Isolation and partial characterization of a rifampicin induced rabbit liver microsomal cytochrome P-450

5-Fluorocytosine antagonizes the action of sterol biosynthesis inhibitors in Candida glabrata

The concentration-dependent antagonistic interaction between 5-fluorocytosine and a sterol biosynthesis inhibitor (SBI) was studied using intact cells and cell-free extracts of Candida glabrata. 5-Fluorocytosine promoted incorporation of radioactivity into 4-desmethylsterols (P < 0.01), and enhanced the relative and absolute increases of ergosterol (P < 0.05) in C. glabrata incubated aerobically with an SBI (miconazole or amorolfine). Further aerobic incubation of C. glabrata with combinations of a nucleic acid or protein synthesis inhibitor (rifampicin or chlortetracycline) and an SBI (miconazole) promoted a similar increase in ergosterol biosynthesis. In contrast, 5-fluorocytosine reduced the incorporation of radioactivity into 4,4-dimethylsterols (P < 0.01), but had no obvious effect on the absolute ergosterol level in C. glabrata incubated statically with miconazole. In cell-free extracts of cultures previously incubated with 5-fluorocytosine, ergosterol synthesis was less sensitive to the action of miconazole. Antagonism between 5-fluorocytosine and the SBI is thus mediated by a reversal of inhibition of intracellular ergosterol synthesis. The possible mechanisms underlying antagonism between 5-fluorocytosine and SBIs that inhibit different sites of the sterol biosynthesis pathway, as well as its clinical relevance to combination therapy, are discussed.

Differential induction of cytochrome P-450 isozymes by rifampicin in the Chinese hamster, Cricetus griseus

Treatment of male and female Chinese hamsters with rifampicin at intraperitoneal doses of 25 and 50 mg/kg did not increase the cytochrome P-450 content of the liver except for a 1.3-fold increase in male hamsters at a dose of 50 mg/kg. Enhancement of the activities of erythromycin N-demethylase and testosterone hydroxylases, except for 15 alpha-hydroxylation, was observed in the livers of both male and female hamsters treated with rifampicin at both doses. Western blot analysis revealed that rifampicin caused no change in the content of CYP3A subfamily proteins in the liver, whereas changes in that of CYP2A subfamily proteins were evident.

Activation volume and energetic properties of the binding of CO to hemoproteins

We have investigated the CO binding to various reduced hemoproteins by stopped-flow rapid mixing as a function of pressure (from 0.1 to 200 MPa) and temperature (from 4 to 35 degrees C). In particular, we studied several varieties of cytochrome P-450: CYP11A1 (scc), CYP2B4 (LM2), CYP3A6 (LM3c), and Cyp2a (7 alpha), as well as chloroperoxidase and lactoperoxidase, and compared the results to data reported for other hemoproteins. Whereas the CO binding activation enthalpy delta H++ and entropy delta S++ (correlated through a compensation effect) varied greatly between the hemoproteins, with no apparent relation to structural features, the pressure effect depended on the nature of the proximal axial heme ligand: the activation volume was very small for cysteine (S-) ligand hemoproteins (delta V++ = +1 to +6 ml mol-1), and markedly negative for histidine (N) ligand hemoproteins (delta V++ = -3 to -36 ml mol-1). Furthermore, the transition state volume of the histidine ligand class enzymes, but not that of the cysteine ligand enzymes, depended on the solvent composition. These results suggest that the CO-binding transition state of the S-ligand class has a molecular conformation similar to the ground state. In the histidine class, however, the transition state appears to involve protein conformational changes and/or solvation processes.

