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W Levin

Publications and source records attributed to W Levin.

At least 109 records · Page 6Linked to original sources
PubMed1986

Reactivity and tumorigenicity of bay-region diol epoxides derived from polycyclic aromatic hydrocarbons.

During the past decade substantial progress has been made in elucidating factors that determine the tumorigenic activity of bay-region diol epoxides, major ultimate carcinogenic metabolites derived from polycyclic aromatic hydrocarbons. Neither high nor low chemical reactivity of the diol epoxides (as measured by rates of uncatalyzed solvolysis) is required for high tumorigenic response. In contrast, aspects of molecular structure such as conformation and absolute configuration strongly influence tumorigenic activity. The role of conformation is illustrated by the observation that those diol epoxides whose hydroxyl groups are pseudoaxial are weak or inactive as tumorigens. Absolute configuration is an important determinant of biological activity of bay-region diol epoxides: in all cases studied to date, the predominantly formed (R,S)-diol-(S,R)-epoxides are generally the most tumorigenic of the four metabolically possible configurational isomers. In the course of investigating the effects of structural factors on tumorigenic activity, we identified the (4R,3S)-diol-(2S,1R)-epoxide of benzo(c)phenanthrene as the most potent tumorigen (in initiation-promotion experiments on mouse skin) of the diol epoxides studied to date. Studies of all four configurationally isomeric diol epoxides derived from benzo(c)phenanthrene led to the striking observation that these diol epoxides exhibit an exceptionally high efficiency of covalent binding, relative to hydrolysis, when allowed to react with calf thymus DNA in aqueous solution. Thus, these diol epoxides should provide an excellent tool for the detailed study of such binding. When the four isomeric benzo(c)phenanthrene diol epoxides are compared, there appears to be no simple correlation between tumorigenic response and either the extent of binding to DNA or the major types of deoxyribonucleoside adducts formed. Deoxyribonucleoside adducts of benzo(c)phenanthrene diol epoxide have also been identified from the DNA of cultured rodent embryo cells after treatment of the cells with tritium-labeled benzo(c)phenanthrene. The distribution of adducts is consistent with predominant metabolic formation of the (4R,3S)-diol-(2S,1R)-epoxide; deoxyadenosine is the major site in the cellular DNA attacked by this epoxide, just as it is in DNA in solution. Further experiments are in progress which we hope will identify more subtle aspects of the DNA binding of benzo(c)phenanthrene diol epoxides that may be uniquely correlated with their tumorigenic activity.

Animals↗
PubMed1986

Ovarian Sertoli-Leydig cell tumour with raised serum alpha fetoprotein. A case report.

A case of ovarian Sertoli-Leydig cell tumour with a raised serum alpha fetoprotein is reported. The patient first presented at the age of 27 years with a history of 6 years' amenorrhoea followed by 3 months irregular vaginal bleeding. A ovarian tumour was found and excised and shown microscopically to be a spindle cell malignant tumour. The patient was treated with chemotherapy and had a complete response. Thirty months after first presentation there was a recurrence in the pelvis which microscopically showed the typical features of a Sertoli-Leydig cell tumour. Six months later a second recurrence had the microscopic appearance of a lipid cell tumour. A raised serum alpha fetoprotein was found at the time of the second recurrence and immunohistochemistry showed this protein in the Leydig and luteinized cells of the recurrent tumours but not in the spindle cells of the original ovarian neoplasm.

Adult↗
PubMed1986

The primary structure of cytochrome P-450d purified from rat liver microsomes: prediction of helical regions and domain analysis.

