MUTATIONS IN A SPECIFIC HUMAN SERUM-ALBUMIN THYROXINE-BINDING SITE DEFINE THE STRUCTURAL BASIS OF FAMILIAL DYSALBUMINEMIC HYPERTHYROXINEMIA

The familiar dysalbuminemic hyperthyroxinemia (FDR) phenotype results hom a natural human serum albumin (HSA) mutant with histidine instead of arginine at amino acid position 218. This mutation results in an en hanced affinity for thyroxine, Site-directed mutagenesis and a yeast p rotein expression system were used to synthesize wild type BSA and FDN HSA as well as several other HSA mutants. Studies oil the binding of thyroxine to these HSA species using equilibrium dialysis and quenchin g of tryptophan 214 fluorescence suggest that the FDH mutation affects a single thyroxine binding site located in the 2A subdomain of HSA. S ite-directed mutagenesis of HSA and thyroxine analogs mere used to obt ain information about the mechanism of thyroxine binding to both wild type and FDH HSA. These studies suggest that the guanidino group of ar ginine at,amino acid position 218 in wild type RSA is involved in an u nfavorable binding interaction with the amino group of thyroxine, wher eas histidine at amino acid position 218 in FDR HSA is involved in a f avorable binding interaction with thyroxine. Neither arginine at amino acid position 222 nor tryptophan at amino acid position 214 appears t o favorably influence the binding of thyroxine to wild type HSA.