Page MW est.:response

[Mark Jason Logan]

If you add urea to SDS-PAGE you decrease the magnitude of the hydrophobic
effect. This phenomenon is responsible (in part) for the stability of
native protein structure as well as the association of individual SDS
molecules with the protein (ratio of approx. one SDS molecule/two amino
acids).  Therefore the minute you add urea in sufficient concentrations to
disrupt the packing of core hydrophobic regions you have also reduced the
driving force for the association of SDS with the protein.  

SDS-PAGE is full of flaws when it comes to MW determinations.  If you
really want to know what the molecular weight of a protein is then use
Mass. Spec.  Many core facilities provide this service for a nominal fee
($44.00 at W.M. Keck Biotechnologies Lab. Yale is but one example)

Hope this helps.

Mark Logan