X-ray scattering studies of Methylophilus methylotrophus (sp. W3A1) electron-transferring flavoprotein. Evidence for multiple conformational states and an induced fit mechanism for assembly with trimethylamine dehydrogenase.
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Small angle x-ray solution scattering has been used to generate a low resolution, model-independent molecular envelope structure for electron-transferring flavoprotein (ETF) from Methylophilus methylotrophus (sp. W(3)A(1)). Analysis of both the oxidized and 1-electron-reduced (anionic flavin semiquinone) forms of the protein revealed that the solution structures of the protein are similar in both oxidation states. Comparison of the molecular envelope of ETF from the x-ray scattering data with previously determined structural models of the protein suggests that ETF samples a range of conformations in solution. These conformations correspond to a rotation of domain II with respect to domains I and III about two flexible "hinge" sequences that are unique to M. methylotrophus ETF. The x-ray scattering data are consistent with previous models concerning the interaction of M. methylotrophus ETF with its physiological redox partner, trimethylamine dehydrogenase. Our data reveal that an "induced fit" mechanism accounts for the assembly of the trimethylamine dehydrogenase-ETF electron transfer complex, consistent with spectroscopic and modeling studies of the assembly process. (+info)
Formation of W(3)A(1) electron-transferring flavoprotein (ETF) hydroquinone in the trimethylamine dehydrogenase x ETF protein complex.
(58/1035)
The electron-transferring flavoprotein (ETF) from Methylophilus methylotrophus (sp. W(3)A(1)) exhibits unusual oxidation-reduction properties and can only be reduced to the level of the semiquinone under most circumstances (including turnover with its physiological reductant, trimethylamine dehydrogenase (TMADH), or reaction with strong reducing reagents such as sodium dithionite). In the present study, we demonstrate that ETF can be reduced fully to its hydroquinone form both enzymatically and chemically when it is in complex with TMADH. Quantitative titration of the TMADH x ETF protein complex with sodium dithionite shows that a total of five electrons are taken up by the system, indicating that full reduction of ETF occurs within the complex. The results indicate that the oxidation-reduction properties of ETF are perturbed upon binding to TMADH, a conclusion further supported by the observation of a spectral change upon formation of the TMADH x ETF complex that is due to a change in the environment of the FAD of ETF. The results are discussed in the context of ETF undergoing a conformational change during formation of the TMADH x ETF electron transfer complex, which modulates the spectral and oxidation-reduction properties of ETF such that full reduction of the protein can take place. (+info)
Role of furanocoumarin derivatives on grapefruit juice-mediated inhibition of human CYP3A activity.
(59/1035)
With juices of grapefruit and related fruits, possible relationships between contents of six different furanocoumarins and extents of inhibition of microsomal CYP3A activity have been studied in vitro. Microsomal CYP3A-mediated testosterone 6beta-hydroxylation was inhibited by the addition of a fruit juice (2.5%, v/v) from eight different grapefruit sources, two sweeties, three pomelos, and one sour orange, whereas no clear inhibition was observed with two sweet orange juices. The inhibitory component in grapefruit juice resides mainly in the precipitate rather than in the supernatant after centrifugation. Higher amounts of (R)-6',7'-dihydroxybergamottin (DHB) were distributed in the supernatant, whereas GF-I-1, GF-I-2, GF-I-4, and the newly isolated GF-I-5 and GF-I-6 were detected predominantly in the precipitate. Mixing of five representative furanocoumarins at their detectable levels in grapefruit juice reproduced roughly the inhibitory potencies of grapefruit juice, but omission of any of the components resulted in decreased potencies. These results suggested that all the major furanocoumarins contributed to the CYP3A inhibitory properties of grapefruit juice. Furthermore, all six furanocoumarins showed stronger CYP3A inhibitory potencies after preincubation in the presence of NADPH, suggesting that both competitive and mechanism-based inhibition occur in a grapefruit juices-drug interaction. (+info)
Radiolytic studies of trimethylamine dehydrogenase. Spectral deconvolution of the neutral and anionic flavin semiquinone, and determination of rate constants for electron transfer in the one-electron reduced enzyme.
