An in-vitro study on the metabolism of rokitamycin and possible interactions of the drug with rat liver microsomes

This in-vitro study was designed to identify the enzyme(s) involved in the major metabolic pathway of rokitamycin, i.e. the formation of leucomycin A7, and to assess possible interactions of the drug with rat liver microsomes. Formation of leucomycin A7 was NADPH-independent and was not appreciably inhibited by anti-rat NADPH cytochrome P-450 reductase serum or cimetidine, a nonspecific inhibitor of cytochrome P-450 isoforms. Eadie-Hofstee plots for the formation of leucomycin A7 were indicative of apparently monophasic behaviour for six rat liver microsomes tested. The mean (+/- s.d.) kinetic parameters, Km, Vmax and Vmax/Km, for the formation of leucomycin A7 from rokitamycin were 47+/-13 microM, 390+/-56 nmol min(-1) (mg protein)(-1) and 8.6+/-1.6 mL min(-1) (mg protein)(-1), respectively. Three esterase inhibitors (100 microM), bis-nitrophenylphosphate, physostigmine and metrifonate inhibited the formation of leucomycin A7 by more than 60%. Metabolism of rokitamycin was inhibited by terfenadine, but not by mequitazine, whereas chlorpheniramine and theophylline activated the formation of leucomycin A7. Rokitamycin, leucomycin A7, leucomycin V, erythromycin and clarithromycin were weak inhibitors of CYP3A-catalysed 3-hydroxylation of quinine with mean IC50 values ranging from 71 to >100 microM. It is concluded that in rat liver microsomes the formation of leucomycin A7 from rokitamycin is catalysed mainly by an esterase (possibly cholinesterase, EC3.1.1.8), but not by cytochrome P-450 enzyme(s). Although in this in-vitro animal study CYP3A activity was barely inhibited by rokitamycin, the possibility cannot be totally discounted in man when rokitamycin is co-administered with drugs metabolized by CYP3A.

Identification and cloning of GCA1, a gene that encodes a cell surface glucoamylase from Candida albicans

Adherence of yeast cells of Candida albicans to human oesophageal cells is greater when cells are grown in 500 mM D-galactose in comparison to D-glucose at the same concentration. Moreover, a 190 kDa mannoprotein (MP190) from a yeast cell wall preparation is highly expressed when cells are grown in the presence of galactose but less so in glucose. We now report on the identification of the MP190 and the isolation of its encoding gene. MP190 was purified, and three internal peptides were isolated and sequenced. Each of the three peptides showed significant homology (65-85%) with a glucoamylase (GAM1) from the yeast, Schwanniomyces occidentalis. In order to isolate the C. albicans homologue of GAM1 (GCA1), we probed a genomic library with a 0.9-kb internal fragment of the S. occidentalis GAM1 and isolated a 2.3-kb clone that corresponded to the 5' region of the gene. Polymerase chain reaction (PCR) amplification was used to isolate the remainder of the open reading frame. GCA1 encodes a 946 amino acid protein containing three putative hydrophobic, membrane-spanning domains and 15 potential N-glycosylation sites. Both Gca1p and GAM1 are novel to the family of glycosyl hydrolases. Northern analysis indicated that GCA1 is transcribed to a greater extent in galactose than in sucrose or glucose. Also, using reverse transcriptase (RT)-PCR, we observed expression of GCA1 in a rat model of oral candidiasis, indicating that Gca1p is expressed during disease development.

Evidence for involvement of polymorphic CYP2C19 and 2C9 in the N-demethylation of sertraline in human liver microsomes

AIMS

The present study was designed to define the kinetic behaviour of sertraline N-demethylation in human liver microsomes and to identify the isoforms of cytochrome P450 involved in this metabolic pathway.

METHODS

The kinetics of the formation of N-demethylsertraline were determined in human liver microsomes from six genotyped CYP2C19 extensive (EM) and three poor metabolisers (PM). Selective inhibitors of and specific monoclonal antibodies to various cytochrome P450 isoforms were also employed.

RESULTS

The kinetics of N-demethylsertraline formation in all EM liver microsomes were fitted by a two-enzyme Michaelis-Menten equation, whereas the kinetics in all PM liver microsomes were best described by a single-enzyme Michaelis-Menten equation similar to the low-affinity component found in EM microsomes. Mean apparent Km values for the high-and low-affinity components were 1.9 and 88 microm and V max values were 33 and 554 pmol min-1 mg-1 protein, respectively, in the EM liver microsomes. Omeprazole (a CYP2C19 substrate) at high concentrations and sulphaphenazole (a selective inhibitor of CYP2C9) substantially inhibited N-demethylsertraline formation. Of five monoclonal antibodies to various cytochrome P450 forms tested, only anti-CYP2C8/9/19 had any inhibitory effect on this reaction. The inhibition of sertraline N-demethylation by anti-CYP2C8/9/19 was greater in EM livers than in PM livers at both low and high substrate concentrations. However, anti-CYP2C8/9/19 did not abolish the formation of N-demethylsertraline in the microsomes from any of the livers.