Effect of phenobarbital and other model inducers on cytochrome P450 isoenzymes in primary culture of dog hepatocytes

1. The effects of phenobarbital (PB), beta-naphthoflavone (beta-NF), omeprazole (Omep) and rifampicin (Rif) on drug-metabolizing activities in dog hepatocytes, cultured with William's medium E, were examined. 2. The drug metabolizing activities of the hepatocytes decreased during culture; 7-ethoxycoumarin O-deethylase (ECOD) activity was nearly 70% of initial value at 72 h, but 7-methoxycoumarin O-demethylase (MCOD), 7-propoxycoumarin O-depropylase (PCOD), progesterone 6 beta-hydroxylase (6 beta-OH-P), progesterone 16 alpha-hydroxylase (16 alpha-OH-P), progesterone 21-hydroxylase (21-OH-P), 7-ethoxyresorufin O-deethylase (EROD) activities and total cytochrome P450 content were approx. 50%. 3. When the hepatocytes were cultured with PB, the enzyme activities increased time- and dose-dependently. MCOD, ECOD and PCOD activities increased 5-8 fold with 2 mM PB in 96 h. Similar results were obtained for 6 beta-OH-P, 16 alpha-OH-P and 21-OH-P activities, and total cytochrome P450. The effect of PB was abolished when 2.5 microM cycloheximide or 0.1 microM actinomycin D was included in the culture. 4. Treatment of hepatocytes with 40 microM beta-NF for 72 h resulted in 25-fold elevation of EROD activity. beta-NF enhanced PCOD activity approx. six-fold, while ECOD increased only slightly, and 7-MCOD negligibly. 5. Omep (100 microM) increased EROD activity nearly 10-fold, and 25 microM Rif increased 6 beta-OH-P activity approx. 8-fold, but ECOD only slightly. 6. Western blot analysis of microsomes from cultured dog hepatocytes with anti-rat CYP 2B1 antibodies indicated that PB increased an immunochemically-reactive protein. The protein showed the same mobility as the major dog P450 isozyme (cytochrome P450 PBD-2 or CYP 2B11) purified from liver microsomes of PB-treated male beagle dog. In a similar manner, induction of cytochrome P450 PBD-1 (CYP 3A12) by PB was confirmed.

Expression of cytochrome P450 3A in amphibian, rat, and human kidney

Family 3A mammalian liver cytochromes P450 (3A1, rat; 3A3/4, human) catalyze the 6 beta-hydroxylation of endogenous steroids and are steroid inducible. Our recent finding that A6 cells (a toad kidney epithelial cell line) contain corticosterone 6 beta-hydroxylase activity as a steroid-inducible microsomal cytochrome P450 raised the possibility that corticosterone 6 beta-hydroxylase activity in the A6 cells is catalyzed by a member of the 3A family. We found that incubation of A6 cell microsomes from dexamethasone-induced cells with antibodies against family 3A proteins specifically inhibited corticosterone 6 beta-hydroxylase activity. Microsomes from A6 cells analyzed on immunoblots developed with family 3A specific antibodies revealed immunoreactive proteins and treatment of A6 with corticosterone or dexamethasone increased the amounts of 3A immunoreactive protein(s). Furthermore, A6 RNA hybridized with 3A cDNAs on Northern blots and genomic DNA from A6 cells hybridized with a 3A cDNA on a Southern blot. Thus, toad kidney A6 cells express a family 3A P450 that is immunochemically, functionally, and genetically related to the mammalian liver 3A proteins. Prompted by these findings in amphibian kidney, we examined mammalian kidney for evidence of family 3A proteins. Immunocytochemical studies of frozen cryostat sections of normal adult rat kidney incubated with 3A1 antibody showed immunoreactivity only with collecting duct. Immunoblot analysis of human kidney microsomes found three protein bands representing 3A3/4, 3A5, and a 53-kDa Mr protein immunoreactive with human 3A antibody. An unexpected finding was the polymorphic expression of 3A3/4 in human kidney with only one of seven (14%) adult human kidneys tested expressing this protein while 3A5, a protein which is polymorphically expressed in adult human livers, was routinely present in the adult human kidney samples tested. Since human fetal liver contains a family 3A P450 we examined human fetal kidney microsomes by immunoblot analysis with human liver 3A antibody and found expression of a protein tentatively identified as 3A7. Thus, like A6 amphibian cells, family 3A P450 proteins and mRNAs are prominent, functional components in the kidney of mammals, including man.