The complete amino acid sequence of rat liver microsomal cytochrome P-450d (induced by isosafrole) was deduced by microsequence analysis of the tryptic peptides after separation by reverse-phase HPLC and alignment by comparison to the cDNA sequence reported by K. Kawajiri, O. Gotoh, K. Sogawa, Y. Tagashira, M. Muramatsu, and Y. Fujii-Kuriyama (1984, Proc. Natl. Acad. Sci. USA 81, 1649-1653). Results from the two approaches are in complete agreement with the exception of two residues, Lys-30 (Arg in cDNA) and Phe-261 (Ser in cDNA). As previously reported by us (L. H. Botelho, D. E. Ryan, P.-M. Yuan, R. Kutney, J. E. Shively, and W. Levin (1982, Biochemistry 21, 1152-1155) the NH2-terminal sequence of the mature protein lacks the NH2-terminal Met residue. Comparison of the rat cytochrome P-450d sequence with the mouse cytochrome P3-450 cDNA sequence reported by S. Kimura, F.J. Gonzalez, and D.W. Nebert (1984, Nucl. Acids Res. 12, 2917-2928) reveals a high sequence homology with a total of 32 amino acid differences including six conferring charge changes. Prediction of the secondary structure of cytochrome P-450d yields a maximum of 17 helices, two of which may be poly(Pro)-like helices adjacent to potential membrane-spanning alpha-helices. Four of the alpha-helices are sufficiently hydrophobic to traverse the endoplasmic reticulum. The remaining helices are largely amphiphilic. Analysis of the helices in reference to predicted membrane topology suggests that cytochrome P-450d either has one large and one small globular domain separated by a transmembrane domain and anchored by NH2-terminal and COOH-terminal transmembrane domains, or has one large globular domain anchored at both ends by transmembrane domains.

Amino Acid Sequence↗
PubMed1986

N-demethylation of N-nitrosodimethylamine catalyzed by purified rat hepatic microsomal cytochrome P-450: isozyme specificity and role of cytochrome b5.

Metabolism of the potent hepatocarcinogen N-nitrosodimethylamine (NDMA) was evaluated in reconstituted monooxygenase systems containing each of 11 purified rat hepatic cytochrome P-450 isozymes. The reaction has an absolute requirement for cytochrome P-450, NADPH-cytochrome P-450 reductase, and NADPH, as well as a partial dependence on dilauroylphosphatidylcholine. Of the cytochrome P-450 isozymes evaluated, only cytochrome P-450j, purified from livers of ethanol- or isoniazid-treated rats, had high catalytic activity for the N-demethylation of NDMA. At substrate concentrations of 0.5 and 5 mM, rates of NDMA metabolism to formaldehyde catalyzed by cytochrome P-450j were at least 15-fold greater than the rates obtained with any of the other purified isozymes. At the pH optimum (approximately 6.7) for the reaction, the Km,app and Vmax were 3.5 mM and 23.9 nmol/min/nmol cytochrome P-450j, respectively. With hepatic microsomes from ethanol-treated rats, which contain induced levels of cytochrome P-450j, the Km,app and Vmax were 0.35 mM and 3.9 nmol/min/nmol cytochrome P-450, respectively. Inclusion of purified cytochrome b5 in the reconstituted system containing cytochrome P-450j caused a six-fold decrease in Km,app (0.56 mM) of NDMA demethylation with little or no change in Vmax (29.9 nmol/min/nmol cytochrome P-450j). Trypsin-solubilized cytochrome b5, bovine serum albumin, or hemoglobin had no effect on the kinetic parameters of the reconstituted system, indicating a specific effect of intact cytochrome b5 on the Km,app of the reaction. These results demonstrate high isozyme specificity in the metabolism of NDMA to an ultimate carcinogen and further suggest an important role for cytochrome b5 in this biotransformation process.

Animals↗
PubMed1986

Gestational trophoblastic disease: the significance of vaginal metastases.

Five patients with gestational trophoblastic disease whose presenting symptom was hemorrhage from vaginal metastases have been added to our previous report. The clinical features, management, and responses to treatment are outlined. All the patients required suturing of the bleeding lesions under general anesthetic to arrest the hemorrhage. In addition one patient needed selective arterial embolization. This did not compromise the response to chemotherapy. We confirm our previous view that the presence of vaginal metastases should be classified as a high-risk factor and that these patients be treated with multiple agent chemotherapy from the outset.

Adult↗
PubMed1986

Microhysteroscopy in evaluation of the endocervix in endometrial carcinoma.

