Expression in yeast of three allelic cDNAs coding for human liver P-450 3A4. Different stabilities, binding properties and catalytic activities of the yeast-produced enzymes

Functional characterization of zebrafish cytochrome P450 1 family proteins expressed in yeast

BACKGROUND

Zebrafish express five cytochrome P450 1 genes: CYP1A, CYP1B1, CYP1C1, CYP1C2, inducible by aryl hydrocarbon receptor agonists, and CYP1D1, a constitutively expressed CYP1A-like gene. We examined substrate selectivity of CYP1s expressed in yeast.

METHODS

CYP1s were expressed in W(R) yeast, engineered to over-express P450 reductase, via pYES/DEST52 and via pYeDP60. Microsomal fractions from transformed yeast were examined for activity with fluorogenic substrates, benzo[a]pyrene and testosterone. Modeling and docking approaches were used to further evaluate sites of oxidation on benzo[a]pyrene and testosterone.

RESULTS

CYP1s expressed in yeast dealkylated ethoxy-, methoxy-, pentoxy- and benzoxy-resorufin (EROD, MROD, PROD, BROD). CYP1A and CYP1C2 had the highest rates of EROD activity, while PROD and BROD activities were low for all five CYP1s. The relative rates of resorufin dealkylation by CYP1C1, CYP1C2 and CYP1D1 expressed via pYeDP60 were highly similar to relative rates obtained with pYES/DEST52-expressed enzymes. CYP1C1 and CYP1C2 dealkylated substituted coumarins and ethoxy-fluorescein-ethylester, while CYP1D1 did not. The CYP1Cs and CYP1D1 co-expressed with epoxide hydrolase oxidized BaP with different rates and product profiles, and all three produced BaP-7,8,9,10-tetrol. The CYP1Cs but not CYP1D1 metabolized testosterone to 6β-OH-testosterone. However, CYP1D1 formed an unidentified testosterone metabolite better than the CYP1Cs. Testosterone and BaP docked to CYP homology models with poses consistent with differing product profiles.

CONCLUSIONS

Yeast-expressed zebrafish CYP1s will be useful in determining further functionality with endogenous and xenobiotic compounds.

GENERAL SIGNIFICANCE

Determining the roles of zebrafish CYP1s in physiology and toxicology depends on knowing the substrate selectivity of these enzymes.

In vitro modulatory effects on three major human cytochrome P450 enzymes by multiple active constituents and extracts of Centella asiatica

ETHNOPHARMACOLOGICAL RELEVANCE

Centella asiatica (CA) has been widely cultivated as a vegetable or spice in China, Southeast Asia, India, Sri Lanka, Africa, and Oceanic countries and traditionally used for wound healing and maintaining normal blood pressure.

AIM OF THE STUDY

The present study was carried out to examine the potential modulatory effects of three commercially available active components (asiaticoside, asiatic acid and madecassic acid) and four extracts (aqueous, ethanol, dichloromethane and hexane) of CA on three major cDNA-expressed human cytochrome P450 (CYP) isoforms.

MATERIALS AND METHODS

High-performance liquid chromatography (HPLC)-based enzyme assays, namely tolbutamide 4-methyhydroxylase, dextromethorphan O-demethylase and testosterone 6beta-hydroxylase assays were developed to probe activities of CYP2C9, CYP2D6 and CYP3A4, respectively. Probe substrates were incubated with or without each active component and extract for each isoform, followed by examination of the kinetics parameters, IC(50) and K(i), to characterize modulatory effects.

