Nature of Thyroxine-binding Globulin in Human Serum

Abstract

THE term thyroxine-binding globulin is often used to denote a serum protein fraction with a mobility intermediate between those of α₁ - and α₂-globulins in paper or moving-boundary electrophoresis in veronal buffer, pH 8.6, ionic strength 0.05–0.10 (ref. 1). However, with the introduction of supporting media such as agar and starch gels and the use of different buffers, ionic strength or pH for paper electrophoresis, binding of thyroid hormones has also been detected in the ‘prealbumin’ zone². Although the proteins of the ‘prealbumin’ zone obtained under different conditions are not likely to be identical, a strong thyroxine-binding property was detected in human serum prealbumin rich in tryptophan³, as obtained from Prof. Schultze⁴. From chemical and immunological observations I concluded that the tryptophanrich prealbumin, present as a complex in the α-globulin fraction, might be responsible for much of thyroxine-binding in human serum⁵. Since then, an α-globulin-type of thyroxine-binding protein has been obtained in this laboratory (unpublished results); the nature of this protein does not support the hypothesis that the tryptophan-rich prealbumin is involved in binding thyroxine. One possible explanation for this discrepancy is that Schultze's prealbumin preparation contained traces (1–2 per cent) of thyroxine-binding globulin which were responsible for the production of antibodies to it present in antisera against this prealbumin. In fact, multiple precipitation lines were observed in immunological studies using rabbit anti-human prealbumin serum⁵. The experiments to be described support this explanation and throw further light on the nature of the thyroxine-binding globulin in human serum.