Non-covalent bonds are present in the alpha2M-peptide complexes. alpha2M was complexed to FITC-labeled peptide at a 1:50 molar ratio as described in Materials and methods. alpha2M-peptide complexes treated with SDS (lanes 3 and 4), with SDS and 2-ME (lanes 5 and 6) or untreated (lanes 1 and 2) were analyzed by SDS-PAGE (left panels for each treatment; lanes 1, 3 and 5) and immunoblotting with anti-FITC antibody (right panels; lanes 2, 4 and 6). Dissociation of the native alpha2M protein (720 kDa) by SDS denaturation produces a dimeric subunit (360 kDa). The disulphide bonds holding the dimer together can be cleaved using 2-ME. The monomeric subunits are 180 kDa.