ApoB-100-containing Lipoproteins Are Major Carriers of 3-Iodothyronamine in Circulation

Abstract

3-Iodothyronamine (T1AM) 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 T1AM is also bound to serum factors. We report here that T1AM is largely bound to a single protein component of human serum. Using T1AM-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 T1AM in human serum resides in the apoB-100-containing low density lipoprotein fraction. T1AM reversibly binds to apoB-100-containing lipoprotein particles with an equilibrium dissociation constant (KD) of 17 nm and a T1AM/apoB-100 stoichiometry of 1:1. Competition binding assays demonstrate that this binding site is highly selective for T1AM. Intracellular T1AM uptake is significantly enhanced by apoB-100-containing lipoprotein particles. Modest enhancements to apoB-100 cellular uptake and secretion by T1AM were observed; however, multidose T1AM treatment did not affect lipid or lipoprotein inventory in vivo. Thus, it appears that apoB-100 serves as a carrier of circulating T1AM and affords a novel mechanism by which T1AM gains entry to cells. 3-Iodothyronamine (T1AM) 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 T1AM is also bound to serum factors. We report here that T1AM is largely bound to a single protein component of human serum. Using T1AM-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 T1AM in human serum resides in the apoB-100-containing low density lipoprotein fraction. T1AM reversibly binds to apoB-100-containing lipoprotein particles with an equilibrium dissociation constant (KD) of 17 nm and a T1AM/apoB-100 stoichiometry of 1:1. Competition binding assays demonstrate that this binding site is highly selective for T1AM. Intracellular T1AM uptake is significantly enhanced by apoB-100-containing lipoprotein particles. Modest enhancements to apoB-100 cellular uptake and secretion by T1AM were observed; however, multidose T1AM treatment did not affect lipid or lipoprotein inventory in vivo. Thus, it appears that apoB-100 serves as a carrier of circulating T1AM and affords a novel mechanism by which T1AM gains entry to cells.