CONCLUSIONS

The polymorphic enzyme CYP2C19 catalyses the high-affinity N-demethylation of sertraline, while CYP2C9 is one of the low-affinity components of this metabolic pathway.

Avirulence of Candida albicans CaHK1 mutants in a murine model of hematogenously disseminated candidiasis

Deletion of both alleles of the Candida albicans CaHK1 gene, which causes cells to flocculate when grown at pH 7.5, a pH comparable to that of mammalian blood, abolishes the ability of the yeast to establish a successful infection in a murine model of hematogenously disseminated candidiasis. Within 72 h all mice inoculated with the parental C. albicans strain had died. The mice infected with either the heterozygote or revertant strain, either of which harbors only one functional CaHK1 allele, also succumbed to the infection, although survivors were observed for up to 16 days postinfection. However, mice inoculated with the Deltacahk1 null strain survived for the course of the infection. These results indicate that CaHK1 is required for the virulence of C. albicans in a murine model of hematogenously disseminated candidiasis. In contrast, CaHK1 is not required for the virulence of C. albicans in a rat model of vaginal candidiasis.

A further interaction study of quinine with clinically important drugs by human liver microsomes: determinations of inhibition constant (Ki) and type of inhibition

Our previous study showed that several drugs inhibited quinine 3-hydroxylation, a cytochrome P450 (CYP) 3A4-mediated reaction, in vitro. In this extended study, 13 drugs were selected and tested by human liver microsomes in order to further determine their respective inhibition constant (Ki) and type of inhibition. According to the apparent Ki values, the inhibitory rank order of these tested drugs was as follows: ketoconazole > doxycycline > omeprazole > tetracycline > troleandomycin (with pre-incubation) > primaquine > troleandomycin (without pre-incubation) > nifedipine > erythromycin > verapamil > oleandomycin > diltiazem > cimetidine > hydralazine. Among these drugs, doxycycline, tetracycline, ketoconazole, nifedipine and hydralazine were judged as mixed inhibitors; whereas, the remaining other drugs tested were judged as competitive inhibitors. When the plasma/serum concentrations possibly attained after their usual therapeutic doses were taken into account, tetracycline, doxycycline, omeprazole, ketoconazole, nifedipine, troleandomycin and erythromycin are likely to be inhibitors of quinine metabolism in patients when these drugs are co-administrated with quinine.

An in vitro study on the metabolism and possible drug interactions of rokitamycin, a macrolide antibiotic, using human liver microsomes

This in vitro study was designed to identify the enzyme(s) involved in the two major metabolic pathways of rokitamycin [formations of leucomycin A7 (LMA7) from rokitamycin and of leucomycin V (LMV) from LMA7] and to assess possible drug interactions using human liver microsomes. Formation of LMA7 or LMV was NADPH-independent. Anti-rat NADPH cytochrome P-450 (CYP) reductase serum, specific inhibitors, or substrates of CYP isoforms showed no effects on the formation of LMA7 or LMV. The mean Vmax and Vmax/Km for the formation of LMA7 from rokitamycin were much greater (P <.01) than those for the formation of LMV from LMA7. Two esterase inhibitors, bis-nitro-phenylphosphate and physostigmine (100 microM), inhibited the formation of LMA7 or LMV by more than 85%, whereas no appreciable inhibition occurred by several substrates of carboxylesterase (EC 3.1.1.1). Except the moderate inhibition produced by promethazine and terfenadine, theophylline, mequitazine, chlorpheniramine, and diphenhydramine showed little or no inhibition for the formation of LMA7 or LMV. Rokitamycin, LMA7, LMV, erythromycin, and clarithromycin (up to 500 microM) had no appreciable inhibition for CYP1A2-, 2C9-, and 2D6-mediated catalytic reactions. However, rokitamycin, LMA7, erythromycin, and clarithromycin inhibited the CYP3A4-catalyzed triazolam alpha-hydroxylation with IC50 (Ki) values of 5.8 (2.0), 40, 33 (20), and 56 (43) microM, respectively. It is concluded that the formations of LMA7 from rokitamycin and of LMV from LMA7 are catalyzed mainly by human esterase enzyme [possibly cholinesterase (EC3.1.1.8)]. However, whether rokitamycin would inhibit the CYP3A-mediated drug metabolism in vivo requires further investigations in patients.