Developmental expression of rabbit cytochrome P450 CYP1A1, CYP1A2 and CYP3A6 genes. Effect of weaning and rifampicin

Developmental expression of CYP1A1, CYP1A2 and CYP3A6 in the rabbit have been studied. Cytochromes P450IA1, P450IA2 and P450IIIA6 exhibited comparable patterns of developmental expression. Present at low level (less than 0.05 nmol/mg) in the new born animal up to week 3, these proteins sharply accumulated between weeks 3 and 4 to reach a maximum by week 4 (P450IA1, 0.2 nmol/mg; P450IA2, 0.8 nmol/mg; P450IIIA6, 0.12 nmol/mg) and decreased in the adult (P450IA1, 0.2 nmol/mg; P450IA2, 0.4 nmol/mg; P450IIIA6, 0.09 nmol/mg). Cytochromes P450IA1 and P450IA2 were not expressed in the untreated fetus. Onset of CYP3A6 gene expression occurred at day 30 of gestation and both transcription and mRNA accumulation were transplacentally inducible by rifampicin only shortly before birth, i.e. after treatment of the females between days 28 and 30 of gestation. Both long (1.85 kb) and short (1.7 kb) mRNA transcripts were expressed in untreated or rifampicin-treated fetuses. CYP3A6 gene expression was also induced by rifampicin in 1-week-old and 2-week-old animals. Developmental expression of CYP1A1 and CYP1A2 genes was shown to be closely related to the diet change accompanying weaning which occurs at weeks 3-4. In animals subjected to either delayed (week 6) or early (week 2) weaning, sharp accumulation of messages, proteins and related activities were delayed or anticipated accordingly with respect to normal weaning. Artificially scheduled weaning gave similar results when repeated with biological-grade lucern (grown in the absence of chemical fertilizers, pesticides, etc.), the main constituent of commercial rabbit chow. While CYP3A6 gene expression could be brought forward by early weaning at week 2, both message and protein did not exhibit increased accumulation after delayed weaning at week 6, and remained at the low level of the new born animal. Treatment of 1-week-old and 2-week-old animals with triiodothyronine or of 3-week-old animals with propylthiouracil, an antithyroid factor, did not modify the normal pattern of developmental expression of genes CYP1A1, CYP1A2 and CYP3A6. It is concluded that (a) the onset of CYP3A6 gene expression in the fetus occurs at day 30 of gestation, (b) expression of this gene may be induced transplacentally by rifampicin, (c) CYP1A1, CYP1A2 and CYP3A6 gene expression is sharply activated at weaning, and (d) thyroid hormones appear not to be responsible for the pattern of developmental expression of these genes in the rabbit.

The interaction between cyclosporin A and erythromycin studied by means of an isolated perfused rat liver model

The isolated perfused rat liver model was successfully used to study the clinical interaction observed between cyclosporin A (CsA) and erythromycin (ER). Three experimental studies were performed. In a "Control group", isolated rat livers were perfused with 60 micrograms 3H-CsA for 20 min. In an "ER group", isolated rat livers were simultaneously perfused with 60 micrograms 3H-CsA and 6.0 mg ER. In an "ER ind group", rats were treated with ER for 5 days, conditions under which the P450IIIA gene subfamily, principally involved in CsA metabolism, was specifically induced, and isolated rat livers were perfused with 60 micrograms 3H-CsA for 20 min. Biological parameters of the liver model such as biliary flow, oxygen consumption and pH were similar whatever group of animals. Biliary excretion of total radiolabel (sum of CsA and all the metabolites = CsAT) was similar in both the "Control" and "ER" groups but significantly higher in the "ER ind" group with respectively 8.05 +/- 0.81%, 8.45 +/- 1.93% and 14.54 +/- 2.12% of the entire amount of infused drug excreted during 2 hr. Using a high performance liquid chromatography method which specifically resolves unchanged CsA from its metabolites, we demonstrated that, although similar amounts of radiolabelled material were excreted in both the "Control" and "ER" groups, the CsA/metabolite ratio was lower than 1.0 in the "Control group" and higher than 1.0 in the "ER group", suggesting an inhibition of CsA metabolism by ER in the "ER group". These data could explain the increase in CsA blood levels observed in the clinic following administration of high ER doses and the reversing effect when ER administration is stopped.