RESULTS

CYP2C9 was more susceptible to inhibitory effects by CA extracts compared to CYP2D6 and CYP3A4. Moderate degree of inhibition was observed in ethanol (K(i)=39.1 microg/ml) and dichloromethane (K(i)=26.6 microg/ml) extracts implying potential risk of interaction when CYP2C9 substrates are consumed with CA products. The two extracts however showed negligible inhibition towards CYP2D6 and CYP3A4 (IC(50)'s of 123.3 microg/ml and above). Similarly CA aqueous and hexane extracts did not significantly inhibit all three isoforms investigated (IC(50)'s of 117.9 microg/ml and above). Among the active constituents investigated, asiatic acid and madecassic acid appeared to selectively inhibit CYP2C9 and CYP2D6 more than CYP3A4. Of particular interest is the potent inhibitory effect of asiatic acid on CYP2C9 (K(i)=9.1 microg/ml). This signifies potential risk of interaction when substrates for this isoform are taken together with CA products with high asiatic acid content. Inhibitions of asiatic acid with the other isoforms and that of madecassic acid with all isoforms were only moderate (K(i)'s ranged from 17.2 to 84.4 microg/ml). On the other hand, the IC(50) values for asiaticoside were high (1070.2 microg/ml or above) for all three isoforms, indicating negligible or low potential of this compound to modulate CYP enzymatic activity.

CONCLUSION

Centella asiatica extracts and active constituents inhibited CYP2C9, CYP2D6 and CYP3A4 activities with varying potency with CYP2C9 being the most susceptible isoform to inhibition. Significant inhibition was observed for asiatic acid and CA ethanol and dichloromethane extracts, implying involvement of semipolar constituents from CA in the effect. This study suggested that CA could cause drug-herb interactions through CYP2C9 inhibition.

Identification and functional characterization of eight CYP3A4 protein variants

The genetic component of the inter-individual variability in CYP3A4 activity has been estimated to be between 60% and 90%, but the underlying genetic factors remain largely unknown. A study of 213 Middle and Western European DNA samples resulted in the identification of 18 new CYP3A4 variants, including eight protein variants. A total of 7.5% of the population studied was found to be heterozygous for one of these variants. In a bacterial heterologous expression system, two mutants, R130Q and P416L, did not result in detectable P450 holoprotein. One mutant, T363M, expressed at significantly lower levels than wild-type CYP3A4. G56D, V170I, D174H and M445T were not significantly different when compared with wild-type CYP3A4 in expression or steroid hydroxylase activity. L373F displayed a significantly altered testosterone metabolite profile and a four-fold increase in the Km value for 1'-OH midazolam formation. The results suggest a limited contribution of CYP3A4 protein variants to the inter-individual variability of CYP3A4 activity in Caucasians. Some variants may, however, play a role in the atypical response to drugs or altered sensitivity to carcinogens.

Cytochrome b5 augments the 17,20-lyase activity of human P450c17 without direct electron transfer

In the biosynthesis of steroid hormones, P450c17 is the single enzyme that catalyzes both the 17alpha-hydroxylation of 21-carbon steroids and the 17,20-lyase activity that cleaves the C17-C20 bond to produce C19 sex steroids. Cytochrome b5 augments the 17,20-lyase activity of cytochrome P450c17 in vitro, but this has not been demonstrated in membranes, and the mechanism of this action is unknown. We expressed human P450c17, human P450-oxidoreductase (OR), and/or human cytochrome b5 in Saccharomyces cerevisiae and analyzed the 17alpha-hydroxylase and 17,20-lyase activities of the resulting yeast microsomes. Yeast expressing only P450c17 have 17alpha-hydroxylase and trace 17,20-lyase activities toward both Delta4 and Delta5 steroids. Coexpression of human OR with P450c17 increases the Vmax of both the 17alpha-hydroxylase and 17,20-lyase reactions 5-fold; coexpression of human b5 with P450c17 also increases the Vmax of the 17,20-lyase reactions but not of the 17alpha-hydroxylase reactions. Simultaneous expression of human b5 with P450c17 and OR, or addition of purified human b5 to microsomes from yeast coexpressing human P450c17 and OR, further increases the Vmax of the 17,20-lyase reaction without altering 17alpha-hydroxylase activity. Genetically engineered yeast and mixing experiments demonstrate that OR is both necessary and sufficient for microsomal 17,20-lyase activity. Addition of purified human holo-b5, apo-b5, or cytochrome c to microsomes containing both human P450c17 and OR demonstrate that the stimulatory action of b5 does not require electron transfer from b5 to P450c17. These data suggest that human b5 acts principally as an allosteric effector that interacts primarily with the P450c17.OR complex to stimulate 17, 20-lyase activity.