CYP2C19 genotype status and effect of omeprazole on intragastric pH in humans

OBJECTIVE

Omeprazole is metabolized by genetically determined S-mephenytoin 4'-hydroxylase (CYP2C19) in the liver. This study aimed to determine whether the effect of omeprazole on intragastric pH depends on CYP2C19 genotype status.

METHODS

CYP2C19 genotype status for 2 mutations associated with the poor metabolizer phenotype was determined by a polymerase chain reaction-restriction fragment length polymorphism method in 16 healthy volunteers. Helicobacterpylori status was determined by serology and the [13C]urea breath test. After a single oral administration of 20 mg omeprazole or a placebo, intragastric pH values were recorded for 24 hours. Plasma levels of omeprazole and its 2 metabolites and gastrin were measured before and 1, 2, 3, 5, 7, 10, and 24 hours after administration.

RESULTS

Fifteen of the 16 subjects were H pylori negative. Five of the 15 subjects were homozygous extensive metabolizers, 4 were heterozygous extensive metabolizers, and 6 were poor metabolizers. After omeprazole administration, significant differences in mean intragastric pH values and plasma levels of gastrin, omeprazole and its metabolites were observed among the 3 groups, whereas no significant differences in these parameters were observed with the placebo administration.

CONCLUSIONS

The effect of omeprazole on intragastric pH significantly depends on CYP2C19 genotype status. The genotyping test of CYP2C19 may be useful for an optimal prescription of omeprazole.

A study on the metabolism of etoposide and possible interactions with antitumor or supporting agents by human liver microsomes

The metabolism of etoposide was investigated by using human liver microsomes and nine recombinant human cytochrome P450 (CYP) isoforms to identify the CYP isoform(s) involved in the major metabolic pathway (3'-demethylation) of etoposide as well as to evaluate the possible metabolic interactions with several antitumor or supporting agents. The 3'-demethylation of etoposide followed a Michaelis-Menten one-enzyme kinetic behavior in six human liver microsomal samples. The relationships were assessed with six different human liver microsomes between the 3'-demethylation of etoposide and metabolic activities for substrate probes of the respective CYP isoforms, showing a significant correlation (r = 0. 932, P < .01) only with 6beta-hydroxylation of testosterone, a marker substrate for CYP3A4. Inhibitor/substrate probes for CYP3A4, ketoconazole, troleandomycin, verapamil and cyclosporin, or supporting agents, vincristine and prednisolone, inhibited etoposide 3'-demethylation by human liver microsomes. p-Nitrophenol, a substrate for CYP2E1, also inhibited etoposide 3'-demethylation. Among the nine recombinant human CYP isoforms, CYP3A4 exhibited the highest catalytic activity with respect to etoposide 3'-demethylation, compared with the minor activities of CYP1A2 and 2E1. Collectively, these data suggest that etoposide 3'-demethylation is mediated mainly by CYP3A4 and to a minor extent by CYP1A2 and 2E1. Furthermore, some supporting agents (vincristine and prednisolone) and the substrates of CYP3A4, which may be coadministered with etoposide during the cancer chemotherapies, inhibit the etoposide 3'-demethylation activity in vitro. The results may provide clinical implications with respect to the possible metabolic interactions between etoposide and other drugs studied herein in patients with cancer undergoing etoposide concurrently with either of them.

Up-regulation of two Candida albicans genes in the rat model of oral candidiasis detected by differential display

Candida albicans is an opportunistic fungal pathogen responsible for the largest percentage of fungal-mediated oral and oesophageal disease. In this regard, knowledge concerning patterns of gene expression during the establishment and/or maintenance of infection may be the key to the design of new strategies for treatment, as well as providing insight into pathogenesis. To address this issue, experiments were performed that utilized differential display to compare the spectrum of C. albicans genes expressed during oral infection versus growth in in vitroculture. Experimentally, the rat model of oral candidiasis served as the in vivo source. After initiation of infection and subsequent harvesting of C. albicans from the rat oral cavity, RNA was isolated, and used with a small number of primers in reverse-transcriptase polymerase chain reaction (RT-PCR) and differential display experiments. Fragments unique to in vivo samples were subcloned and sequenced. Southern blot analysis verified the origin of seven fragments as fromC. albicans. Additionally, specific RT-PCR confirmed that two of these fragments represented genes that were up-regulated during C. albicans in vivo growth in the rat model. Database searches indicated the fragments share homology with a member of the C. albicans agglutinin gene family and to a bacterial gene (gidB) possibly involved in cell division.

