"Covalent" avidin & streptavidin on ELISA plates seems impossible in
theory and very do-able, i.e. it works in practical applications.
Since avidin/SA is a tetramer which can not possible have all
4 subunits coupled to the plate, it really does not matter much
that say 1 unit is covalently coupled - the other 3 may come off.
In practical usage, the commercial SA plates (Pierce etc) do
not leach avidin/Streptavidin - i.e. for all practical purposes
it is covalent. You could coat into plates that react with the
protein covalently, but as I said, it is probably not worth the