[Interaction of thyroid hormones with immunoglobulins isolated from human blood serum. II. Structural elements and localization of thyroxine-binding segment in the immunoglobulin M molecule].

The structural characteristics and topography of the thyroxine T4-binding site on the human immunoglobulin M (IgM) molecule have been studied using affinity binding with T4 structural analogs blocking the formation of the T4-IgM complex with proteins displaying an affinity for individual structural components of IgM and proteolytic fragmentation followed by determination of the T4-binding activity of the isolated fragments. It has been found that IgM has a poor selectivity towards the binding of T4 structural analogs which is characteristic of transport proteins but not of autoantibodies. The T4-binding region of IgM lies outside the variable portion of the Fab-fragment and is apparently constituted by the Cmu 1-domain of the heavy chain and the constant part of the light chain linked via a disulphide bridge. The T4-binding site located within this region possesses no stereospecificity and contains structural elements complementary to the iodine atoms of the outer ring and the aliphatic chain of the iodothyronine molecule.