High-affinity binding of tetraiodothyroacetic acid by a prealbumin in normal rabbit serum

A protein that binds tetraiodothyroacetic acid (tetrac) with high specificity has been detected in normal rabbit serum. Scatchard plots revealed the protein to have a principal binding site with both high capacity and high affinity for tetrac (KA 4.8 X 10(10) M-1. Binding of tetrac by the protein is partially inhibited by barbital. During polyacrylamide gel electrophoresis at pH 8.0, the tetrac binding protein has a mobility characteristic of a prealbumin. As judged from competitive binding studies, the protein also binds tetraiodothyropropionic acid (tetraprop) firmly, but less so than tetrac. The apparent affinity of the protein for the triiodinated analogues of tetrac and tetraprop is only about 1% that for tetrac, and that for thyroxine (T4) only 0.6% that for tetrac; 3,5,3'-triiodothyronine is not bound at all. The protein can be utilized in a competitive protein binding assay for tetrac in human serum, after removal of cross-reacting T4.