Streptavidinisaproteinthathassimilarbindingpropertiestoeggwhiteavidin.Itisisolatedfromstreptomycesavidinii.Streptavidinhasamolecularweightof60kDaandhas4subunits.Eachsubunitcanbindonemoleculeofbiotin.Biotinisawater-solublevitamin.Streptavidinhasanextremelyhighbindingaffinity(Kd=10-15)forbiotin.Streptavidinconjugatedtoalkalinephosphatasehasprovenusefulinthedetectionofantigenscoupledwithbiotinylatedsecondaryantibodies.Thereareseveraladvantageswhenusingastreptavidin-conjugateversusanABCcomplex.Incontrasttoavidin,streptavidinisnotglycosylatedandisthereforeunchargedatneutralpH(6.5versus10).Thislowersnonspecificbackgroundstaining.Streptavidinalsolackscarbohydratesidechainsthatmaybeanothercauseofnon-specificbackground.Anotherkeyadvantageofstreptavidinisthesignificantincreaseinsensitivity(probablyduetolesssterichindrance),thusfacilitatinganincreaseinoverallbindingcapacity.Streptavidin-ALPismuchmorestablethantheABCcomplex.TheABCcomplexmustbefreshlymade30minutespriortouse,andisstableonlyforafewdays.Incontrast,theStreptavidin-ALPreagentisverystable,color-coded,andcanbestoredforupto1-2years.Biocare’sStreptavidin-ALPcomesinaready-to-useformat,thussavingtimeandpotentialmistakes.