毒理學基礎(chǔ) Chapter 2 mechanism 英文版

MechanismsofToxicity

UnderstandingMechanismsofToxicity-Whydoesthishelp?ProvidesarationalbasisforinterpretingdescriptivetoxicitydataHelpstoestimateprobabilitythatachemicalwillcausetoxicityUsedtodesigndrugsthatwillbemorespecificandlesshazardousHelpsustounderstandbasicphysiologicalandbiochemicalpathways(neurotransmission,DNArepair,inflammation)StepsLeadingtoToxicityStep1:DeliveryStep2:InteractionwithtargettissueormoleculeStep3disruptionofcellfunctionorstructureStep4initiationofrepairmechanismatmolecular,cellularandtissuetoxicityDisruption>Repair=ToxicityDelivery:FromthesiteofexposuretothetargetIntensityoftoxiceffectsDependsonconcentrationandpersistenceofultimatetoxicantatthesiteofaction(theoretically)Ultimatetoxicant:chemicalspeciesthatreactswiththetargetmolecule(protein,DNA,lipidetc.)parentcompound,metaboliteoftheparentcompound,orROSgeneratedduringbiotransformationConcentrationoftheUTatthetargetdependsontheeffectivenessoftheprocessesthatoritsconcentrationatthetargetsite.AbsorptionvsPresystemicEliminationAbsorption:transferofthechemicalfromthebodysurfacesystemiccirculationAffectedby:SurfaceareaConcentrationCharacteristicsofepilayerLipidsolubility:>lipophilicity=absorptionAbsorptionvsPresystemicEliminationPresystemiceliminationCanweeliminateagentbeforeitdoesharm?GItractliver(firstpass)eliminationviabilekidneyLungmaydothesameThesetissuesmaytakethebruntofthedamageMechanismsthatfacilitate

distributiontoatargetPorosityofcapillaryendothelium(hepaticsinusoids,renalperitubularcapillaries-largefenestrae50-150nm)Specializedmembranetransport(endocytosisoftoxicant-proteincomplexesbyrenalproximaltubularcells)Reversibleintracellularbinding(bindingtothepigmentmelanin.Melaninispolyanionicandmaybindorganicandinorganiccations.CapillaryendotheliumRenalCapillaries#1Stopsallcellsandplatelets#2Stopslargeplasmaproteins#3Stopsmedium-sizedproteins,notsmallonesMechanismsopposing

DistributiontoatargetBindingtoplasmaproteins(albumin)Specializedbarriers(BBBandplacenta,testis)Distributiontostoragesites(leadaccumulatesinbone,lipophiliccompoundsmayaccumulateinadipose)AssociationwithintracellularbindingproteinsExportfromcellsExcretionvs.ReabsorptionExcretion:removalofthexenobioticandreturntotheexternalenvironmentPrimarilykidney,liverBreastmilk,bileandintestinallumenareminorroutesVolatile,non-reactivegasespulmonarycapillariesandexhaledExcretionvs.ReabsorptionReabsorption:“recycling”oftoxicantMayoccurintherenaltubulesastoxicantdiffusesbackacrosstubularcellsMayalsooccurforGItoxicantsbybiliary,gastricandintestinalexcretionthroughreabsorptionacrossintestinalmucosaDependentonlipophilicityFororganicacidsandbases,diffusionisinverselyrelatedtotheextentofionization,becausethenon-ionizedmoleculeismorelipidsoluble.(ionized=excreted,nonionized=absorbed)Toxicationvs.DetoxicationToxication:biotransformation(changing)ofthecompoundtoformamoretoxicmetaboliteAkametabolicactivationResultsinreactivespeciessuchas:Electrophiles(+charge,electrondeficient)Freeradical(unpairedelectronNucleophiles(-charge,reactwithelectrophiles)Redox-activereactantsToxicationvs.DetoxicationDetoxication:biotransformationthatresultsinlessreactivemetaboliteofthechemicalDetoxicationof:ToxicantswithnofunctionalgroupsAddafunctionalgroupandthenconjugatewithanacidNucleophiles:ConjugationElectrophilesConjwithglutathioneFreeradicalsSOD,Gpx,catalaseTakenfrom:CasarettandDoull’sToxicology,TheBasicScienceofPoisons6thEditionCurtisKlaassenEditorRemovalofH2O2Catalase(peroxisomes)GlutathioneperoxidaseCytosolandmitochondriaUsesGSHasacofactorHydroxylRadical?OHFormedby:RadiolysisofwaterRxnofH2O2withFe2+(FentonReaction)RxnofO2-withH2O2(HaberWeissRxn)Howis?OHdamaging?CauseslipidperoxidationRemovesHfromFAfreelipidradicalFreelipid+O2LOO?Reactswithproteins,DNA,lipidsPeroxynitratePeroxynitrateONOO-formedby:

O2-+NO?(nitricoxide)ONOO-Attackslipids,proteins,andDNANOproducedbymanytissues,potentvasodilatorDefensesagainstOxygenFreeRadicalsEnzymes:SOD:convertssuperoxide(O2-)toH2O2andO2Catalase:2H2O2