CYP3A5 is the major cytochrome P450 3A expressed in human colon and colonic cell lines

CYP3A is known to be expressed in liver, small intestine and colon. However, its isoform distribution (CYP3A4, 3A5 and 3A7) and inducibility have not been clearly elucidated in the colon. Therefore, we analyzed CYP3A in human colon and compared its expression and inducibility to the human colonic cell lines HT29 and Caco2, which were used as models. Patients suffered either from sigmoiditis or colonic adenocarcinoma. Patients as well as HT29 and Caco2 cells were treated with rifampicin. CYP3A protein expression was analyzed in the colon of patients and in the cells by immunoblot and by isoelectric focusing enabling separation of CYP3A isoforms, while mRNA expression was determined using specific reverse transcription-polymerase chain reaction. In both human colon and cells, CYP3A5 was the main isoform expressed at the protein and mRNA levels. Rifampicin treatment had no effect on CYP3A expression. HT29 and Caco2 cells exhibiting the same CYP3A expression and inducibility might therefore be useful in vitro models for studying xenobiotic metabolism in human colon.

Thermodynamic studies of substrate binding and spin transitions in human cytochrome P-450 3A4 expressed in yeast microsomes

An approach to the quantitative spectral analysis of substrate binding and inactivation of cytochrome P-450 in microsomes is described. The method is based on the application of the principal component analysis technique on the Soret-region spectra measured at different temperatures at various concentrations of substrate. This approach allowed us to study the thermodynamic parameters of substrate binding and spin transitions in human cytochrome P-450 3A4 expressed in yeast (Saccharomyces cerevisiae) microsomes. These parameters are discussed in comparison with the values reported earlier by Ristau et al. [(1979) Acta Biol. Med. Ger. 38, 177-185] for rabbit liver cytochrome P-450 2B4 in solution with benzphetamine as a substrate. Our analysis shows the substrate-free states of 2B4 and 3A4 to be very similar. However, substrate binding seems to perturb haem-protein interactions in 3A4 in contrast with 2B4, where the effect of substrate binding on the thermodynamic parameters of spin transitions was insignificant. The implication of the results for the mechanism of substrate-induced spin shift is discussed.

The role of individual susceptibility in cancer burden related to environmental exposure

Individual susceptibility to cancer may result from host factors including differences n metabolism, DNA repair, altered expression of protooncogenes and tumor suppressor genes, and nutritional status. Since most carcinogens require metabolic activation before binding to DNA, variations in an individual's metabolic phenotype that have detected in enzymes involved in activation and detoxification should play an essential role in the development of environmental cancer. This phenotypic metabolic variation has now been related to genetic polymorphisms, and many genes encoding carcinogen-metabolizing enzymes have been identified and cloned. Consequently, allelic variants or genetic defects that give rise to the observed variation and new polymorphisms have been recognized. Development of simple polymerase chain reaction (PCR)-based assays has enabled identification of an individual's genotype for a variety of metabolic polymorphisms. Thus, recent knowledge of the genetic basis for individual metabolic variation has opened new possibilities of studies focusing on increased individual susceptibility to environmentally induced cancer, which are reviewed with special reference to smoking-induced lung cancer. Cancer susceptibility due to chemical exposure is likely to be determined by an individual's phenotype for a number of enzymes (both activating and detoxifying) relevant to that of a single carcinogen or mixtures of carcinogens. Given the number and variability in expression of carcinogen-metabolizing enzymes and the complexity of chemical exposures, assessment of a single polymorphic enzyme (genotype) may not be sufficient. Mutations in the p53 gene are among the most common genetic changes in human cancer. The frequency and type p53 mutations can act as a fingerprint of carcinogen exposure and may therefore provide information about external etiological agents, intensity of exposure, and host factors affecting the tumorigenesis process. In human lung cancer, p53 mutations (both the mutation pattern and frequency) have been linked with tobacco smoking; the type of mutation most frequently observed is G:C to T:A transversion, a mutation preferentially induced by benzo[a]pyrene diol epoxide. An association between the presence of this transversion and the genotype deficient in glutathione S-transferase M1-mediated detoxification has been observed in lung cancer. Taken together, these findings suggest that determination of metabolic at risk genotypes in combination with levels of DNA adducts in target (surrogate) tissues and the p53 mutation pattern should allow the identification of susceptible individuals and subgroups in carcinogen-exposed populations.