Selective inactivation by 21-chlorinated steroids of rabbit liver and adrenal microsomal cytochromes P-450 involved in progesterone hydroxylation

The inactivation by 21-chlorinated steroids of rabbit liver cytochromes P-450 involved in the hydroxylation of progesterone has been investigated in intact microsomes encompassing two phenotypes of 21-hydroxylase activity, two phenotypes of 16 alpha-hydroxylase activity, and three phenotypes of 6 beta-hydroxylase activity. In liver microsomes from outbred New Zealand White male rabbits exhibiting a high content of cytochrome P-450 1, 21,21-dichloropregnenolone caused a time- and NADPH-dependent loss of 21-hydroxylase activity. This loss of activity exhibited a number of characteristics of mechanism-based inactivation, including irreversibility, saturation with increasing inhibitor concentrations, and protection by substrate, and was also documented with purified P-450 1 in a reconstituted system. 21,21-Dichloropregnenolone caused no time-dependent loss of 6 beta-hydroxylase activity in microsomes from the New Zealand White rabbits or from control or rifampicin-treated rabbits of the inbred B/J strain. In contrast, in the microsomes from the B/J rabbits, some inactivation of the 16 alpha-hydroxylase was observed (k = 0.04 min-1), regardless of the rifampicin treatment. The other two compounds tested, 21-chloropregnenolone and 21,21-dichloroprogesterone, were less effective than the dichloropregnenolone as inactivators of cytochrome P-450 1. On the other hand, 21,21-dichloroprogesterone, but not 21,21-dichloropregneolone, caused a rapid time-dependent loss of 21-hydroxylase activity in rabbit adrenal microsomes. The results indicate that the introduction of a dichloromethyl group into a substrate bearing a methyl group normally hydroxylated by only one or a few forms of cytochrome P-450 may be a rational means of designing selective inhibitors of the enzyme.

Complete sequence of cytochrome P450 3c cDNA and presence of two mRNA species with 3' untranslated regions of different lengths

Two cDNAs (pLM3c 4.1 and pLM3c 6.1) coding for rabbit cytochrome P450 3c were sequenced. cDNA 4.1 (1768 bp) exhibits an open reading frame from nucleotides 74 to 1576 encoding the 501 amino acid residues of the entire protein. cDNA 6.1 (189 bp) appears to encode the last 24 amino acids. Comparative amino acid sequence analysis indicated that P450 PCN1, PCN2, and HLp from rat and man, were 70, 67, and 73% homologous, respectively, to P450 3c. According to the cytochrome P450 nomenclature, the P450 3c gene is termed P450IIIA4. Comparison of the nucleotide sequences indicated that cDNA 6.1 was 100% homologous to cDNA 4.1. However, whereas a poly(A) tract started 23 nucleotides after the AATAAA consensus sequence in cDNA 6.1, cDNA 4.1 had a 3' untranslated region extending 101 bp beyond the polyadenylation signal, which lacked poly(A). This observation is consistent with the previous finding that both cDNA 4.1 and 6.1 hybridized with two distinct species of poly(A)RNA (1700 and 1850 bases) from rabbit liver. The extreme 3'-end 79-bp fragment of cDNA 4.1 therefore was isolated by subcloning in pUC12 (clone p18-Rsa I) and used to probe Northern blots of poly(A)RNA from control and rifampicin-treated rabbit liver. In contrast to cDNA 4.1 and 6.1, p18-Rsa I cDNA hybridized only with the largest (1850 bases) mRNA species. We conclude that rabbit liver contains two P450 3c mRNA species differing in the length of their 3' untranslated region.