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Trimethylamine dehydrogenase from the pseudomonad Methylophilus methylotrophus has been examined using the technique of pulse radiolysis to rapidly introduce a single reducing equivalent into the enzyme. Using enzyme that has had its iron-sulfur center rendered redox-inert by prior reaction with ferricenium hexafluorophosphate, we determined the spectral change associated with formation of both the anionic and neutral forms that were generated at high and low pH, respectively, of the unique 6-cysteinyl-FMN of the enzyme. With native enzyme, electron transfer was observed within the radiolytically generated one-electron reduced enzyme but only at low pH (6.0). The kinetics and thermodynamics of this electron transfer in one-electron reduced enzyme may be compared with that studied previously in the two-electron reduced enzyme. In contrast to previous studies with two-electron reduced enzyme in which a pK(a) of approximately 8 was determined for the flavin semiquinone, in the one-electron reduced enzyme the semiquinone was not substantially protonated even at pH 6. 0. These results indicate that reduction of the iron-sulfur center of the enzyme significantly decreases the pK(a) of the flavin semiquinone of the active site. This provides further evidence, in conjunction with the strong magnetic interaction known to exist between the centers in the two-electron reduced enzyme, that the two redox-active centers in trimethylamine dehydrogenase are in intimate contact with one another in the active site of the enzyme. (+info)
In vivo antitumor activity and host toxicity of methoxymorpholinyl doxorubicin: role of cytochrome P450 3A.
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Methoxymorpholinyl doxorubicin (MMDX; PNU 152243) is a promising doxorubicin derivative currently undergoing clinical evaluation. Previous in vitro studies suggested that the compound undergoes hepatic biotransformation by cytochrome P450 (CYP) 3A into a more cytotoxic metabolite(s). The present study examined the role of CYP3A-mediated metabolism in the in vivo antitumor activity and host toxicity of MMDX in the mouse model and investigated the potential for increasing the therapeutic effectiveness of the drug by inducing its hepatic CYP-catalyzed activation. We found that MMDX cytotoxicity for cultured M5076 tumor cells was potentiated 22-fold by preincubating the drug with NADPH-supplemented liver microsomes from untreated C57BL/6 female mice. A greater (50-fold) potentiation of MMDX cytotoxicity was observed after its preincubation with liver microsomes isolated from animals pretreated with the prototypical CYP3A inducer pregnenolone-16alpha-carbonitrile. In contrast, in vivo administration of the selective CYP3A inhibitor troleandomycin (TAO) reduced both potentiation of MMDX cytotoxicity and the rate of CYP3A-catalyzed N-demethylation of erythromycin by isolated liver microsomes (55.5 and 49% reduction, respectively). In vivo antitumor activity experiments revealed that TAO completely suppressed the ability of 90 microg/kg MMDX i.v., a dose close to the LD10, to delay growth of s.c. M5076 tumors in C57BL/6 mice and to prolong survival of DBA/2 mice with disseminated L1210 leukemia. Moreover, TAO administration markedly inhibited the therapeutic efficacy of 90 microg/kg MMDX i.v. in mice bearing experimental M5076 liver metastases; a complete loss of MMDX activity was observed in liver metastases-bearing animals receiving 40 microg/kg MMDX i.v. plus TAO. However, pregnenolone-16alpha-carbonitrile pretreatment failed to enhance MMDX activity in mice bearing either s.c. M5076 tumors or experimental M5076 liver metastases. Additional experiments carried out in healthy C57BL/6 mice showed that TAO markedly inhibited MMDX-induced myelosuppression and protected the animals against lethal doses of MMDX. Taken together, these findings demonstrate that an active metabolite(s) of MMDX synthesized via CYP3A contributes significantly to its in vivo antitumor activity and host toxicity. (+info)
Inhibitory effect of natural furanocoumarins on human microsomal cytochrome P450 3A activity.
