You have purified the receptor for a hormone by affinity chromatography. During gel filtration chromatography under native conditions the receptor elutes between pyruvate decarboxylase (250 kDa) and glutamine synthetase (620 kDa). During SDS-PAGE, in the absence of reducing agents, the receptor migrates as a single band of approximately 230 kDa. When SDS-PAGE is carried out in the presence of 2-mercaptoethanol the receptor migrates as two bands of approximately 95 and 135 kDa. Explain this result.

In this case, the heterotetramer is composed of two heterodimers, each composed of two subunits that held together by disulfide bonds. These heterodimers bind together through hydrophobic interactions in order to form a tetramer. The Sodium Dodecyl Sulfate (SDS) is a denaturing detergent that dissociates both heterodimers by interfering with hydrophobic interactions that hold heterodimers together. It is for that reason that only a single band of 230 kDa is observed (135 kDa + 95 kDa = 230 kDa). Moreover, 2-Mercaptoethanol breaks disulfide bonds, thereby separating the two subunits in the heterodimer. In consequence, after the use of this chemical compound (2-Mercaptoethanol), two different bands with 135 kDa and 95 kDa can be observed.