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Human reductive halothane metabolism in vitro is catalyzed by cytochrome P450 2A6 and 3A4.
Drug Metab Dispos. 1996 Sep; 24(9):976-83.DM

Abstract

The anesthetic halothane undergoes extensive oxidative and reductive biotransformation, resulting in metabolites that cause hepatotoxicity. Halothane is reduced anaerobically by cytochrome P450 (P450) to the volatile metabolites 2-chloro-1,1-difluoroethene (CDE) and 2-chloro-1,1,1-trifluoroethane (CTE). The purpose of this investigation was to identify the human P450 isoform(s) responsible for reductive halothane metabolism. CDE and CTE formation from halothane metabolism by human liver microsomes was determined by GC/MS analysis. Halothane metabolism to CDE and CTE under reductive conditions was completely inhibited by carbon monoxide, which implicates exclusively P450 in this reaction. Eadie-Hofstee plots of both CDE and CTE formation were nonlinear, suggesting multiple P450 isoform involvement. Microsomal CDE and CTE formation were each inhibited 40-50% by P450 2A6-selective inhibitors (coumarin and 8-methoxypsoralen) and 55-60% by P450 3A4-selective inhibitors (ketoconazole and troleandomycin). P450 1A-, 2B6-, 2C9/10-, and 2D6-selective inhibitors (7,8-benzoflavone, furafylline, orphenadrine, sulfaphenazole, and quinidine) had no significant effect on reductive halothane metabolism. Measurement of product formation catalyzed by a panel of cDNA-expressed P450 isoforms revealed that maximal rates of CDE formation occurred with P450 2A6, followed by P450 3A4. P450 3A4 was the most effective catalyst of CTE formation. Among a panel of 11 different human livers, there were significant linear correlations between the rate of CDE formation and both 2A6 activity (r = 0.64, p < 0.04) and 3A4 activity (r = 0.64, p < 0.03). Similarly, there were significant linear correlations between CTE formation and both 2A6 activity (r = 0.55, p < 0.08) and 3A4 activity (r = 0.77, p < 0.005). The P450 2E1 inhibitors 4-methylpyrazole and diethyldithiocarbamate inhibited CDE and CTE formation by 20-45% and 40-50%, respectively; however, cDNA-expressed P450 2E1 did not catalyze significant amounts of CDE or CTE production, and microsomal metabolite formation was not correlated with P450 2E1 activity. This investigation demonstrated that human liver microsomal reductive halothane metabolism is catalyzed predominantly by P450 2A6 and 3A4. This isoform selectivity for anaerobic halothane metabolism contrasts with that for oxidative human halothane metabolism, which is catalyzed predominantly by P450 2E1.

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Authors+Show Affiliations

Spracklin DK
Department of Anesthesiology, University of Washington, Seattle 98195, USA.
Thummel KE
No affiliation info available
Kharasch ED
No affiliation info available

MeSH

Anesthetics, InhalationAryl Hydrocarbon HydroxylasesCarbon MonoxideChlorofluorocarbonsCoumarinsCytochrome P-450 CYP2A6Cytochrome P-450 CYP3ACytochrome P-450 Enzyme InhibitorsCytochrome P-450 Enzyme SystemHalothaneHumansIn Vitro TechniquesIsoenzymesKetoconazoleKineticsMicrosomes, LiverMixed Function OxygenasesRecombinant Proteins

Pub Type(s)

Journal Article
Research Support, Non-U.S. Gov't
Research Support, U.S. Gov't, P.H.S.

