Thyroid hormone binding to lipid-free apolipoprotein (apo) A-II, C-I, C-II, and C-III isolated from human plasma was investigated by photoaffinity labeling with [125I]T4 and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Both the monomeric and polymeric forms were specifically labeled. Inhibition by 10μM unlabeled L-T4 was ≥50 %, suggesting affinity constants in the nM to μM range; the least inhibition was seen with apoA-II. Unlabeled D-T4 and reverse T3 (rT3) gave the same inhibition as unlabeled L-T4. Inhibitors of thyroid hormone binding to plasma proteins showed a different inhibitor potency with each apolipoprotein and a pattern different from that seen with T4 binding globulin (TBG) and transthyretin (TTR). Also in contrast to TBG, where only unsaturated nonesterified fatty acids (NEFA) are effective inhibitors, both unsaturated and saturated NEFA as well as other lipids inhibited T4 labeling. The flavonoid EMD 21388 was ineffective, confirming that it is a selective inhibitor of T4 binding to TTR. T4 binding to the apoCs was confirmed by the quenching of tryptophan fluorescence by unlabeled L-T4. (ApoA-II was not studied since it lacks tryptophan.) Since the self-association of apolipoproteins involves interaction between amphipathic α-helices, and since the polymeric forms show specific T4 binding properties as in the parent monomer, the T4-binding domain appears to be outside the α-helical domain, as previously seen with apoA-I. The position of the T4 binding sites of apoA-I, A-II, C-I, C-II, C-III, and E in the N-terminal, exon 3-coded regions (and in the exon 2-coded region of A-IV) is associated with amino acid homology, which is also shared with the T4 binding domains of TBG, TTR, and serum albumin.