Expression of five forms of microsomal cytochrome P-450 in primary cultures of rabbit hepatocytes treated with various classes of inducers

In order to investigate the expression of five different forms of microsomal cytochrome P-450 including P-450 2 (P450IIBI), 3b (P450IIC3), 3c (P450IIIA4), 4 and 6 (P450IA2 and A1), hepatocytes were isolated from untreated rabbit and maintained in primary monolayer cultures in serum free modified Waymouth medium in the absence and in the presence of various classes of inducers including phenobarbital (PB), rifampicin (RIF), dexamethasone (DEX) and B-naphthoflavone (BNF). In untreated cultures the level of the various forms of P-450, determined by immunoblot with the use of specific antibodies, generally declined with time but at markedly different rates. In cultures treated with the inducers decline of the various forms was either unaffected, reduced, or even reversed, so that 96 hr after plating some of these forms appeared to be induced several-fold with respect to the untreated cultures. The forms 2 and 3c were co-induced by PB, RIF or DEX; as in vivo, BNF induced forms 4 and 6. Induction of forms 2, 3c, 4 and 6 was accompanied by stimulation of related monooxygenase activities, benzphetamine demethylase, progesterone 6B hydroxylase and benzpyrene hydroxylase and ethoxyresorufin deethylase, respectively. In all cases, induction was accompanied by an increased rate of de novo synthesis of the protein, determined by radio-immunoprecipitation assay with the use of specific antibodies on [3H]-Leu labeled cell lysate. Both induction and increased de novo synthesis were time- and inducer concentration-dependent. In cultures treated with RIF or BNF de novo synthesis of P-450 3c or of P-450 4 and 6 was correlated with the level of their specific mRNA quantitated from northern blots probed with either pLM3c-4.1 or pLM6.1, two plasmids containing inserted cDNA coding for P-450 3c or P-450 6, respectively. We conclude from these experiments that rabbit hepatocytes in primary monolayer cultures represent suitable models for studying regulation induction and pharmacological implications of the microsomal cytochromes P-450.

Intraspecies individuality for the metabolism of steroids

A variety of regulatory factors contribute to differences in the rates of 6 beta-hydroxylation, 16 alpha-hydroxylation and 21-hydroxylation of progesterone as catalysed by liver microsomes prepared from individual rabbits. It is likely that the 6 beta-hydroxylation of progesterone is catalysed primarily by cytochrome P-450 3c, an enzyme that exhibits allosteric activation by alpha-napthoflavone, and by a form of P-450 3b, 6 beta+, that is expressed in some rabbits in an autosomal dominant manner. The mechanism of activation for P-450 3c appears to reflect an effector mediated increase of the affinity of the enzyme for substrate as judged by substrate binding studies. A second form of P-450 3b, 6 beta-, catalyses a major portion of hepatic progesterone 16 alpha-hydroxylation and exhibits activation by a variety of C21 steroids of which 5 beta-pregnane-3 beta,20 alpha-diol is the most efficacious. P-450 1, which catalyses the 21-hydroxylation of progesterone, is expressed at 10-fold higher levels in the 21H phenotype than the 21L phenotype, and the former is inherited as an autosomal dominant characteristic. A cDNA encoding a P-450 1-related gene product exhibits a predicted amino acid sequence that is 95% homologous to that of P-450 1. The P-450 1-related gene product is expressed in liver to a similar degree in both 21H and 21L rabbits.

Cloning of a cDNA coding for P-450 LM3c from rabbit liver microsomes and regulation of its expression

Liver cytochromes P-450 LM3c in the rabbit and P-450p in the rat are two related forms, inducible by macrolide antibiotics such as triacetyloleandomycin (TAO) and glucocorticoids such as dexamethasone. We prepared a cDNA library from TAO induced rabbit liver mRNA and characterized a cDNA (pLM3c-4.1) that hybridized to pDex 3.22, a cDNA complementary to cytochrome P-450p mRNA. Northern blots of liver poly(A)RNA from untreated or TAO, erythromycin and rifampicin treated animals, revealed two mRNA species of approximately 1700 and 1850 nucleotides in length, that hybridized to LM3c cDNA and to pDEX 3.22. The level of both mRNAs was increased five fold over control by macrolide antibiotics but unaffected by both phenobarbital and B-naphthoflavone. After 5 days of TAO treatment LM3c mRNA had increased 5 fold while LM3c protein had increased 25 fold. However, the rate of P-450 LM3c gene transcription measured in isolated liver nuclei remained unchanged throughout five days of TAO treatment. We conclude that TAO may induce cytochrome P-450 LM3c by post-transcriptional effects.