Streptavidinisaproteinthathassimilarbindingpropertiestoeggwhiteavidin.Itisisolatedfromstreptomycesavidinii.Streptavidinhasamolecularweightof60kDaandhas4subunits.Eachsubunitcanbindonemoleculeofbiotin.Biotinisawater-solublevitamin.Streptavidinhasanextremelyhighbindingaffinity(Kd=10-15)forbiotin.StreptavidinconjugatedtohorserADIshperoxidasehasprovenusefulinthedetectionofantigenscoupledwithbiotinylatedsecondaryantibodies.Thereareseveraladvantageswhenusingastreptavidin-conjugatesversusanABCcomplex.Incontrasttoavidin,streptavidinisnotglycosylatedandisthereforeunchargedatneutralpH(6.5versus10).Thislowersnonspecificbackgroundstaining.Streptavidinalsolackscarbohydratesidechainsthatmaybeanothercauseofnon-specificbackground.Anotherkeyadvantageofstreptavidinisthesignificantincreaseinsensitivity(probablyduetolesssterichindrance),thusfacilitatinganincreaseinoverallbindingcapacity.Streptavidin-HRPismuchmorestablethantheABCcomplex.TheABCcomplexmustbefreshlymade30minutespriortouseandisstableonlyforafewdays.Incontrast,theStreptavidin-HRPreagentisverystable,color-coded,andcanbestoredforupto1-2years.Biocare’sStreptavidin-HRPcomesinaready-to-useformat,thussavingtimeandpotentialmistakes.