Microhysteroscopy (MH) was used for the assessment of endocervical involvement in patients with endometrial carcinoma. Between 1 July 1984 and 28 February 1985, fifteen cases were seen. Of these, nine patients had had fractional curettage and all were deemed to have endocervical involvement. As a result of the use of MH, five of these stage II cases were down-staged. The remaining four cases had their staging confirmed by MH and directed endocervical curettage. The six patients who had no initial endocervical assessment were found to have Stage I disease, using the MH alone. The MH findings were corroborated and confirmed on histological examination of the surgical specimens. MH should be the initial investigation in patients with suspected endometrial carcinoma and all patients with postmenopausal bleeding.

Adult↗
PubMed1986

The use of concomitant chemotherapy and radiotherapy prior to surgery in advanced stage carcinoma of the vulva.

Six cases of primary advanced squamous carcinoma of the vulva were treated with concomitant chemotherapy and radiotherapy (C + RT). In five patients it was given in preparation for surgery. The radiation was delivered to the whole pelvis in 10 equal daily fractions of either 2.0 or 2.5 Gy. Mitomycin C (10 mg/m2) was given on Day 1, and 5-fluorouracil (1,000 mg/m2) was given daily from Days 1 to 4, inclusive. All six patients had satisfactory early tumor response. Three patients received one course of C + RT and three were given two courses. One patient died suddenly of unknown causes 6 days after completing the C + RT. One patient with fixed groin nodes was treated with palliative intent. She maintained a complete local response but died 6 months later of liver metastases. The remaining four patients underwent surgery without healing complications and are alive with no evidence of disease at 1, 4, 14, and 26 months. In our experience vulval carcinomas can be reduced in size and extent by prior chemotherapy and radiotherapy and require less extensive surgery.

Adult↗
PubMed1986

The value of cancer antigen-125 as a tumor marker in malignant germ cell tumors of the ovary.

The value of cancer antigen-125 (CA-125) as a tumor marker for malignant germ cell tumors (MGCT) of the ovary was investigated and compared with the other recognized tumor markers (human chorionic gonadotrophin (hCG), alpha-fetoprotein (AFP), carcinoembryonic antigen (CEA) and lactate dehydrogenase (LDH) isoenzymes. In the 10 months following June 1984, 4 new cases with MGCT and 1 patient with active disease on treatment were evaluated. In all cases prior to planned surgery the levels of CA-125 were significantly elevated. The serum values ranged from 154 to 617 U/ml (normal less than 20 U/ml). In 1 case (pure dysgerminoma) CA-125 was the only tumor marker. In 3 patients (2 mixed germ cell tumors and 1 immature teratoma) serum LDH (LD 1, 2, and 3) was elevated, and AFP was elevated in 1 of these. In the fifth case (mixed germ cell tumor), on treatment, serum AFP was used to monitor the disease. Four patients underwent cytoreductive surgery followed by combination chemotherapy. The changes in the serum levels of CA-125 paralleled those of the other tumor markers while on therapy. In our experience CA-125 is an invaluable indicator of the clinical status of the patient and could be a new tumor marker in patients with MGCT.

Adolescent↗
PubMed1986

Bacterial and mammalian cell mutagenicity of four optically active bay-region 3,4-diol-1,2-epoxides and other derivatives of the nitrogen heterocycle dibenz[c,h]acridine.

