Thyroxine-Binding Protein Represents the Major Vitamin D-Binding Protein in the Plasma of the Turtle, Trachemys scripta

Structural homology between the high-affinity thyroxine (T₄)-binding protein (TBP) in the plasma of the turtle, Trachemys scripta, and vitamin D-binding proteins (DBP) of mammals prompted an investigation of plasma vitamin D binding in the turtle. Several lines of evidence indicate that the TBP represents the primary binding protein for 25-OH-cholecalciferol (D₃) in the turtle plasma. D₃-binding protein in whole plasma migrates in the same position as TBP by size-exclusion chromatography and polyacrylamide gel electrophoresis; it is electrophoretically distinct from sex hormone-binding proteins. D₃ binding to purified TBP alone is enhanced (up to sevenfold) in the presence of plasma proteins, including albumin; with this correction, the D₃-binding activity of plasma corresponds to expected TBP titers. Plasma selectively stripped of TBP by affinity chromatography and purified turtle albumin have only trace D₃-binding activity. Variations in physiological state (thryoidal status, age, and sex) previously associated with variable T₄ binding land TBP levels) in T. scripia show correlated variability in plasma D₃ binding. D₃ binding is also highly correlated with T₄ binding (r = 0.81; P < 0.001) for plasma samples taken from 30 adults representing 10 different species of Trachemys. D₃ binding in plasma exhibits a high-affinity site (K_a = 2.3 × 10⁸ M^-1) and a second lower-affinity (K_a = 2 × 10⁶ M^-1), higher-capacity site; capacities are highly variable. Purified TBP has a comparable high affinity (K_a = 2.8 × 10⁸ M^-1), with a capacity close to 1 mol/mol. Binding of T₄ and D₃ are not competitive, indicating separate binding sites for the two ligands; in fact, T₄ tends to enhance the affinity and the capacity for D₃. A single protein in turtle plasma ("TBP/DBP") functions in the transport of two different hormones normally served by two distinct binding proteins (representing different multigene families) in mammals. These results have implications for the mediation of T₄ effects on growth. Copyright 1994, 1999 Academic Press