In vivo reconstitution of highly active Candida maltosa cytochrome P450 monooxygenase systems in inducible membranes of Saccharomyces cerevisiae

To establish a system for functional characterization of individual Candida maltosa cytochrome P450 monooxygenases, the NADPH-cytochrome P450 reductase from this yeast species was co-expressed in Saccharomyces cerevisiae with each of the following cytochrome P450 forms; P450Cm1 (CYP52 A3), P450Cm2 (CYP52 A4), and P450AlK2A (CYP52 A5). For this purpose, a multicopy plasmid was constructed that contained two independent expression units controlled by the galactose-inducible GAL10 promoter. As shown by spectral and immunological methods, large amounts of the desired monooxygenase components could be simultaneously produced in the respective S. cerevisiae transformants. It was important, however, to adjust semi-anaerobic cultivation conditions during induction by galactose to minimize a mutual impairment of cytochrome P450 and NADPH-cytochrome P450 reductase formation. Compared to the specific cellular content of the host-own enzyme, a 75- to 100-fold overproduction of the reductase component was obtained resulting in P450/reductase molar ratios of about 1:3 in the microsomal fractions prepared from the co-expression strains. At the same time, the rates of cytochrome P450-dependent lauric acid hydroxylation increased more than 10-fold, showing a proper reconstitution of the C. maltosa monooxygenase systems in S. cerevisiae. Using intact cells, an efficient biotransformation of lauric acid to omega-hydroxylauric acid and dodecanedioic acid was found. S. cerevisiae cells coexpressing cytochrome P450 and NADPH-cytochrome P450 reductase were characterized by a marked proliferation of the endoplasmic reticulum. Immunoelectron microscopy revealed a colocalization of the monooxygenase components produced to these newly formed membrane structures.

High affinity of ergopeptides for cytochromes P450 3A. Importance of their peptide moiety for P450 recognition and hydroxylation of bromocriptine

The interaction between rat and human liver cytochromes P450 with a series of lysergic acid derivatives and ergopeptide alkaloids was studied by difference visible spectroscopy. Ergopeptides, like bromocriptine, ergocryptine and dihydroergotamine, strongly interacted with rat liver microsomes with the appearance of a difference spectrum which is characteristic of their binding to a protein site close to the heme. The intensity of this spectrum was clearly dependent on the amounts of P450s 3A in the microsomes and was at its maximum in dexamethasone-treated rat microsomes. All the ergopeptides studied exhibited a high affinity for rat P450s 3A (Ks around 1 microM), although lysergic acid derivatives not bearing the tripeptide moiety failed to give significant interactions with these P450s. A cyclic azatripeptide exhibiting a structure very similar to that of the tripeptide moiety of ergopeptides also interacted with P450s 3A with appearance of an intense type I difference spectrum. Very similar results were observed with two allelic forms of human liver P450 3A4, P450 NF25 and P450 hPCN1, produced in yeast. In both cases all the ergopeptides studied showed high affinities for the P450s (Ks 0.6-2.2 microM) and an intense shift from the low-spin to the high-spin state upon substrate binding (60-100% spin shift). Lysergic acid derivatives not bearing the tripeptide group of ergopeptides also completely failed to interact with P450s 3A4. Liver microsomes from rats pretreated with dexamethasone, a specific inducer of P450 3A, were found to be particularly active for the hydroxylation of bromocriptine, which occurs at the level of its tripeptide moiety. Human liver microsomes as well as P450 NF25 and P450 hPCN1 also exhibited a high activity for bromocriptine hydroxylation at this level. These results show that ergopeptides exhibit a particularly high affinity for P450s of the 3A subfamily. The tripeptide moiety of ergopeptides is essential for their recognition by P450s 3A and binds at a site close to P450 heme, producing type-I difference spectra. Accordingly, at least one of the studied ergopeptides, bromocriptine, is hydroxylated by P450s 3A at the proline ring of the cyclopeptide moiety. As cyclosporine is known to be a good substrate of P450s 3A, these results suggest that P450s 3A may be especially prone in a general manner to recognize and oxidize peptides or pseudopeptides.