNA was cloned and sequenced (AY034877). Three isoenzymes, ES10, ES4 and ES3, account for more than 95% of rat liver microsomal carboxylesterase activity. They obey Michaelis-Menten kinetics for hydrolysis of retinyl palmitate with Km values of about 1 micro m and specific activities between 3 and 8 nmol.min-1.mg-1 protein. D50580 and AY034877 also hydrolyzed retinyl palmitate. Gene-specific oligonucleotide probing of multiple-tissue Northern blot indicates differential expression in various tissues. Multiple genes are highly expressed in liver and small intestine, important tissues for retinoid metabolism. The level of expression of any one of the six different carboxylesterase isoenzymes will regulate the metabolism of retinyl palmitate in specific rat cells and tissues.

Stereoselective hydrolysis of xenobiotic esters by different cell lines from rat liver and hepatoma and its application to chiral prodrugs for designated growth suppression of cancer cells

Stereoselectivity in the hydrolysis of racemic ethyl 2-phenylacetate derivatives by cultured cells of noncancerous cell lines from rat liver (BRL, BRL 3A, Clone 9, and ARLJ301-3), spontaneously or oncogene transformed rat liver cell lines (ARLJ301-3TR1, Anr4, Anr9-1, and Anr13-1), and cancer cell lines from rat hepatoma (H4-II-E, McARH7777, and MH1C1) and sarcoma (XC) was studied. A strong (R)-enantiomer preference was found in the hydrolysis of ethyl 2-hydroxy-(2c) and 2-methoxy-2-phenylacetate (3c) by the noncancerous and oncogene-transformed cells and an (S)-enantiomer preference for ethyl N-acylphenylalaninates with all the present cell lines. These inclinations were, however, not recognized with ethyl 2-methoxy-2-phenylpropanoate and ethyl N-difluoroacetyl- or N-trifluoroacetylphenylalaninate. Moreover, the R preference for 3c was reversed in the reaction by hepatoma cells. Thus, the stereoselectivity is influenced by both structure of acyl group and species of cell lines. The hepatoma cells were considerably different from the noncancerous or oncogene-transformed cells in stereoselectivity. This fact was consistent with the order of colony formation in soft agar cultures (index of malignancy) and the resemblance in actively stained esterase patterns in gel electrophoresis. The stereoselective hydrolysis leads to cell-specific activation of anticancer prodrugs. This has been confirmed for the first time by the stereoselectivity of Anr4 and H4-II-E cells in the hydrolysis of a chiral mustard ester, bis(2-chloroethyl)aminophenyl 2-methoxy-2-phenylacetate (14) and by the difference of IC50 values of (R)- and (S)-14 against the two cell lines.

Determination of rat plasma esterase-1 (ES-1) activity by scanning densitometry of gradient polyacrylamide gels with zymogram detection

There is no specific assay for rat plasma esterase-1 (ES-1) activity. Plasma contains many esterases, while known substrates do not discriminate between esterases. With gel electrophoresis, plasma esterase isozymes can be separated. Thus, a method consisting of gradient polyacrylamide gel electrophoresis, visualization of the enzyme with a staining technique based on substrate conversion, and densitometric scanning of the stained gel has been developed for quantitative measurement of rat plasma ES-1 activity. ES-1 activities were expressed as total peak areas. Reproducibility of the method was found to be about 10% (expressed as apparent between-gel coefficient of variation). When the ES-1 zone areas was expressed relative to that of a plasma ES-1 standard, reproducibility was about 3%. The kinetics of catalysis of alpha-naphthyl acetate hydrolysis by ES-1 could be determined with the gel scanning assay; the Km was 0.76 mM. At the alpha-naphthyl acetate concentration of 2.69 mM, total peak areas of the ES-1 zone were linearly associated with the staining time (up to at least 40 min) and amount of plasma (up to 26.25 microL). The pH of the staining buffer influences the ES-1 zone area, the largest areas being obtained when the pH ranged between 7.0 and 7.8. With propionate as acyl moiety of the alpha-naphthyl ester substrate, ES-1 zone areas were higher than with either acetate, butyrate or hexanoate.

Aspirin hydrolyzing esterases from rat liver cytosol

Unlike most esterases, which are predominantly bound to the microsomal fraction, the enzymes hydrolyzing acetylsalicylic acid are present in an equal amount in the cytosol. Two soluble isozymes were purified to homogeneity from rat liver and characterized as serine esterases with a Mr of 35,000. Both had the wide substrate spectrum characteristic of enzymes active in detoxication. Both had a very low Km for acetylsalicylate. Three other cytoplasmic enzymes active with aspirin were observed but these differed in their high Mr (about 220,000) and their lack of reactivity with antibody to one of the homogeneous isozymes.

Genetic variation and biochemical properties of esterase-18 (ES-18) in the laboratory rat (Rattus norvegicus): a new locus of esterase cluster 2 in linkage group V

A new liver-specific rat carboxylesterase isozyme (EC 3.1.1.1) designated esterase-18 (ES-18) is described. Genetic variation of ES-18 was examined in 93 inbred strains and substrains and a structural locus Es-18 was suggested, coding for either the presence (Es-18a) or the absence (Es-18b) of the isozyme. Linkage studies involving two backcross series revealed that Es-18 resides in cluster 2 of LGV. No recombination between Es-18 and other cluster 2 loci was found in 19 lines of two RI strain sets or in the backcross series.