2H2O+

O2Gpx:H2O2+2GSH

2H2O+GSSGDefensesagainstOxygenFreeRadicalsScavengersofROS:VitE(tocopherol)electronacceptor/blocksfreeradicalchainrxnVitC(ascorbate)watersoluble,reactswithsuperoxideandhydroxylradicalRetinoids:precursorsofVitA,lipidsolchainbreakingAONO?:mayproteosomalactivity(removedamageproteins)ReactionoftheUltimateToxicantwiththeTargetmoleculeMostoftenthetoxicantwillreactwithnucleicacids(DNA,RNA),protein,orlipidstocausetoxiceffectsTypesofreactionNonconvalentbidingUsuallyreversibleduetolowbindingenergyCovalentbindingPracticallyirreversibleHydrogenabstractionCanresultinradicalsElectrontransferEnzymaticreactionSometoxicantsactthisway(e.gdiphtheriatoxin)EffectsofToxicantsonTargetMoleculesDysfunctionofTargetMoleculesDestructionoftheTargetMoleculesNeoantigenFormationHowcanToxicantsaffectTargetMolecules?1.DysfunctionoftargetmoleculesMimickingendogenousligandsBlockingneurotransmissionAffectingionchannelsInhibitingenzymesHowcanToxicantsaffectTargetMolecules?2.DestructionofTargetMoleculesAdductformationCrosslinkingandfragmentationFreeradicalattackHowcanToxicantsaffectTargetMolecules?3.NeoantigenformationToxicantattackmayformanewmoleculethatcausesanimmuneresponseWhatiscellsignalingSignalingmoleculeslikegrowthfactors,hormones,cytokines,neurotransmitters—canactivatetranscriptionfactorsThesecancontrolanumberofactionsinthecellsuchasproliferation,generegulationandapoptosisFig3-7Doesthetoxicantalterasignalpathwaythatdealswithcellproliferation?XenobioticsthatfacilitatephosphorylationofsignaltransducersoftencausemitosisandtumorformationDoesthetoxicantcauseaincellproliferation?MaygetadecreaseofreplacementcellswhenthecellisinjuredImpairmentofcellularmaintenanceATPsynthesisCa2+regulationProteinsynthesisMicrotubuleformationMembranefunctionCalciumRegulationThecellregulatescalciumbyPlasmamembraneTransportmechanismsCalciumissequesteredinthemitochondriaandendoplasmicreticulumMitochondrialPermeabilityTransition(MPT)IncreaseininnermitochondrialmembranepermeabilitymaybeduetoMitoCauptakeROS,RNSDepletionofATPRepairordysrepairOncedamageoccurs,whatarethewaysthatcellscanrepair?Somereactionsarereversible(oxidationofproteinthiolsandmethylationofDNA)hydrolyticremovalandreplacementRepairProteinRepairThiolgroupsareessential!OxidationcanbereversedbyGlutaredoxinandThioredoxinLipidRepairGpx,GrxNADPHneededtorecycletheseRepairDNArepairFairlystableduetohistoneassnAnumberofrepairmechanismsexistDNAphotolyase-cleavespyrimidinedimersExcisionrepair—canremovebasesandleaveanemptysitewithmaybereplacedwithnewbasesPARP(polyADP-ribosepolymerase)MitoDNAdon’thavehistones—moresusceptibleTissuerepairIntissueswithcellscapableofmultiplyingCellsareremovedbynecrosisorapoptosisregenerationApoptosisCellsuicideRemovalwithoutinflammationCellshrinksOrderlyremovalofdiscretecellsorsmallareasofcellGoodfortissuethathascellsthatcanmultiplyandreplaceNotsogoodfornonreplaceablecellsSideReactionstoTissueInjuryInflammationinitiatedbyresidentmacrophagesCharacterizedbychangesinporosityofmicrocirculationProductionofROSandRNSInflammatorycytokinesandcellsLeukocytesFibroblastsPlateletsSideReactionstoTissueInjuryAlteredProteinSynthesis:AcutePhaseProteinsMacrophagesandendothelialcellsofinjuredtissuereleasecytokinesCytokinesalterproteinsynthesisIL-1,IL-6andTNFactoncellreceptorstoortranscriptionalactivityof+or–acutephaseproteinse.g.hepaticacutephaseproteinssuchasC-reactiveprotein,aresecretedintothecirculationandareindicativeofinflammation,tissueinjuryorneoplasm.WhenrepairfailsNecrosis:celldeath-gameoverCanbehaltedbyrepairmechanisms,apoptosisandproliferationCellswellingInflammationUsuallylargeareasofcellsWhenrepairfailsFibrosisPathologicconditionExcessivedepositionofextracellularmatrix/abnormalcompositionTGF-isamajormediatorFibrosis—liverandlungWhenrepairfailsCarcinogenesismutationistheinitiatingeventFailureofDNArepairFailureofapoptosismutationofprotooncogenesFailuretoterminatecellproliferationmutationoftumorsuppressorgenesFailureofDNArepairMutationistheinitiatingeventincarcinogenesisDirectassault(adducts,breaks)Indirectassault(ROS,RNS)Mutatedcellsthatsurvivecanpassonmutation(heritable)MutatedcellsusuallyhavegrowthadvantagetumorandrapidgrowthMutationofprotooncogenesProtooncogenesarehighlyconservedgenesthatencodeforproteinsinvolvedwithcellgrowthandproliferationTheyinclude:GrowthfactorsGrowthfactorreceptorsIntracellularsignaltransducersNuclearTFsTumorSuppressorGenesTumorsuppressorgenesencodeforproteinsthatinhibitprogressionofcellsincelldivisionP53isinthiscategory53kDaproteintransactivatesgeneswhoseproductsarrestcellcycleorpromoteapoptosisandrepressesgenesthatencodeantiapoptoticproteinsCellCycleControlGrowthstimulation-cellsgofromG0-G1DN