The mutagenic activities of the enantiomers of the diastereomeric pair of bay-region 3,4-diol-1,2-epoxides of the nitrogen heterocycle, dibenz[c,h]acridine, have been evaluated in histidine-dependent strains of Salmonella typhimurium and in an 8-azaguanine-sensitive line of Chinese hamster cells. In strains TA 98 and TA 100 of S. typhimurium the pair of enantiomers with [1R,2S,3S,4R] and [1S,2R,3R,4S] absolute configuration and the benzylic hydroxyl group trans to the epoxide oxygen are 2 to 4 times more mutagenic than the [1S,2R,3S,4R] and [1R,2S,3R,4S] isomers in which the benzylic hydroxyl and epoxide oxygen are cis. In both strains of bacteria there is very little difference in mutagenic activity between the enantiomers of each diastereomer. In contrast to these results in bacteria, the bay-region 3,4-diol-1,2-epoxide isomer with [1R,2S,3S,4R] absolute configuration is 5 to 7 times more mutagenic to Chinese hamster V79 cells than are the other 3 isomers. The enantiomeric pair of bay-region tetrahydro-1,2-epoxides of dibenz[c,h]acridine are at least 7 times more mutagenic than the diol-epoxides in the Salmonella assay, and no difference in mutagenic activity is observed between enantiomers. In the Chinese hamster V79 cell system, however, the tetrahydro-1,2-epoxide with [1R,2S] absolute configuration is 2- to 3-fold more mutagenic than its enantiomer with [1S,2R] absolute configuration. Homogeneous rat liver epoxide hydrolase does not catalyze the hydration of the diol-epoxide isomers to nonmutagenic products, although the tetrahydroepoxides, especially the tetrahydro-3,4-epoxide, are metabolized by the enzyme. Results of metabolic activation experiments with the bacterial mutagenesis system and microsomes from Aroclor 1254-treated rats are consistent with the mutagenicity data described above and support the concept that dibenz[c,h]acridine is metabolically activated to a bay-region diol-epoxide. Notably: (a) 3,4-dihydrodibenz[c,h]acridine, the expected precursor of a bay-region tetrahydroepoxide, is metabolized to a potent mutagen; (b) racemic dibenz[c,h]acridine 3,4-dihydrodiol is metabolized to products which are several-fold more mutagenic than are products of the metabolism of dibenz[c,h]acridine or its 1,2- or 5,6-dihydrodiols; and (c) the tetrahydro-3,4-diol, which lacks the isolated bay-region double bond, is not metabolically activated to a bacterial mutagen.

Acridines↗
PubMed1986

Identification of a human liver cytochrome P-450 homologous to the major isosafrole-inducible cytochrome P-450 in the rat.

The rat 3-methylcholanthrene-inducible family of liver cytochromes P-450 contains two proteins (P-450c and P-450d) that are immunochemically related, possess 68% total sequence homology, and are induced by a number of toxic or carcinogenic compounds. To determine whether equivalent isozymes of hepatic cytochrome P-450 are expressed in humans, as they are in several mammalian species, we performed immunoblot analyses on microsomes prepared from 14 human liver specimens and found that each one contained a 52.5-kDa protein (termed HLd) that reacted with antibodies specific for rat P-450d. In addition, one specimen contained a 54-kDa protein (termed HLc) that reacted with antibodies specific for rat P-450c. HLd was purified through the use of immunoaffinity chromatography and was found to be 56% homologous to rat P-450d and 61% homologous to the equivalent isozyme in the rabbit (P-450 LM4) through their first 18 NH2-terminal amino acids. Finally, levels of immunoreactive HLd varied more than 10-fold among these patients but were unrelated to the patients' drug treatments, smoking habits, or amount of immunoreactive HLp, a human liver cytochrome P-450 related to the glucocorticoid-inducible family of rat cytochromes P-450. We conclude that, in man, there is a cytochrome P-450 family composed of two isozymes (HLc and HLd) that are immunochemically and structurally related to the 3-methylcholanthrene-inducible family observed in several other species.

Adult↗
PubMed1986

Tumorigenicity of optical isomers of the diastereomeric bay-region 3,4-diol-1,2-epoxides of benzo(c)phenanthrene in murine tumor models.

