title: Human liver microsomal enflurane defluorination catalyzed by cytochrome P-450 2E1.
abstract: The volatile anesthetic agent enflurane undergoes oxidative metabolism in human liver, yielding both inorganic and organic fluoride metabolites. Numerous studies conducted in animals indicate that the enzyme cytochrome P-450 2E1 is a major catalyst for the defluorination reaction. However, the P-450 enzyme catalyzing enflurane metabolism in humans has not been identified. Therefore, experiments were conducted to determine whether hepatic P-450 2E1 is a catalyst for the reaction in humans, and whether other constitutive or inducible isoforms might also be involved. Purified human liver P-450 2E1, reconstituted with cytochrome b5 and P-450 reductase, catalyzed enflurane defluorination at a rate of 9.3 nmol F-/nmol P-450/30 min, in contrast to a mean liver microsomal rate of 2.0 nmol F-/nmol P-450/30 min. The microsomal rate of defluorination for individual human livers correlated significantly with the microsomal content of P-450 2E1 protein (r = 0.92), the rate of p-nitrophenol hydroxylation (r = 0.86), and the rate of chlorzoxazone 6-hydroxylation (r = 0.90). In addition, specific anti-P-450 2E1 IgG, at a concentration of 10 mg IgG/nmol P-450 inhibited the microsomal reaction by 80%. Finally, a series of P-450 isoform-specific chemical inhibitors of oxidative metabolism--furafylline (1A2), sulfaphenazole (2C9/10), quinidine (2D6), troleandomycin (3A3/4), and diethyldithiocarbamate (2E1)--were screened for their ability to block human microsomal enflurane defluorination. Only diethyldithiocarbamate, a mechanism-based inhibitor of P-450 2E1, inhibited the reaction; this occurred to a degree comparable to the effect of anti-P-450 2E1 antibody.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Pharmaceutics, University of Washington, Seattle 98195.
authors.forename: K E
authors.initials: KE
authors.lastname: Thummel
authors.forename: E D
authors.initials: ED
authors.lastname: Kharasch
authors.forename: T
authors.initials: T
authors.lastname: Podoll
authors.forename: K
authors.initials: K
authors.lastname: Kunze
mesh_terms: D000818:Animals; D015153:Blotting, Western; D002384:Catalysis; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000276:immunology / Q000378:metabolism*; D004737:Enflurane / Q000276:immunology / Q000378:metabolism*; D006801:Humans; D007074:Immunoglobulin G / Q000276:immunology; D066298:In Vitro Techniques; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology* / Q000276:immunology; D051381:Rats
chemical_list: D065607:Cytochrome P-450 Enzyme Inhibitors; D007074:Immunoglobulin G; D003577:Cytochrome P-450 Enzyme System; D004737:Enflurane