(62/1035)
To investigate the possible drug interaction with herbal medicine, furanocoumarin derivatives isolated from several Umbelliferous crude drugs were examined for their inhibitory effects on a typical human drug metabolizing enzyme, cytochrome P450 3A (CYP3A). Most furanocoumarins tested at 0.1 mM reduced microsomal testosterone 6beta-hydroxylation as an index of CYP3A activity to less than 50% of the control. In particular, the dimer and trimer derivatives of furanocoumarins showed striking inhibition, whose potencies were similar to that of a typical CYP3A inhibitor, ketoconazole. Preincubation of dimer types of furanocoumarins increased suppression but not most of the monomer derivatives, suggesting that the inhibition on CYP3A activity was caused by at least plural mechanisms. These results raised the possibility that the furanocoumarin containing herbal medicines may alter pharmacokinetics of co-ingested drugs similar to the case with grapefruit juice. (+info)
A protein factor of rat liver mitochondrial matrix involved in flavinylation of dimethylglycine dehydrogenase.
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The involvement of rat liver mitochondria in the flavinylation of the mitochondrial matrix flavoenzyme dimethylglycine dehydrogenase (Me2GlyDH) has been investigated. Me2GlyDH was synthesized as an apoenzyme in the rabbit reticulocyte lysate (RL) transcription/translation system and its flavinylation was monitored by virtue of the trypsin resistance of the holoenzyme. The rate of holoenzyme formation in the presence of FAD was stimulated with increasing efficiency by the addition of solubilized mitoplasts, mitochondrial matrix and DEAE-purified matrix fraction. Apo-Me2GlyDH was also converted into holoenzyme when the solubilized mitoplasts were supplemented with FMN and ATP. This observation is consistent with the existence of a mitochondrial FAD synthetase generating the FAD needed for holoenzyme formation from its precursors. Holoenzyme formation in the presence of FAD increased linearly with the concentration of matrix protein in the assay, and depended on the amount of externally added Me2GlyDH with saturation characteristics. These findings suggest the presence of a protein factor in the mitochondrial matrix which stimulates Me2GlyDH flavinylation. This factor was different from both mitochondrial heat shock protein (Hsp)70, as shown by immunodepletion experiments, and mitochondrial Hsp60, as demonstrated by the capability of a DEAE-purified matrix fraction devoid of Hsp60 to accelerate flavinylation of both RL translated and purified Me2GlyDH. (+info)
Mibefradil is a P-glycoprotein substrate and a potent inhibitor of both P-glycoprotein and CYP3A in vitro.
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Mibefradil, a calcium T- and L-channel blocker developed for use in hypertension, was recently removed from the market after reports of severe drug-drug interactions. Mibefradil is known to inhibit various cytochrome P450 enzymes involved in drug metabolism, particularly CYP3A. However, the extent and the severity of the observed drug interactions in humans suggest that inhibition of additional systems important to drug disposition, such as the drug transporter P-glycoprotein (P-gp), may also have contributed to the severity of the mibefradil interactions. A polarized epithelial cell line, LLC-PK1, which does not express P-gp, and the derived L-MDR1 cell line, which overexpresses human P-gp, were used to study the effects of mibefradil on drug transport. A markedly greater basal-to-apical versus apical-to-basal transport of [H3]mibefradil was seen in the L-MDR1, but not in the LLC-PK1 cells, suggesting that the drug is a substrate of P-gp. Using a human intestinal cancer-derived cell line Caco-2, which constitutively expresses P-gp, mibefradil was shown to be a potent inhibitor of P-gp-mediated digoxin transport, with an IC50 of 1.6 microM. Additionally, the effect of mibefradil on CYP3A was assessed using human liver microsomes. Mibefradil inhibited CYP3A-mediated nifedipine oxidase activity with an IC50 of 0.8 microM, and a Ki of 0.6 microM. Thus, mibefradil is not only a P-gp substrate, but also a potent inhibitor of both P-gp and CYP3A. These data suggest that the severity of drug interactions seen with mibefradil use is due to the dual inhibition of both P-gp and CYP3A. (+info)