Tumorigenic activities of the (+)- and (-)-enantiomers of the diastereomeric, bay-region benzo(c)phenanthrene 3,4-diol-1,2-epoxides were evaluated in two mouse tumor models. In an initiation-promotion experiment on mouse skin, a single topical application of 10, 25, or 75 nmol of the compounds was followed by 20 weeks of promotion with 12-O-tetradecanoylphorbol-13-acetate. Of the four optical isomers of the bay-region diol epoxides, (-)-(R,2S,3S,4R)-3,4-dihydroxy-1,2-epoxy-1,2,3,4-tetrahydrogenzo(c )phenanthrene [(-)-diol epoxide-2] and (+)-(1R,2S,3R,4S)-3,4-dihydroxy-1,2-epoxy-1,2,3,4-tetrahydrobenzo(c) -phenanthrene [(+)-diol epoxide-1] had equally high tumor-initiating activity while (+)-[1S,2R,3R,4S]-3,4-dihydroxy-1,2-epoxy-1,2,3,4-tetrahydrobenzo (c)phenanthrene [(+)-diol epoxide-2] had less than one-half of the activity of (-)-diol epoxide-2 and (+)-diol epoxide-1. (-)-(1S,2R,3S,4R)-3,4-Dihydroxy-1,2-epoxy-1,2,3,4-tetrahydrobenzo(c) -phenanthrene [(-)-diol epoxide-1] was inactive at the doses tested. In newborn mice, (-)-diol epoxide-2 was almost 10-fold more active in producing lung tumors (average number of lung tumors/mouse) than the next most active compound, (+)-diol epoxide-2, at a total dose of 10 nmol. The enantiomers of diol epoxide-1 were inactive at this dose. When the total dose of each optical isomer was increased to 50 nmol, (-)-diol epoxide-1 was still inactive, and (+)-diol epoxide-1 produced a significant number of lung tumors (0.9 lung tumor/mouse), but this isomer still had less than 10% of the activity of the (+)- and (-)-diol epoxide-2 isomers. (-)-Diol epoxide-2, but none of the other optical isomers, also produced a significant incidence of hepatic tumors at the higher dose, and this compound was found to be the most tumorigenic bay-region diol epoxide ever tested in newborn mice. Racemic diol epoxide-1 had approximately 1% of the tumorigenic activity of racemic diol epoxide-2 in newborn mice, but both racemates had equal tumor-initiating activity on mouse skin. These results dramatically illustrate the complexities involved in ranking the relative tumorigenic activities of compounds in different tumor models.

Animals↗
PubMed1986

High tumorigenicity of the 3,4-dihydrodiol of 7-methylbenz[c]acridine on mouse skin and in newborn mice.

The tumorigenicities of 7-methylbenz[c]acridine (7MB[c]ACR) and its five metabolically possible trans-dihydrodiols were determined in two mouse tumor models. In initiation-promotion studies on mouse skin, a single topical application of 0.15 to 0.75 mumol of compound was followed 9 days later by twice weekly applications of 12-O-tetradecanoylphorbol-13-acetate for 20 wk. Comparison of the average number of skin tumors per mouse indicated that 7MB[c]ACR 3,4-dihydrodiol, the metabolic precursor of a bay-region diol-epoxide, was 4- to 6-fold more active than the parent compound as a tumor initiator. The 1,2-, 5,6-, 8,9-, and 10,11-dihydrodiols of 7MB[c]ACR had no significant tumor-initiating activity at the doses tested. In newborn mice, a total dose of 0.35 mumol of compound was administered i.p. during the first 15 days of life, and tumorigenic activity was determined when the mice were 32 to 36 wk old. 7MB[c]ACR 3,4-dihydrodiol induced about 8-fold more pulmonary tumors per mouse and 9-fold more hepatic tumors per male mouse than the parent aza-substituted hydrocarbon. The other four dihydrodiols of 7MB[c]ACR had no significant tumorigenic activity. The high tumorigenic activity of 7MB[c]ACR 3,4-dihydrodiol in both tumor models suggests that a bay-region 3,4-diol-1,2-epoxide may be an ultimate carcinogenic metabolite of 7MB[c]ACR. 7MB[c]ACR was at least 5-fold more active as a tumor initiator on mouse skin than was the unsubstituted aza-aromatic compound, benz[c]acridine. This latter result indicates that substitution of a methyl group at position 7 of benz[c]acridine leads to enhanced tumor-initiating activity, as has been previously demonstrated for benz[a]anthracene and its 7-methyl derivative.

