SM_c12Biosignaling

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1、c12Biosignaling.qxd11/29/124:12PMPageS-131chapterBiosignaling121.HormoneExperimentsinCell-FreeSystemsInthe1950s,EarlW.Sutherland,Jr.,andhiscolleaguescarriedoutpioneeringexperimentstoelucidatethemechanismofactionofepinephrineandglucagon.Givenwhatyouhavelearnedin

2、thischapterabouthormoneaction,interpreteachoftheexperimentsdescribedbelow.IdentifysubstanceXandindicatethesignificanceoftheresults.(a)Additionofepinephrinetoahomogenateofnormalliverresultedinanincreaseintheactivityofglycogenphosphorylase.However,ifthehomogenate

3、wasfirstcentrifugedatahighspeedandepinephrineorglucagonwasaddedtotheclearsupernatantfractionthatcontainsphosphory-lase,noincreaseinthephosphorylaseactivityoccurred.(b)Whentheparticulatefractionfromthecentrifugationin(a)wastreatedwithepinephrine,substanceXwaspro

4、duced.Thesubstancewasisolatedandpurified.Unlikeepinephrine,substanceXactivatedglycogenphosphorylasewhenaddedtotheclearsupernatantfractionofthecentrifugedhomogenate.(c)SubstanceXwasheat-stable;thatis,heattreatmentdidnotaffectitscapacitytoactivatephos-phorylase.(

5、Hint:WouldthisbethecaseifsubstanceXwereaprotein?)SubstanceXwasnearlyidenticaltoacompoundobtainedwhenpureATPwastreatedwithbariumhydroxide.(Fig.8–6willbehelpful.)AnswerSubstanceXiscyclicAMP.Epinephrinestimulatesglycogenphosphorylasebyactivatingtheenzymeadenylylcy

6、clase,whichcatalyzesformationofcAMP,thesecondmessenger.(a)Adenylylcyclaseisamembrane-boundprotein;centrifugationsedimentsitintotheparticulatefraction.(b)CyclicAMPdirectlystimulatesglycogenphosphorylase.(c)CyclicAMPisheat-stable;itcanbepreparedbytreatingATPwithb

7、ariumhydroxide.2.EffectofDibutyrylcAMPversuscAMPonIntactCellsThephysiologicaleffectsofepinephrineshouldinprinciplebemimickedbyadditionofcAMPtothetargetcells.Inpractice,additionofcAMPtointacttargetcellselicitsonlyaminimalphysiologicalresponse.Why?Whenthestructur

8、allyrelatedderivativedibutyrylcAMP(shownbelow)isaddedtointactcells,theexpectedphysiologicalresponseisreadilyapparent.Explainthebasisforthedifferenceincellularresponsetothese