Identity of the residues responsible for the species-restricted complement inhibitory function of human CD59

The membrane-anchored glycoprotein CD59 inhibits assembly of the C5b-9 membrane attack complex (MAC) of human complement. This inhibitory function of CD59 is markedly selective for MAC assembled from human complement components C8 and C9, and CD59 shows little inhibitory function toward MAC assembled from rabbit and many other non-primate species. We have used this species selectivity of CD59 to identify the residues regulating its complement inhibitory function: cDNA of rabbit CD59 was cloned and used to express human/rabbit CD59 chimeras in murine SV-T2 cells. Plasma membrane expression of each CD59 chimera was quantified by use of a 5'-TAG peptide epitope, and each construct was tested for its ability to inhibit assembly of functional MAC from human versus rabbit C8 and C9. These experiments revealed that the species selectivity of CD59 is entirely determined by sequence contained between residues 42 and 58 of the human CD59 polypeptide, whereas chimeric substitution outside this peptide segment has little effect on the MAC inhibitory function of CD59. Substitution of human CD59 residues 42-58 into rabbit CD59 resulted in a molecule that was functionally indistinguishable from native human CD59, whereas the complementary construct (corresponding residues of rabbit CD59 substituted into human CD59) was functionally indistinguishable from rabbit CD59. Based on the solved solution structure of CD59, these data suggest that selectivity for human C8 and C9 resides in a cluster of closely spaced side chains on the surface of CD59 contributed by His44, Asn48, Asp49, Thr51, Thr52, Arg55, and Glu58 of the polypeptide.

Mutual inhibition between quinine and etoposide by human liver microsomes. Evidence for cytochrome P4503A4 involvement in their major metabolic pathways

The mutual inhibition between quinine and etoposide with their major metabolic pathways (i.e. quinine 3-hydroxylation and etoposide 3'-demethylation) was examined in vitro by human liver microsomes. Etoposide inhibited quinine 3-hydroxylation in a concentration-dependent manner with a mean IC50 of 65 microM. The mean maximum inhibition by etoposide (100 micro) of quinine 3-hydroxylation was about 60%. Similarly, etoposide 3'-demethylation was inhibited by quinine in a concentration-related manner with a mean IC50 value of 90 microM. The mean maximum inhibition by quinine (100 M) of etoposide 3'-demethylation was about 52%. An excellent correlation (r = 0.947, p < 0.01) between quinine 3-hydroxylase and etoposide 3'-demethylase activities in six different human liver microsomes was observed. Two inhibitors of CYP3A4, ketoconazole (1 microM) and troleandomycin (100 microM), inhibited quinine 3-hydroxylation by about 90% and 80%, and etoposide 3'-demethylation by about 75% and 65%, respectively. We conclude that quinine and etoposide mutually inhibit the metabolism of each other, consistent with the previous finding that CYP3A4 catalyzes the metabolism of both substrates.

The In vitro hepatic metabolism of quinine in mice, rats and dogs: comparison with human liver microsomes

The major metabolic pathway of quinine in the human has been shown to be 3-hydroxylation mediated mainly by human cytochrome P450 (CYP) 3A4. In this extended in vitro study, quinine 3-hydroxylation was further investigated using microsomes from mouse, rat, dog and human livers and was compared among them in terms of the in vitro enzyme-kinetic parameters and quinine-drug interaction screenings. In all species, 3-hydroxyquinine was the principal metabolite of quinine. There was intra- and interspecies variability among all the kinetic parameters, and dogs exhibited a closer resemblance to humans in terms of the mean kinetic data. Ketoconazole and troleandomycin inhibited the 3-hydroxylation of quinine in all species. Both alpha-naphthoflavone and diazepam showed an interspecies difference in quinine 3-hydroxylation: a trend toward an activation in dog and human, and a significant inhibition in mouse and rat, liver microsomes. Antisera raised against rat CYP3A2 strongly inhibited quinine 3-hydroxylation by about 96, 84 and 92% with mouse, rat and dog liver microsomes, respectively, but neither anti-rat 2C11 and 2E1 antisera did so with rat liver microsomes. Primaquine, doxycycline and tetracycline substantially inhibited the formation of 3-hydroxyquinine in rat, dog and human species, but proguanil had no such effect in any species. Chloroquine inhibited quinine 3-hydroxylation with rat and dog liver microsomes but not with human liver microsomes. There was a significant correlation (r = 0.986, P < .001) between the CYP3A contents and the formation rates of 3-hydroxyquinine in eight human liver microsomal samples. It is concluded that 3-hydroxyquinine is a main metabolite of quinine and that CYP3A/Cyp3a is a principal isoform involved in this metabolic pathway in the respective (rat, dog and human/mouse) species tested. The dog and possibly the rat may be qualitatively and quantitatively suitable animal models for exploring the quinine 3-hydroxylase activity and for screening quinine-drug interactions in vitro, at certain inconsistency with the human liver microsomal data.