Acridines↗
PubMed1985

Microheterogeneity of a male-specific rat hepatic cytochrome P-450: existence of three allozymic forms.

Preparations of hepatic cytochrome P-450 h [D. E. Ryan, et al. (1984) J. Biol. Chem. 259, 1239] and cytochrome P-450 2c [D. J. Waxman (1984) J. Biol. Chem. 259, 15481] from outbred Sprague-Dawley rats were analyzed using two-dimensional electrophoresis and in situ peptide mapping. Both preparations consisted of the same isozyme which was previously characterized as a developmentally regulated, male-specific cytochrome P-450 active in the 16 alpha-hydroxylation of steroids. Each preparation evidenced microheterogeneity which was shown, in part, to result from the existence of two genetically determined variant forms of cytochrome P-450 h/2c. Analyses of hepatic microsomes from several inbred strains of rat revealed that each was characterized by a single variant form of this isozyme, with some strains expressing a variant that was not present in Sprague-Dawley rats. Genetic crosses indicated that these electrophoretic variants represent allozymic forms of cytochrome P-450 h/2c which are codominantly expressed at a single autosomal locus. Additional microheterogeneity of each allozymic form of cytochrome P-450 h/2c was shown to result from a specific in vitro modification that may involve limited proteolysis near its C terminus by a microsome-bound protease.

Alleles↗
PubMed1985

Monoclonal antibodies distinguish among isozymes of the cytochrome P-450b subfamily.

Polyclonal antibody has been shown previously to react identically with cytochromes P-450b and P-450e purified from Long Evans rats and a strain variant of cytochrome P-450b purified from Holtzman rats (P-450bH). In the present study, an array of 12 different monoclonal antibodies produced against cytochrome P-450b has been used to distinguish among these closely related phenobarbital-inducible rat hepatic cytochromes P-450. In immunoblots and enzyme-linked immunosorbent assays, 10 monoclonal antibodies bind to cytochromes P-450b, P-450e, and P-450bH; one monoclonal antibody (B50) recognizes cytochromes P-450b and P-450bH but not cytochrome P-450e; and one monoclonal antibody (B51) is specific for cytochrome P-450b. In addition, one monoclonal antibody (BEF29) reacts strongly with cytochrome P-450f, and another antibody (BEA33) reacts weakly with cytochrome P-450a. No cross-reactions with cytochromes P-450c, P-450d, and P-450g-P-450j were detected with any of the monoclonal antibodies in these assays. Six spatially distinct epitopes on cytochrome P-450b were identified, and differences in antibody reactivity provided evidence for three additional overlapping epitopes. Several monoclonal antibodies are potent inhibitors of testosterone and benzphetamine metabolism supported by cytochrome P-450b in a reconstituted system. B50 and BE52 do not inhibit metabolism of the two substrates by microsomes from untreated rats, but inhibit benzphetamine N-demethylation and testosterone metabolism to 16 alpha- and 16 beta-hydroxytestosterone as well as androstenedione formation 67-94% by microsomes from phenobarbital-treated rats. No other pathways of testosterone metabolism are inhibited by these monoclonal antibodies. The differential inhibition of microsomal metabolism of benzphetamine and testosterone by these monoclonal antibodies is a reflection of the content and inducibility of cytochromes P-450b and P-450e as well as other cytochrome P-450 isozymes.

Animals↗
PubMed1985

Differential stereoselectivity of cytochromes P-450b and P-450c in the formation of naphthalene and anthracene 1,2-oxides. The role of epoxide hydrolase in determining the enantiomer composition of the 1,2-dihydrodiols formed.

