THYROID HORMONE INTERACTIONS WITH CELLS AND PROTEINS

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Thyroxine-binding globulin (TBG) and cortisol binding globulin (CBG) are members of the serine protease family (serpins) which include the acute phase proteins, alpha 1-antitrypsin and anti 1-antichymotrypsin. Synthesis of the latter two proteins in hepatocytes is increased by the cytokine IL-6, but synthesis of TBG and CBG are decreased. The transcription rate of the TBG gene is decreased by IL-6, while that of the CBG gene is unaltered. The decrease in CBG synthesis in the acute phase reaction, resulting in a local increase in free cortisol levels, may play a role in combating inflammation. Continuation of work on thyroxine binding to apolipoproteins of HDL by photoaffinity labeling has now demonstrated binding not only to apoA-I but also to the minor apolipoproteins, including apoA-II, apoA-IV, apoC-I, II and III, and apoE. Thus, T4 binding may be a general property of apolipoproteins and may have a role in the intracellular targeting of thyroid hormone. The affinity labeling reagents, N-bromoacetyl thyroxine and N-bromoacetyl triiodothyronine have been synthesized and purified by new methods which lead to the preparation of more pure and better characterized products than were previously available. These reagents are widely used for affinity labeling of thyroid hormone binding proteins.