Changes in structures of milk proteins upon photo-oxidation

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1、10968J.Agric.FoodChem.2007,55,1096810976ChangesinStructuresofMilkProteinsuponPhoto-oxidationTRINEK.DALSGAARD,DANIELOTZEN,,JACOBH.NIELSEN,ANDLOTTEB.LARSEN*DepartmentofFoodScience,UniversityofAarhus,P.O.Box50,DK-8830Tjele,DenmarkandInterdisciplinaryNanoscienceCentre(iNANO)DepartmentofMo

2、lecularBiology,GustarWiedsVej10C,UniversityofAarhus,8000AarhusC,DenmarkChangesinproteinstructuresasaresultofriboflavin-inducedphoto-oxidationwerestudiedforsixmilkproteins:R-casein,
-casein,κ-casein,lactoferrin,R-lactalbumin,and
-lactoglobulin.Themilkproteinsshowedsignificantvariabilityi

3、nsensitivitytophoto-oxidation.Afterphoto-oxidation,anincreaseincarbonylcontentbecauseofoxidationoftryptophan,histidine,andmethionine,aswellasformationofdityrosine,wasobservedforallproteinsstudied,althoughatverydifferentlevels.Generally,theincrementwashighestforR-and
-caseinandwaslowes

4、tforlactoferrin.Lossoftryptophanbecauseofphoto-oxidationwaswell-correlatedwiththeformationofthetryptophanoxidationproducts,N-formylkynurenineandkynurenine.Changesatthetertiaryproteinstructurelevelwereobservedafterphoto-oxidationoftheglobularproteins,wheretryptophanfluorescenceemissioni

5、ndicatedunfoldingofR-lactalbuminand
-lactoglobulin,whereaslactoferrinachievedamorecompacttertiarystructure.ChangesinsecondarystructurewereobservedforR-lactalbuminand
-lactoglobulin,whereasthesecondarystructureoflactoferrindidnotchange.PolymerizationofR-and
-caseinandoflactoferrinwasob

6、served,whereasκ-casein,R-lactalbumin,and
-lactoglobulinshowedlittletendencytopolymerizeafterphoto-oxidation.Labilitytowardphoto-oxidationisdiscussedaccordingtothestructuralstabilitiesoftheglobularproteins.KEYWORDS:Photo-oxidation;milkproteins;riboflavin;proteinstructure;proteinoxidatio

7、nINTRODUCTIONreducecleavageoftheproteinsthatarenecessaryfortheclottingprocessinmilk.Reactiveoxidativespeciesarefoundinallfoodstuffs,Proteinoxidationinmilkcanbeinitiatedeitherenzymatically,includingmilkandmilk-derivedproducts.Traditionally,theforexample,bythelactoperoxidasesystem(3),by

8、photo