As is the case for cytochrome P-450c, arene 1,2-oxides have been identified as initial metabolites when naphthalene and anthracene are oxidized by cytochrome P-450b in a highly purified, reconstituted system. Overall rates of metabolism by cytochrome P-450b are greater than 3-fold and greater than 50-fold lower than the respective rates of metabolism by cytochrome P-450c. For both hydrocarbons, the (-)-(1S,2R)-oxide predominates (74%) with cytochrome P-450b as the terminal oxidant, based on trapping the labile arene oxides as N-acetyl-L-cysteine S-conjugates of known absolute configuration. This result is in marked contrast to data obtained with cytochrome P-450c where the (+)-(1R,2S)-oxides predominate (73-greater than 95%). In the absence of added epoxide hydrolase, the metabolically formed arene oxides rapidly isomerize to phenols. Addition of increasing amounts of epoxide hydrolase to the incubation medium results in the formation of trans-1,2-dihydrodiols at the expense of phenols from the common arene oxide intermediates. Evaluation of the kinetic parameters (Km and kcat) for the hydration of the (+)- and (-)-enantiomers of both arene oxides by epoxide hydrolase has indicated that the (+)-(1R,2S)-enantiomers exhibit lower values of Km (approximately 1 microM) whereas the values of kcat are similar for both enantiomers of a given arene oxide. These parameters have allowed construction of a mathematical model which predicts the enantiomer composition of the dihydrodiols formed from naphthalene in reconstituted systems containing specific epoxide hydrolase concentrations. The data reported argue against a selective functional coupling mechanism between cytochrome P-450c and epoxide hydrolase in the metabolism of naphthalene and anthracene to the 1,2-dihydrodiols.

Animals↗
PubMed1985

Properties of liver microsomal cholesterol 5,6-oxide hydrolase.

Cholestane 3 beta,5 alpha, 6 beta-triol has been identified as the exclusive product formed on hydration of cholesterol 5,6 alpha- and 5,6 beta-oxide catalyzed by cholesterol oxide hydrolase in liver microsomes obtained from five mammalian species. Highest activities were present in microsomes from rats and humans. Both acid- and base-catalyzed hydrolysis of the two epoxides also produce this product, presumably due to preference for pseudo-axial opening of the oxirane ring to form product with a trans-AB ring junction. Although the beta-oxide is more reactive than the alpha-oxide upon acid-catalyzed hydration, the alpha-oxide is a 4.5-fold better substrate than the beta-oxide as indicated by values of Vmax/Km. The kinetic parameters Vmax and Km for the reaction catalyzed by rat liver microsomes are 1.68 +/- 0.15 X 10(-7) M min-1 and 10.6 +/- 1.5 microM for the alpha-oxide and 1.32 +/- 0.11 X 10(-7) M min-1 and 37.2 +/- 5.5 microM for the beta-oxide at 0.35 mg protein/ml, pH 7.4, 6.35% (v/v) CH3CN, and 37 degrees C. Several imino compounds are competitive inhibitors for the enzyme from rat liver. The most effective of these is 5,6 alpha-iminocholestanol (Ki = 0.085 microM) which was known to be a good inhibitor from previous studies. Inhibition by aziridines is consistent with the participation of acid catalysis in the mechanism of action of the enzyme. Cholesterol oxide hydrolase is a distinct enzyme from oxidosqualene cyclase as well as microsomal epoxide hydrolase (EC 3.3.2.3) and the recently reported mouse hepatic microsomal epoxide hydrolase that catalyzes the hydration of trans-stilbene oxide.

Animals↗
PubMed1985

Absolute configuration of the 5,6-oxide formed from 7,12-dimethylbenz[a]anthracene by cytochrome P450c.

The absolute configurations of the enantiomeric 5,6-arene oxides of 7,12-dimethylbenz[a]anthracene (DMBA) were recently assigned such that the late eluting enantiomer from a chiral HPLC column has 5R,6S absolute configuration. [Mushtaq et al. (1984) BBRC 125, 539]. The authors further concluded that the 5R,6S-enantiomer predominates on metabolism of DMBA by cytochrome P450c in liver microsomes from 3-methylcholanthrene-treated rats. Their chemical assignment of absolute configuration is incorrect. Thus, metabolism of DMBA by these microsomes as well as by homogeneous cytochrome P450c produces 5,6-oxide highly enriched (95%) in the 5S,6R-enantiomer in accord with theoretical predictions.

9,10-Dimethyl-1,2-benzanthracene↗