A Practical Approach, Second Edition=Ronald D. Ho.pdf

530 DEVELOPMENTAL REPRODUCTIVE TOXICOLOGY: A PRACTICAL APPROACH, SECOND EDITIONstereospecific actions of these enzymes require a 1,4-cis-cis-pentadiene structure in the substratemolecule. This moiety is transformed into 1-hydroperoxy-2-trans, 4-cis-pentadiene. Subsequently,this relatively unstable hydroperoxy derivative of arachidonic acid undergoes catalysis, giving riseto a wide array of biologically active metabolites, such as leukotrienes, hydroxyeicosatetraenoicacids (HETEs), lipoxins, and hepoxilins. 29–31 The substrate requirements of LO are less stringentthan those of PGS. Thus, certain LOs not only use free PUFAs but can also peroxidize di- and triglycerides,lipoproteins, and even membrane-bound lipids. LOs are ubiquitous in plants and animals,occurring in several mammalian organs, including intrauterine tissues. 6,29–31 Various arachidonatemetabolites generated by LOs are essential for fertilization, implantation, normal growth anddevelopment of the conceptus, maintenance of pregnancy, and certain aspects of parturition. 6 Asingle LO protein not only exhibits dioxygenase activity toward PUFAs but also displays cooxidaseactivity toward a large number of xenobiotics. 29–31 The peroxidaselike (cooxidase) activity ofdifferent LOs is not only efficiently supported by PUFAs and their hydroperoxides but also bysubstitutes such as H 2 O 2 , tert-butyl hydroperoxide, and cumene hydroperoxide. 29–31 The principalmechanisms of xenobiotic oxidation include one-electron oxidation of reducing substances to freeradical species by LO. Concurrently, the active ferric LO is converted into inactive ferrous LO.Ferrous iron oxidation by hydroperoxide regenerates the active enzyme for the next catalytic cycle.The intermediate peroxyl radicals of PUFA generated during dioxygenation also serve as oxidantsand trigger epoxidation or sulfoxidation of xenobiotics. Additional mechanisms of LO-mediatedxenobiotic catalysis have been described. 29–315. Lipid PeroxidationLipid peroxidation is a normal biochemical process constantly occurring at a basal rate in essentiallyall cell types. It occurs in tissues as a metal-catalyzed nonenzymatic process, as well as processesmediated by LO and PGS. Lipid peroxidation products are essential for maintenance of pregnancyand for normal growth and development of the conceptus. They are involved in the induction oflabor and in the parturition process at term. Recent data show that increased lipid peroxidation dueto aberrant PGS and/or LO activities in reproductive tissues may be linked to various disorders,such as hypertension and preeclampsia in pregnancy. 32 Uncontrolled lipid peroxidation may alsobe responsible for the developmental or reproductive toxicities of some chemicals. The proposedmechanisms include the generation of reactive oxygen species (ROS), the toxic products of lipidperoxidation, and/or xenobiotic free radicals. 3,4 Lipid peroxidation-coupled cooxidation representsanother pathway for oxidation of certain xenobiotics. 1,6B. Phase II Reactions1. UDP-Glucuronosyl TransferaseGlucuronidation is a Phase II reaction catalyzed by two families of microsomal UDP-glucuronosyltransferases (UGTs: E.C. 2.4.17), with each family made up of several members. 33 The processinvolves transfer of glucuronic acid from UDP-glucuronic acid (UDPGA) to aglycone by UGT.The major site for glucuronidation is the liver. The concentration of UDPGA in the liver is estimatedat about 350 µM. UGT-mediated hepatic glucuronidation of xenobiotics is considered a “lowaffinity–high-capacity”process. UGT is inducible by xenobiotics. Operation of the enzyme in vivorequires at least two transporters. One is for the transport of UDPGA from cytoplasm to the lumenof endoplasmic reticulum, and the other exports glucuronide into the cytoplasm. The enzyme exhibitslatency because of its luminal location, and prior membrane disruption by, e.g., brief sonication ordetergent addition, is needed for full expression of activity. Many endobiotics and xenobiotics or theirmetabolites bearing –OH, –COOH, –NH 2 , and –SH groups undergo glucuronidation. Although most© 2006 by Taylor & Francis Group, LLC