Metabolic interactions of selected antimalarial and non-antimalarial drugs with the major pathway (3-hydroxylation) of quinine in human liver microsomes

AIMS

Nine antimalarial (plus two metabolites of proguanil) and twelve non-antimalarial drugs were tested for their possible interaction with CYP3A4-catalysed 3-hydroxylation of quinine by human liver microsomes in vitro.

METHODS

3-Hydroxyquinine was assayed in the incubation mixture by an h.p.l.c. method using fluorometric detection. The respective IC50 values were estimated for the twenty-one drugs and two metabolites of proguanil tested herein.

RESULTS

Thirteen drugs exhibited an inhibitory effect on the 3-hydroxylation of quinine. According to the respective mean IC50 values, the inhibitory rank order of the drugs was: ketoconazole > troleandomycin (TAO, with preincubation) > doxycycline > omeprazole > primaquine > tetracycline = TAO (without preincubation) > nifedipine > erythromycin > verapamil > cimetidine > diltiazem > oleandomycin > hydralazine. Other drugs or metabolites showed little or no inhibition of quinine metabolism (mean IC50 > 200 or 500 microM). Among the antimalarial drugs, doxycycline showed relatively potent inhibition of quinine 3-hydroxylation with a mean IC50 value of 17 microM, followed by primaquine and tetracycline, with mean IC50 values of 20 and 29 microM, respectively.

CONCLUSIONS

When the plasma/serum concentrations possibly attained after their usual therapeutic doses were taken into account, tetracycline, doxycycline, omeprazole, ketoconazole, nifedipine, TAO and erythromycin are likely to be inhibitors of quinine metabolism in patients when the drugs are co-administrated with quinine.

Structure-activity and mechanism studies on silicon phthalocyanines with Plasmodium falciparum in the dark and under red light

Syntheses for the new photosensitizers HOSiPcOSi(CH3)2(CH2)3N(CH1)1 or 3(CH3)2, Pc 34 and Pc 25, have been developed and the order of activity of these photosensitizers and the previously reported photosensitizer Pc 4, HOSiPcOSi(CH3)2(CH2)3N(CH3)2, in the dark and with broad-band red light toward Plasmodium falciparum in red blood cell (RBC) suspensions has been studied. The order of activity has been found to be Pc 4 > Pc 34 > Pc 25. Thus, the activity of the photosensitizers under both sets of conditions is inversely proportional to the length of their terminal amino alkyl chains. The 50% inhibition dye concentration (IC50) in the dark for the parasites in RBC suspension with Pc 4 is 24 nM and the dye concentration and light fluence that yield > or = 3 log10 of parasite inactivation with Pc 4 are 2 microM and 3 J/cm2, respectively. The synthesis of DNA and proteins by the parasites in culture was strongly inhibited by Pc 4 in the dark while parasite lactate dehydrogenase (pLDH) activity was unaffected. With Pc 4 and light, DNA and protein synthesis of the parasites in culture was strongly inhibited, pLDH activity of the parasites was moderately inhibited and ribosome density of the parasite cells was reduced. Gel electrophoresis studies showed that synthesis of all parasite proteins was inhibited to a similar extent. These results suggest that Pc 4 both in the dark and with light inactivates the cells by disturbing their machinery for the synthesis of not just one but a whole series of proteins. It is concluded that Pc 4 and light may be able to serve as a practical sterilization combination not only for HIV and other viruses but also for malaria parasites in RBC concentrates, and that Pc 4 by itself may have potential as a chemotherapeutic agent toward malaria.

Corticotropin-releasing hormone-binding protein and its possible role in neuroendocrinological research

Corticotropin-releasing hormone-binding protein (CRH-BP), predominately produced by the liver, is a glycoprotein with a molecular weight of 37 kDa. The mature protein consists of 7 exons and 6 introns, with 5 tandem disulfide bridges which are essential for the binding of corticotropin-releasing hormone (CRH). This binding protein is distributed and expressed differently from corticotropin-releasing hormone receptors (CRH-Rs), as is the ligand requirement. Most CRH in plasma is bound to its binding protein, is therefore inactive and unable to bind to its receptor. Other competitives can reverse the binding, liberating CRH. Together with the CRH neuropeptides and CRH receptors, CRH-binding protein plays a role in the hypothalamic-pituitary-adrenal axis, in immune/inflammatory reactions as an auto/paracrine proinflammatory regulator, in pregnancy, as well as in some pathological conditions.

