SM_c23HormonalRegulationandIntegrationofMammalianMetabolism

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1、c23HormonalRegulationandIntegrationofMammalianMetabolism.qxd12/12/124:13PMPageS-266chapterHormonalRegulationandIntegrationof23MammalianMetabolism1.PeptideHormoneActivityExplainhowtwopeptidehormonesasstructurallysimilarasoxytocinandvasopressincanhavesuchdifferenteffects(seeFig.23–9).AnswerOxytocinand

2、vasopressinarerecognizedbydifferentreceptors,typicallyfoundindifferentcelltypes.Thesereceptorsarecoupledtodifferentdownstreameffectsintheirtargetcells.2.ATPandPhosphocreatineasSourcesofEnergyforMuscleDuringmusclecontraction,thecon-centrationofphosphocreatineinskeletalmuscledropswhiletheconcentration

3、ofATPremainsfairlyconstant.However,inaclassicexperiment,RobertDaviesfoundthatifhefirsttreatedmusclewith1-fluoro-2,4-dinitrobenzene(p.98),theconcentrationofATPdeclinedrapidlywhiletheconcentrationofphosphocreatineremainedunchangedduringaseriesofcontractions.Suggestanexplanation.AnswerMusclecontraction

4、resultsinanethydrolysisofATP.AlthoughtheamountofATPinmuscleisverysmall,thesupplycanberapidlyreplenishedbyphosphorylgrouptransferfromthephosphocreatinereservoir,catalyzedbycreatinekinase:PhosphocreatineADP34creatineADPBecausethisreactionisrapidrelativetotheuseofATPbymuscle,[ATP]remainsinasteadystat

5、e.Theeffectofpretreatmentwithfluoro-2,4-dinitrobenzenesuggeststhatthisisaneffec-tiveinhibitorofcreatinekinase.Underworkingconditions,thesmallamountofmuscleATPisquicklydepletedandcannotbereplenished.3.MetabolismofGlutamateintheBrainBraintissuetakesupglutamatefromtheblood,transformsitintoglutamine,the

6、nreleasesitintotheblood.Whatisaccomplishedbythismetabolicconversion?Howdoesittakeplace?Theamountofglutamineproducedinthebraincanactuallyexceedtheamountofglutamateenteringfromtheblood.Howdoesthisextraglutaminearise?(Hint:youmaywanttoreviewaminoacidcatabolisminChapter18;recallthatNH3isverytoxictothebr

7、ain.)AnswerAmmoniaisverytoxictonervoustissue,especiallythebrain.ExcessNH3inbraincellsisremovedbythetransformationofglutamatetoglutamine,catalyzedbyglutaminesyn-thetase.Glutamineisthenexportedtothebloo