Language

eng

PubMed ID

8886607

Citation

Spracklin, D K., et al. "Human Reductive Halothane Metabolism in Vitro Is Catalyzed By Cytochrome P450 2A6 and 3A4." Drug Metabolism and Disposition: the Biological Fate of Chemicals, vol. 24, no. 9, 1996, pp. 976-83.
Spracklin DK, Thummel KE, Kharasch ED. Human reductive halothane metabolism in vitro is catalyzed by cytochrome P450 2A6 and 3A4. Drug Metab Dispos. 1996;24(9):976-83.
Spracklin, D. K., Thummel, K. E., & Kharasch, E. D. (1996). Human reductive halothane metabolism in vitro is catalyzed by cytochrome P450 2A6 and 3A4. Drug Metabolism and Disposition: the Biological Fate of Chemicals, 24(9), 976-83.
Spracklin DK, Thummel KE, Kharasch ED. Human Reductive Halothane Metabolism in Vitro Is Catalyzed By Cytochrome P450 2A6 and 3A4. Drug Metab Dispos. 1996;24(9):976-83. PubMed PMID: 8886607.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Human reductive halothane metabolism in vitro is catalyzed by cytochrome P450 2A6 and 3A4. AU - Spracklin,D K, AU - Thummel,K E, AU - Kharasch,E D, PY - 1996/9/1/pubmed PY - 1996/9/1/medline PY - 1996/9/1/entrez SP - 976 EP - 83 JF - Drug metabolism and disposition: the biological fate of chemicals JO - Drug Metab Dispos VL - 24 IS - 9 N2 - The anesthetic halothane undergoes extensive oxidative and reductive biotransformation, resulting in metabolites that cause hepatotoxicity. Halothane is reduced anaerobically by cytochrome P450 (P450) to the volatile metabolites 2-chloro-1,1-difluoroethene (CDE) and 2-chloro-1,1,1-trifluoroethane (CTE). The purpose of this investigation was to identify the human P450 isoform(s) responsible for reductive halothane metabolism. CDE and CTE formation from halothane metabolism by human liver microsomes was determined by GC/MS analysis. Halothane metabolism to CDE and CTE under reductive conditions was completely inhibited by carbon monoxide, which implicates exclusively P450 in this reaction. Eadie-Hofstee plots of both CDE and CTE formation were nonlinear, suggesting multiple P450 isoform involvement. Microsomal CDE and CTE formation were each inhibited 40-50% by P450 2A6-selective inhibitors (coumarin and 8-methoxypsoralen) and 55-60% by P450 3A4-selective inhibitors (ketoconazole and troleandomycin). P450 1A-, 2B6-, 2C9/10-, and 2D6-selective inhibitors (7,8-benzoflavone, furafylline, orphenadrine, sulfaphenazole, and quinidine) had no significant effect on reductive halothane metabolism. Measurement of product formation catalyzed by a panel of cDNA-expressed P450 isoforms revealed that maximal rates of CDE formation occurred with P450 2A6, followed by P450 3A4. P450 3A4 was the most effective catalyst of CTE formation. Among a panel of 11 different human livers, there were significant linear correlations between the rate of CDE formation and both 2A6 activity (r = 0.64, p < 0.04) and 3A4 activity (r = 0.64, p < 0.03). Similarly, there were significant linear correlations between CTE formation and both 2A6 activity (r = 0.55, p < 0.08) and 3A4 activity (r = 0.77, p < 0.005). The P450 2E1 inhibitors 4-methylpyrazole and diethyldithiocarbamate inhibited CDE and CTE formation by 20-45% and 40-50%, respectively; however, cDNA-expressed P450 2E1 did not catalyze significant amounts of CDE or CTE production, and microsomal metabolite formation was not correlated with P450 2E1 activity. This investigation demonstrated that human liver microsomal reductive halothane metabolism is catalyzed predominantly by P450 2A6 and 3A4. This isoform selectivity for anaerobic halothane metabolism contrasts with that for oxidative human halothane metabolism, which is catalyzed predominantly by P450 2E1. SN - 0090-9556 UR - https://www.unboundmedicine.com/medline/citation/8886607/Human_reductive_halothane_metabolism_in_vitro_is_catalyzed_by_cytochrome_P450_2A6_and_3A4_ L2 - http://dmd.aspetjournals.org/cgi/pmidlookup?view=long&amp;pmid=8886607 DB - PRIME DP - Unbound Medicine ER -