[Studies on the hormone-binding function of thyroid hormone-binding proteins by heat treatment]

Thyroxine-binding globulin (TBG), a major thyroid hormone-binding protein, is denatured by heat and subsequently loses its binding activity. This study aimed to investigate changes in the function of thyroid hormone-binding proteins utilizing a method of heat treatment. The first aim of this study was to examine how the relationship between thyroid hormones and binding proteins changes with heat treatment at 56 degrees C in serum from normal subjects. When approximately half of the native TBG was denatured by heat, the mean percentage of T4 bound to thyroxine-binding prealbumin (TBPA), measured by a single immunoprecipitation method, was increased from 15.3% to 25.7% and the percentage of T4 bound to albumin was increased from 4.7% to 7.5%. Serum free T4 concentration were increased to 166.0 +/- 17.1% (Mean +/- SD) from the basal value. By heating at 58 degrees C, T4 bound to TBPA was denatured, but T4 bound to albumin showed a significant increase, and serum free T4 concentrations were increased to 396.0 +/- 62.1%. Because T4 binding ratios of TBPA and albumin were increased after TBG denaturation, free hormone concentrations did not increase very greatly. The second aim of this study was to examine the properties of TBG in patients with non familial partial deficiency of TBG (NFPD-TBG). When the serum sample of NFPD-TBG was heated, the half time of NFPD-TBG was 14.1 +/- 5.5 min, which was significantly shorter than that of normal TBG (45.1 +/- 7.0 min, P < 0.001). Isoelectric focusing (IEF) of NFPD-TBG showed that the pIs were not different from those of normal TBG. When serum from NFPD-TBG was mixed with normal serum, the half time was still shorter than the normal serum samples. The data suggest that the molecular structure of NFPD-TBG is not abnormal, but a certain factor which alters the heat stability of TBG exists in the serum from NFPD-TBG.