Avirulence of Candida albicans FAS2 mutants in a mouse model of systemic candidiasis

Disruption of both alleles of the Candida albicans FAS2 gene abolishes the ability of the organism to establish infection in a murine model of systemic candidiasis. Within 72 h all mice inoculated with 10(6) CFU of the parental C. albicans strain had died. In contrast, all animals inoculated with the mutant strain CFD2 survived for the course of the experiment (21 days). Animals infected with either mutant strain CFD1 or CFD3, in which only one FAS2 allele was disrupted, also succumbed to infection, but mortality was not observed until 4 days postinfection and survivors remained for up to 20 days postinfection. The results demonstrate that FAS2 is required for successful C. albicans infection.

Identification of human cytochrome P450 isoforms involved in the 3-hydroxylation of quinine by human live microsomes and nine recombinant human cytochromes P450

Studies using human liver microsomes and nine recombinant human cytochrome P450 (CYP) isoforms (CYP1A1, 1A2, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1 and 3A4) were performed to identify the CYP isoform(s) involved in the major metabolic pathway (3-hydroxylation) of quinine in humans. Eadie-Hofstee plots for the formation of 3-hydroxyquinine exhibited apparently monophasic behavior for all of the 10 different microsomal samples studies. There was interindividual variability in the kinetic parameters, as follows: 1.8-, 3.2- and 3.5-fold for K(m) Vmax and Vmax/K(m), respectively. The mean +/- S.D. values for K(m), Vmax and Vmax/K(m) were 106.1 +/- 19.3 microM, 1.33 +/- 0.48 nmol/mg protein/min and 12.8 +/- 5.1 microliters/mg protein/min, respectively. With 10 different human liver microsomes, the relationships between the 3-hydroxylation of quinine and the metabolic activities for substrates of the respective CYP isoforms were evaluated. The 3-hydroxylation of quinine showed an excellent correlation (r = 0.986, P < .001) with 6 beta-hydroxylation of testosterone, a marker substrate for CYP3A4. A significant correlation (r = 0.768, P < .01) between the quinine 3-hydroxylase and S-mephenytoin 4'-hydroxylase activities was also observed. However, no significant correlation existed between the 3-hydroxylation of quinine and the oxidative activities for substrates for CYP1A2 (phenacetin), 2C9 (diclofenac), 2D6 (desipramine) and 2E1 (chlorzoxazone). Ketoconazole and troleandomycin (inhibitors of CYP3A4) inhibited the 3-hydroxylation of quinine by human liver microsomes with respective mean IC50 values of 0.026 microM and 28.9 microM. Anti-CYP3A antibodies strongly inhibited quinine 3-hydroxylation, whereas weak inhibition was observed in the presence of S-mephenytoin or anti-CYP2C antibodies. Among the nine recombinant human CYP isoforms, CYP3A4 exhibited the highest catalytic activity with respect to the 3-hydroxylation of quinine, compared with the minor activity of CYP2C19 and little discernible or no effect of other CYP isoforms. Collectively, these data suggest that the 3-hydroxylation of quinine is mediated mainly by CYP3A4 and to a minor extent by CYP2C19. Other CYP isoforms used herein appear to be of negligible importance in this major pathway of quinine in humans.

Function and expression of flavohemoglobin in Saccharomyces cerevisiae. Evidence for a role in the oxidative stress response

We have studied the function and expression of the flavohemoglobin (YHb) in the yeast Saccharomyces cerevisiae. This protein is a member of a family of flavohemoproteins, which contain both heme and flavin binding domains and which are capable of transferring electrons from NADPH to heme iron. Normally, actively respiring yeast cells have very low levels of the flavohemoglobin. However, its intracellular levels are greatly increased in cells in which the mitochondrial electron transport chain has been compromised by either mutation or inhibitors of respiration. The expression of the flavohemoglobin gene, YHB1, of S. cerevisiae is sensitive to oxygen. Expression is optimal in hyperoxic conditions or in air and is reduced under hypoxic and anaerobic conditions. The expression of YHB1 in aerobic cells is enhanced in the presence of antimycin A, in thiol oxidants, or in strains that lack superoxide dismutase. All three conditions lead to the accumulation of reactive oxygen species and promote oxidative stress. To study the function of flavohemoglobin in vivo, we created a null mutation in the chromosomal copy of YHB1. The deletion of the flavohemoglobin gene in these cells does not affect growth in either rhoo or rho+ genetic backgrounds. In addition, a rho+ strain carrying a yhb1(-) deletion has normal levels of both cyanide-sensitive and cyanide-insensitive respiration, indicating that the flavohemoglobin does not function as a terminal oxidase and is not required for the function or expression of the alternative oxidase system in S. cerevisiae. Cells that carry a yhb1(-)deletion are sensitive to conditions that promote oxidative stress. This finding is consistent with the observation that conditions that promote oxidative stress also enhance expression of YHB1. Together, these findings suggest that YHb plays a role in the oxidative stress response in yeast.

