Binding of thyroxine to human plasma low density lipoprotein through specific interaction with apolipoprotein B (apoB-100).
Human plasma low density lipoprotein (LDL), which binds 0.2% of plasma T4, was shown to interact with the hormone through its protein moiety, apolipoprotein B-100. LDL and LDL2, the major subfraction of LDL, were found to have 3 equivalent binding sites for T4 with Ka = 2.5 x 10(6) M-1. Photoaffinity labeling of LDL with inner ring-labeled [125I]T4, followed by SDS-PAGE or agarose-SDS-PAGE of the labeled products, revealed that apoB-100 and its proteolytic cleavage products, apoB-74 and apoB-26, bound [125I]T4. In the presence of 1 or 10 microM T4, labeling was decreased in 7 separate experiments by 40-53% or 65-86%, respectively, consistent with a Ka of approximately 10(6) M-1. Binding of T4 to apoB-100 associated with VLDL was also demonstrated by photoaffinity labeling. The observed thyroid hormone binding property of lipid-complexed apoB-100 and the knowledge that receptors for the apolipoprotein exist in various tissues suggest a possible physiological role in thyroid hormone transport.
Benvenga
S
S
Clinical Endocrinology Branch, NIDDK, Bethesda, MD 20892.
Cahnmann
H
H
Gregg
R
R
Robbins
J
J
D001055:Apolipoproteins B / Q000097:blood / Q000378:metabolism*; D001345:Autoradiography; D004587:Electrophoresis, Agar Gel; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D008077:Lipoproteins, LDL / Q000097:blood / Q000378:metabolism*; D011485:Protein Binding; D013974:Thyroxine / Q000378:metabolism*
D001055:Apolipoproteins B; D008077:Lipoproteins, LDL; D013974:Thyroxine