PURIFICATION AND CHARACTERIZATION OF A 3,5,3'-L-TRIIODOTHYRONINE-SPECIFIC BINDING-PROTEIN FROM BULLFROG TADPOLE PLASMA - A HOMOLOG OF MAMMALIAN TRANSTHYRETIN

We analyzed the nature of the thyroid hormone-binding proteins in bull frog plasma using N-bromoacetyl-[I-125]T3 as an affinity labeling prob e. Polyacrylamide gel electrophoresis under nondenaturing conditions o f the bullfrog N-bromoacetyl-[I-125]T3-labeled plasma proteins reveale d two proteins with specific binding to T3. A labeled protein that mig rated faster than albumin (T-T3BP) was detected only in plasma obtaine d from tadpoles at stages earlier than, but not at the end of, metamor phic climax. Another protein that migrated more slowly than albumin ap peared in the plasma at the late climax stage and was present in the a dult stage. To study the function of T-T3BP during metamorphosis, it w as purified from tadpole plasma to the single protein. The molecular m ass of this protein was estimated to be 56 kilodaltons by gel filtrati on, but only 16 kilodaltons by sodium dodecyl sulfate-polyacrylamide g el electrophoresis, which indicates that the molecule comprised four i dentical subunits. The amino acid composition of T-T3BP and the amino acid sequence of its N-terminal portion were highly homologous with th ose of mammalian transthyretins. These molecular features indicate tha t T-T3BP is a homolog of mammalian transthyretins. However, in contras t to mammalian transthyretins, the affinity of bullfrog transthyretin for T3 was 360 times higher than that for T4. Scatchard analysis revea led that T-T3BP possessed a single class of T3-binding site, with a K( d) of 0.67 nm at 0 C. These results suggest that bullfrog transthyreti n may play an important role in transporting T3 in the blood during me tamorphosis. (Endocrinology