Requirement for the Candida albicans FAS2 gene for infection in a rat model of oropharyngeal candidiasis

The virulence of Candida albicans strains deficient in fatty acid synthase activity by virtue of disruption/deletion of the FAS2 gene was examined in a rat model of oropharyngeal candidiasis. The FAS2 alleles of C. albicans CAI4 (delta ura3::imm434/delta ura3::imm434) were sequentially disrupted with a cassette that included a portion of FAS2 from which a 984 bp fragment containing the FAS condensing reaction domain was deleted and replaced with hisG-URA3-hisG sequences. Verification of fatty acid synthase inactivation was obtained from assays of enzyme activity. Strains in which a single allele was disrupted (CFD1 and CFD3) exhibited an approximately 20% reduction in activity, when compared to wild-type. In addition, fatty acid synthase activity was abolished in a FAS2 null mutant strain (CFD2), and growth of CFD2 occurred only when the growth medium was supplemented with Tween 40 and certain fatty acids. Strain CFD2 was avirulent in the rat model, indicating that fatty acid synthase activity is required for C. albicans oropharyngeal infection. Strains with a single FAS2 allele disruption colonized the oral cavity, but the number of cells recovered from infected animals was approximately fivefold less than for the parental strain. The results suggest that FAS may be exploited as a possible target for the development of new antifungal agents.

Redox dependent interactions of the metal sites in carbon monoxide-bound cytochrome c oxidase monitored by infrared and UV/visible spectroelectrochemical methods

Spectroelectrochemical titration studies involving the binding of the infrared-active probe ligand carbon monoxide (CO) to the heme alpha 3/CuB site of bovine heart cytochrome c oxidase (CcO) have been reexamined. The spectroelectrochemical cell employed was constructed to monitor both the infrared (IR) and visible/Soret spectra of the CcO-CO complex as a function of the overall oxidation state of the enzyme. A number of commonly used electron transfer mediators were employed to shuttle electrons between the redox active sites within the enzyme and the electrode surface. The well-documented shift in the CO infrared stretch band maximum from 1963.3 cm-1 (CcO fully reduced) to 1965.5 cm-1 (CcO partially oxidized) was carefully titrated electrochemically. Deconvolution of the asymmetric CO stretches indicates the existence of two different states of CO vibrators within the enzyme, presumably due to two conformers which are present in a ratio of approximately 5:1. Upon incrementally stepping the potential from the fully reduced state to the partially oxidized state, we found it possible to follow the decrease in the intensity of the original pair of these conformers and the concomitant increase of a resultant pair while maintaining this 5:1 ratio between the conformers. By plotting the change in the deconvoluted CO peak intensities vs the redox potential, as well as the absorbance changes in the visible/Soret spectra vs the redox potential, we found not only that both fit an n = 1 electron process but also that the spectral changes tracked each other identically with experimental error. Furthermore, analysis of the second derivative of the Soret spectra allowed for the qualitative monitoring of the oxidation state of the Fe alpha site which again tracked identically to that of the CO shift in the IR region. These results would seem to confirm earlier suggestions that perturbing the oxidation state of Fe alpha causes a conformational change in the enzyme which affects the binding site for CO, namely heme alpha 3. As a consequence of the CO IR stretching frequencies changing by only 2 cm-1 during this redox titration, with no accompanying changes in half band width, we suggest that it is impossible that this small but significant change seen in the CO stretching frequencies could be due to an oxidation state change in CuB, given the known sensitivity of the CO stretching frequency to perturbations and the close proximity of Cu(B) to the CO binding site at heme alpha 3 (4.5 A). Therefore, it would appear that Cu(B) must remain reduced as long as CO is bound to the heme alpha 3 site. This is consistent with earlier proposals that Fe alpha 3 and Cu(B) are acting together as a two-electron donor to dioxygen.

