Properties of enzymes in hepatocytes that convert 5-HPETE or LTA4 into LTB4.
Rat hepatocyte homogenates convert 5-hydroperoxyeicosatetraenoic acid (5-HPETE) into biologically active leukotriene B4 (LTB4) as well as less active all-trans-LTB4 (i.e., 6-trans-LTB4 and 6-trans-12-epi-LTB4). Here, we present a hypothesis of the reaction mechanism and the minimal structural requirements of the active enzyme based on the following experimental evidence: The ED50 of the inhibitors 5,8,11,14-eicosatetraynoic acid (ETYA) and 5,6-dehydro-eicosatetraenoic acid was approximately 100-fold higher than for 5-lipoxygenase. Propanethiol and O2 were strong inhibitors of LTB4 formation, whereas butylated hydroxytoluene, nordihydroguaiaretic acid, metyrapone, Desferal and CO had no effect. Cytochrome c, catalase, hematin, and a Fe3+/Fe2+ couple, but not iron-free protoporphyrin IX, catalyzed the formation of only all-trans-LTB4. LTB4 formation in hepatocyte homogenates was heat- and trypsin-sensitive whereas all-trans-LTB4 formation was not. We propose that a ferric heme iron forms a ferryl-hydroxo complex upon homolytic scission of the oxygen-oxygen bond in 5-HPETE and the resulting 5,6-trans-epoxide radical is oxidized by the ferryl-hydroxo complex to yield LTA4. A mechanism for hydrolysis of LTA4 is described that results in formation of LTB4 (less than 1% yield) rather than all-trans-LTB4.
Trudell
J R
JR
Department of Anesthesia, Stanford University, School of Medicine, CA 94305-5117.
Gut
J
J
D000818:Animals; D001094:Arachidonate 5-Lipoxygenase / Q000378:metabolism; D016718:Arachidonic Acid; D001095:Arachidonic Acids / Q000037:antagonists & inhibitors / Q000378:metabolism; D001665:Binding Sites; D055598:Chemical Phenomena; D002621:Chemistry; D002851:Chromatography, High Pressure Liquid; D004721:Endoplasmic Reticulum / Q000201:enzymology; D005609:Free Radicals; D017572:Leukotriene A4; D007975:Leukotriene B4 / Q000378:metabolism*; D015289:Leukotrienes / Q000378:metabolism*; D016859:Lipoxygenase Inhibitors; D008099:Liver / Q000201:enzymology* / Q000648:ultrastructure; D008297:Male; D008954:Models, Biological; D015394:Molecular Structure; D010100:Oxygen / Q000494:pharmacology; D051381:Rats; D011916:Rats, Inbred F344
D001095:Arachidonic Acids; D005609:Free Radicals; D017572:Leukotriene A4; D015289:Leukotrienes; D016859:Lipoxygenase Inhibitors; D007975:Leukotriene B4; D016718:Arachidonic Acid; C025348:arachidonic acid 5-hydroperoxide; D001094:Arachidonate 5-Lipoxygenase; D010100:Oxygen