[A new property of known proteins: specific binding of thyroid hormones by human plasma apolipoproteins].

The thyroid hormones--thyroxine (T4) and triiodothyronine (T3)--are capable of associating with human plasma high (HDL), low (LDL) and very low density lipoprotein particles (VLDL). Apolipoproteins (apo) act as the hormone-binding components of the lipoprotein particles. The thyroid hormone binding to apolipoproteins is a time-dependent, reversible, saturable and sensitive to specific inhibitors interaction with the structurally isolated site in the protein which is complementary to the ligand. The number of such sites per macromolecule varies from one (apoA-I) to three (apoB-100). Low affinities (Ka approximately 10(5)-10(6) M-1) for T4 are characteristic of apoA-II, apoA-IV, apoE and apoB-100, while apoA-I and its lipid complex, apoA-I-HDL display sufficiently high affinities for the hormone (Ka approximately 10(7)-10(8) M-1). The active site of apoA-I-HDL contains a chemical group interacting with the ionized phenolic ring hydroxy-group of T4, includes cavities capable of accommodating iodine atoms of the ligand molecule, has a hydrophobic nature and low stereospecificity and is located close to the surface of the protein globule in the N-terminal part of polypeptide chain conformationally associated with the polar surface monolayer of the lipid matrix. Three hormone-binding sites in apoB-100 are located in different regions of the polypeptide chain which are distant from each other and lie outside the sites of heparin and cellular LDL receptor binding. ApoB-100 and apoE stimulate T4 entry into fibroblasts, while apoA-I inhibits the interaction of T4 and T3 with human placental plasma membranes.