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

552 DEVELOPMENTAL REPRODUCTIVE TOXICOLOGY: A PRACTICAL APPROACH, SECOND EDITIONdifference in leukotriene release was observed in placentas examined after labor at term, afterpreterm labor, or after cesarean section. 100 Leukotriene output is low in uncomplicated pretermlabor but markedly increases in chorioamnionitis-associated preterm labor. The fact that 5- and 12-HETE are generated when homogenates were incubated with 14 C-arachidonate signals that functional5- and 12-LO in human placenta is present at spontaneous vaginal delivery or after cesarean section.155,156 In subsequent studies, 100,200 the predominance of the 12-LO pathway was observed. Decreasedformation of 5- and 12-HETE from arachidonic acid was noted in preeclampsic placentas. 200Recently, cellular oxidative stress was proposed as one of the underlying mechanisms ofdevelopmental toxicity of many chemicals. 3,4 Concurrent depletion of cellular GSH plays a majorcontributory role in this process. GSH provides the first line of defense against excessive productionof xenobiotic free radicals within cells. While performing the sacrificial role as an intracellularnucleophilic trap, GSH is extensively consumed. In the absence of GSH participation, an interactionof xenobiotic free radicals with molecular oxygen leads to the liberation of ROS. 3,4 Other investigatorshave also stressed the role played by ROS in the developmental toxicity and teratogenicityof various chemicals. In this regard, it is noteworthy that although heme proteins, such as P450and various peroxidases, lack the ability to oxidize GSH, LO has been found to trigger its directoxidation in the absence of xenobiotics. This phenomenon, first noted with soybean LO in thepresence of linoleic acid, 201,202 is also displayed by HTP-LO in reaction media supplemented withdifferent PUFAs. 203 GSH oxidation by LO is accompanied by superoxide anion production. 201 Moreintense superoxide anion radical generation can be noted during LO-catalyzed linoleate-coupledcooxidation of NAD(P)H, 204 but not of ascorbate, 205 in the absence of oxidizable xenobiotics.The importance of these findings is further heightened by the demonstration that GS • generatedduring PUFA-supported GSH oxidation by LO can attack a C=C bond in xenobiotics, and theprocess ultimately leads to the formation of a GSH-xenobiotic conjugate. This novel mechanismof GSH conjugate formation from xenobiotics has been documented for ethacrynic acid withsoybean LO and HTP-LO in the presence of various PUFAs. 202,203 It is noteworthy that the rate ofsoybean LO-catalyzed ethacrynic acid conjugation with GSH was up to 1650-fold greater than therates reported in the literature for various preparations of purified GST. 202 This novel reaction withHTP-LO proceeds at a biologically significant rate (about 60% of the maximal velocity noted underoptimum assay conditions) under physiologically relevant conditions of pH and concentrations of GSHand arachidonic acid. 203 Further studies have shown that p-aminophenol, a developmental toxicant inanimals, also rapidly undergoes LO-mediated GSH conjugation to generate an ultimate toxic species. 206The study by Joseph et al. 199 has shown that purified HTP-LO cooxidizes pyrogallol, ABTS,benzidine, 3,3′-dimethoxybenzidine, TMPD, TMBZ, and guaiacol in the presence of linoleic acid.The oxidation rate was highest with TMPD and lowest with guaiacol, the most commonly usedperoxidase substrate. Hypervitaminosis-A has been linked with birth defects in animals and humans.Several studies have suggested that the parent molecule per se may not be teratogenic and bioactivationis essential. The study conducted by Datta and Kulkarni 207 suggested that all-trans retinolacetate is an excellent substrate for cooxidation by soybean LO, as well as HTP-LO, in the presenceof linoleic acid. The dioxygenase-generated peroxyl radical of linoleic acid was proposed to attackthe π-electrons of the C=C bond and bring about epoxidation of the all-trans retinol acetate. Earlier,HTP-LO-mediated epoxidation was also shown with the developmental toxicant BP-diol. 106 Significantcovalent binding of radiocarbon from BP-diol and a preponderance of trans-anti-7,8,9,10-tetrahydrotetrol suggested bioactivation of BP-diol by purified LO isolated from nonsmokers’placenta. These observations clearly explain the literature reports of detection of BP adducts inhuman term placentas from nonsmokers. 208 ATB 1 , a substituted coumarin, is a known animal andhuman teratogen. Many reports have documented biologically significant levels of ATB 1 in maternaland cord blood, 209,210 ATB 1 -DNA adducts in placentas and cord blood, 211 and transplacental carcinogenicity.212 It is widely accepted that ATB 1 requires prior epoxidation to cause toxicity. Thenotion that P450 is the sole catalyst for the bioactivation of this toxicant has been questioned. 6 Thepostulate that LO activates ATB 1 finds strong support in the demonstration that purified adult human© 2006 by Taylor & Francis Group, LLC