ApoB-100-containing lipoproteins are major carriers of 3-iodothyronamine in circulation

3-Iodothyronamine (T(1)AM) is a biogenic amine derivative of thyroid hormone present in tissue and blood of vertebrates. Approximately 99% of the circulating thyroid hormones are bound to plasma proteins, including three major thyroid hormone-binding proteins, and the question arises as to whether circulating T(1)AM is also bound to serum factors. We report here that T(1)AM is largely bound to a single protein component of human serum. Using T(1)AM-affinity chromatography, we isolated this protein, and sequence analysis identified it as apolipoprotein B-100 (apoB-100), the protein component of several low density lipoprotein particles. Consistent with this finding, we demonstrate that >90% of specifically bound T(1)AM in human serum resides in the apoB-100-containing low density lipoprotein fraction. T(1)AM reversibly binds to apoB-100-containing lipoprotein particles with an equilibrium dissociation constant (K(D)) of 17 nm and a T(1)AM/apoB-100 stoichiometry of 1:1. Competition binding assays demonstrate that this binding site is highly selective for T(1)AM. Intracellular T(1)AM uptake is significantly enhanced by apoB-100-containing lipoprotein particles. Modest enhancements to apoB-100 cellular uptake and secretion by T(1)AM were observed; however, multidose T(1)AM treatment did not affect lipid or lipoprotein inventory in vivo. Thus, it appears that apoB-100 serves as a carrier of circulating T(1)AM and affords a novel mechanism by which T(1)AM gains entry to cells.