title: Cytochrome P450 isozymes involved in lisofylline metabolism to pentoxifylline in human liver microsomes.
abstract: We describe the kinetics of pentoxifylline formation from lisofylline in human liver microsomes using selective inhibitors of cytochrome P450 isozymes, correlation studies with specific isozyme activities, and cDNA-expressed human CYP1A2 and 2E1. A biphasic model fitted the data best for the formation of pentoxifylline, Km1 = 0.282 +/- 0.135 microM, Vmax1 = 0.003 +/- 0.001 nmol/min/mg protein, Km2 = 158 +/- 42.6 microM and Vmax2 =0.928 +/- 0.308 nmol/min/mg (N = 4). Pentoxifylline formation by the low Km isoform (200 microM lisofylline) required NADPH, was not inhibited by any isozyme-specific P450 inhibitor, and was inhibited only 10% and 20%, respectively, by aminobenzotriazole and N-octamylamine. We concluded that the low Km enzyme was not a cytochrome P450. At 5 microM of lisofylline the CYP1A2 inhibitor, furafylline, inhibited pentoxifylline formation by 58.8%, and the nonspecific CYP2E1 inhibitor, diethyldithiocarbamate, inhibited pentoxifylline formation by 21.7%. When preincubated with furafylline plus diethyldithiocarbamate, inhibition of pentoxifylline formation was increased 71.4%. Microsomal CYP1A2 activity correlated with pentoxifylline formation (r2 = 0.870, p < 0.001). However, CYP2E1 activity did not correlate with pentoxifylline formation (r2 = 0.143, p = 0.181). Baculovirus insect cell expressed human CYP1A2 formed pentoxifylline at 0.987 nmol/min/nmol cytochrome P450 at 5 microM lisofylline. cDNA expressed CYP2E1 did not catalyze formation of pentoxifylline. Diethyldithiocarbamate inhibited pentoxifylline formation by 85.7% in cDNA expressed CYP1A2. We conclude that CYP1A2 is the high affinity enzyme catalyzing pentoxifylline formation from lisofylline.
authors.affiliation: Department of Pharmaceutics, University of Washington, Seattle, WA 98195-7610, USA.
authors.forename: S H
authors.initials: SH
authors.lastname: Lee
authors.forename: J T
authors.initials: JT
authors.lastname: Slattery
mesh_terms: D019388:Cytochrome P-450 CYP1A2 / Q000235:genetics / Q000378:metabolism; D019392:Cytochrome P-450 CYP2E1 / Q000235:genetics / Q000378:metabolism; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000378:metabolism*; D018076:DNA, Complementary / Q000096:biosynthesis; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D007527:Isoenzymes / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D007700:Kinetics; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology* / Q000378:metabolism; D010431:Pentoxifylline / Q000031:analogs & derivatives* / Q000378:metabolism*; D013379:Substrate Specificity
chemical_list: D065607:Cytochrome P-450 Enzyme Inhibitors; D018076:DNA, Complementary; D004791:Enzyme Inhibitors; D007527:Isoenzymes; D003577:Cytochrome P-450 Enzyme System; D019392:Cytochrome P-450 CYP2E1; D019388:Cytochrome P-450 CYP1A2; C025189:lisofylline; D010431:Pentoxifylline