Cytochrome c oxidase catalysis of the reduction of nitric oxide to nitrous oxide

Reduction of nitric oxide (NO) to nitrous oxide (N2O) is catalyzed by bovine heart cytochrome c oxidase (CcO) in anaerobic solutions at pH 7.2 and 20 degrees C. Cyanide inhibits and forms Fea3(3+)CN. The mononitrosyl (Fea3(2+)NO), but not the dinitrosyl (Fea3(2+)NO; CuB+NO), is a likely intermediate in N2O formation. One-electron reduction of NO at Fea3(2+) could yield N2O via HNO. However, a two-electron reduction of the NO ligand to give an intermediate that reacts with a second NO to give N2O and H2O appears more likely. Conversion of NO to N2O is favored by low levels of both NO and O2, higher NO levels can inhibit both cytochrome c oxidase and NO reductase activities. Raising the O2 level will favor catalysis of NO oxidation to NO2 by CcO. The reactions of NO and the specific CcO activity that occur in tissue will be critically dependent on NO, O2, and CcO levels.

GD2 oligosaccharide: target for cytotoxic T lymphocytes

Carbohydrate antigens rarely provide target epitopes for cytotoxic T lymphocytes (CTL). Disialoganglioside GD2 is a glycolipid expressed at high levels in human tumors and a small group of murine lymphomas (EL4, RBL5, RMA, RMA-S, A13, and BALBRVE). Immunization of C57B1/6 mice with irradiated EL4 cells stimulated a specific CTL response and protected these animals from engraftment of EL4 lymphoma. The CTL activity resided in the CD4-CD8+ population, was dependent on T cell receptor alpha/beta, and was not removed by anti-natural killer cell immunoabsorption, but was restricted to GD2 and H-2b bearing targets. CTL activity could be completely inhibited by GD2-oligosaccharide-specific monoclonal antibodies and their F(ab')2 fragments, but not by immunoglobulin G3 myelomas or antibodies against GD3 or GM2. Soluble GD2 did not inhibit specific tumor lysis. RMA-S lymphoma cells (GD2+H-2b-TAP2 deficient) were resistant to GD2-specific CTL. Sialic acid-containing peptides eluted from EL4 lymphoma cells could (a) stabilize H-2 molecules on RMA-S cells and (b) sensitize them for GD2-specific CTL. Control peptides (derived from vesicular stomatitis virus nucleoprotein peptide and GD2-negative lymphomas) could also stabilize H-2 on RMA-S, but were resistant to GD2-specific CTL. These H-2-binding peptides could be purified by anti-GD2 affinity chromatography. We postulate a new class of naturally occurring epitopes for T cells where branched-chain oligosaccharides are linked to peptides with anchoring motifs for the major histocompatibility complex class I pocket. While analogous to the haptens trinitrophenyl and O-beta-linked acetyl-glucosamine, the potential implications of natural carbohydrates as antigenic epitopes for CTL in biology are considerable.

Effects of overall oxidation state on infrared spectra of heme a3 cyanide in bovine heart cytochrome c oxidase. Evidence of novel mechanistic roles for CuB

Effects of changes in oxidation state at the other metal centers on oxidized heme a3 cyanide of bovine heart cytochrome c oxidase have been investigated. Only one CN- binds, giving Fe3+a3CN, in fully-oxidized cytochrome c oxidase and its 1-, 2-, and 3-electron reduction products. Soret/visible spectra for the heme a3 cyanide are independent of overall redox level, whereas distinct shifts in C-N infrared stretch band frequency occur upon reduction, reflecting changes in the polarity of the ligand (CN-) environment. Catalysis of O2 reduction can be critically dependent upon such changes in polarity at the reduction site. These findings indicate that CuB, when reduced, exists in two forms whose relative stabilities are independent of Fea and CuA oxidation states and, when oxidized, is in only one stable form. These results are consistent with the oxidation of Cu+B triggering proton pumping and with the involvement of a CuB ligand in respiratory control. Electron equivalents introduced into the enzyme are distributed equally among Fea, CuA, and CuB, which raises the possibility that all four electrons used in O2 reduction are donated via Cu+B, which is favorably positioned with respect to Fea3 (the O2 binding site) in order to carry out this role in electron transfers.

Avirulence of Candida albicans auxotrophic mutants in a rat model of oropharyngeal candidiasis

The virulence of Candida albicans strain SC5413 and two isogenic derivatives have been investigated in a rat model of oropharyngeal candidiasis. The results demonstrate that both mutant strains are avirulent in this animal model while the parental strain readily initiates infection. Avirulence is not related to altered growth characteristics or the inability of the strains to undergo yeast-to-hyphal morphogenesis. The potential importance of nutritional sufficiency as a virulence factor as well as the possibility of utilizing such strains in the development of an in vitro expression technology system for Candida albicans is discussed.