Purification and partial characterization of a novel thyroxine-binding protein (27K protein) from human plasma

T4-binding globulin (TBG) prepared from human plasma by the standard three-step procedure (T4-agarose affinity chromatography, anion exchange chromatography, and gel filtration) often shows in sodium dodecyl sulfate-polyacrylamide gel electrophoresis in addition to the expected 54K band, another with a mol wt of 27,000 (27K protein). The two proteins can be separated after the three-step procedure by chromatofocusing (because of different isoelectric points, 4.2-4.8 for TBG and 5.0-5.2 for 27K protein) or by T4-aragose chromatography eluting with a linear gradient of T4 (TBG is eluted between 10(-10) and 10(-9) M T4, 27K protein between 10(-8) and 10(-7) M T4). The 27K protein does not appear to be a fragment of TBG since 1) it does not displace [125I]TBG bound to anti-TBG monoclonal antibodies; and 2) absorption of polyclonal antibody reacting with both TBG and 27K protein with sera from TBG-deficient patients completely prevents [125I]27K protein binding, while only slightly affecting [125I]TBG binding. On the other hand, 27K protein is not simply a contaminant devoid of biological activity, but is a T4-binding protein, as supported by the following findings: 1) it covalently binds [125I]T4 by photoaffinity labeling, and this binding can be almost completely prevented by excess T4; 2) equilibrium dialysis shows two equivalent T4-binding sites per 66K, with an association constant of 0.85 X 10(7) M-1, intermediate between albumin and prealbumin; and 3) tryptophanyl fluorescence analysis shows quenching of 37% of the fluorescence when the protein is titrated with T4. The 27K protein appears as a single 27K band in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, pH 8.8, but under nondenaturing nonreducing conditions mostly remains at the origin of the gel; a fraction enters the gel and migrates slightly ahead of albumin. This electrophoretic pattern is distinct from those of albumin, prealbumin, and TBG. In immunoelectrophoresis in agar at pH 8.6, 27K protein moves slightly faster than TBG. The results of equilibrium sedimentation indicate a mol wt of 66,000, suggesting that the 27K protein might exist as a dimer. These data indicate that the 27K protein is a previously unrecognized T4-binding protein with a low affinity for the hormone. Further studies are required to clarify its physiological role in the transport of circulating thyroid hormones.