Comparison of microsomal drug-metabolizing enzymes in 14 rat inbred strains

Drug metabolic capacity in liver microsomes of 14 rat inbred strains was investigated. Cytochrome P-450 content as well as the following enzyme activities were measured: NADPH cyt. c(P-450) reductase (Red.), aminopyrine N-demethylase (APDM), ethoxycoumarin O-deethylase (ECOD), 1-naphthol: UDP-glucuronosyltransferase (NGT) and hydrolysis of acetylsalicylic acid (ASA; measured at pH 5.5 and pH 7.4). All enzymes measured were found to exhibit statistically significant inter-strain differences. In males the enzyme activities varied over a 7.3-fold (ECOD) to 1.4-fold (cytochrome P-450) range. Other inter-strain differences were generally larger than 2-fold: ASA-hydrolysis at pH 5.5 and 7.4 (3.9- and 3.3-fold variation, respectively), NGT and Red. (2.1-fold variation) and APDM (1.8-fold variation). In females similar, but somewhat smaller inter-strain differences were observed. Correlations between different enzyme activities were generally poor (correlation coefficients r less than 0.7). An exception was the correlation between ASA-hydrolysis at pH 5.5 and pH 7.4 (r = 0.79). We conclude that ASA hydrolysis at pH 5.5 and 7.4 is mediated by the same enzyme or by coregulated enzymes and that all other activities are mediated by different or differentially regulated enzymes. Based on analysis of variance and subsequent inter-strain comparisons, all strains appear to express a unique profile of liver microsomal drug metabolism. No two strains are identical with respect to all activities measured. We suggest that differences between inbred rat strains and particularly the difference in balance between different enzymes in various strains can be used advantageously in pharmacological and toxicological experiments.

Esterase-16 (ES-16) of the rat (Rattus norvegicus): identification and genetic characterization

Esterase-16, an esterase present in lung and other tissues of the laboratory rat, has been characterized by its biochemical properties (electrophoretic mobility, substrate pattern, sensitivity to inhibitors) and genetic variation in 107 inbred strains and substrains including 14 RI strains. It was classified as a carboxylesterase (EC 3.1.1.1). The phenotype ES-16A (BN/Han and 63 other strains) was defined as a narrow electrophoretic band migrating between ES-1A and ES-13A, ES-16B (LEW/Han and 42 other strains) exhibited the same electrophoretic mobility as ES-16A but was distinguished by its extremely weak activity. Segregation of ES-16 in RI strains and backcrosses indicated linkage to linkage group V (LGV). The Es-16 locus was tentatively placed into esterase cluster 2 and homology with Es-7 of the house mouse is proposed.

Metabolism of T-2 toxin by blood cell carboxylesterases

Human and rat blood hydrolysed T-2 toxin along two different pathways giving HT-2 toxin and neosolaniol as primary metabolites, respectively. Neosolaniol represents a metabolic pathway different from that obtained by liver. Rat erythrocytes formed neosolaniol as a primary metabolite whereas white blood cells hydrolysed T-2 toxin to HT-2 toxin. Human erythrocytes formed both HT-2 toxin and neosolaniol whereas all human white cells produced only HT-2 as the primary metabolite. The enzymes responsible for hydrolysis of T-2 toxin to HT-2 toxin in white blood cells and T-2 toxin to neosolaniol in red blood cells were all identified as carboxylesterases by use of specific inhibitors. The ratio between trichothecene hydrolysis and 4-nitrophenyl butyrate hydrolysis varied among the different cell fractions indicating that specific isoenzymes are involved.

Genetic identification of rat liver carboxylesterases isolated in different laboratories

Six carboxylesterases previously isolated from rat liver microsomes, characterized in Brussels and in Kiel, were compared with genetically defined liver esterases of various reference strains using polyacrylamide gel electrophoresis and isoelectric focusing. The six liver carboxylesterases were identified as alloenzymic forms of ES-3, ES-4, ES-8/ES-10 and ES-15 according to the genetic nomenclature recommended by van Zutphen (Van Zutphen, L.F.M. (1983) Transplant. Proceed. 15, 1687-1688). The genetic and biochemical characteristics of the four isoenzymes are summarized, and their identity with several other drug-metabolizing esterases/amidases and lipases of rat liver endoplasmic reticulum is discussed.

Biochemical genetics of Es-14 (formerly Es-Si) and a new esterase variation, Es-15, of the laboratory rat (Rattus norvegicus): biochemistry, tissue expression, and linkage to Es-1 in linkage group V

The segregation of rat esterases controlled by loci residing in linkage group V (LGV) has been studied in two backcross series, (LEW/Han X BN/Han)F1 X LEW/Han and (LEW/Han X LE/Han)F1 X LEW/Han. Es-14 (formerly Es-Si) was shown to be closely linked to Es-1. A new esterase locus, Es-15, was described which codes for a liver isozyme. The distribution pattern of three alleles at the Es-15 locus is presented for 52 independent inbred strains. Close linkage of Es-15 to Es-14 and to Es-1 was established, proposing the following gene order: [Es-2, Es-10]-[ES-1, ES-14, ES-15]. The esterase loci on LGV are thus separated into two gene clusters, cluster 1 and cluster 2. These conclusions are supported by the strain distribution patterns of the two RI strain series, LXB and DXE.