The Relation of the Dual Thyroxine/Vitamin D-Binding Protein (TBP/DBP) of Emydid Turtles to Vitamin D-Binding Proteins of Other Vertebrates

Abstract

The relation of a dual binding protein, involved in the transport of both thyroxine (T 4) and vitamin D 3, in the blood of emydid turtles to blood proteins in other vertebrates was examined immunologically. Binding studies with 25-OH-[ 3H]cholecalciferol (D 3) confirmed the presence of a D 3-binding protein (DBP) in the plasma of a wide variety of chelonian species representing both major suborders, as well as other species of reptiles, amphibians, birds, and mammals. Analysis by polyacrylamide gel electrophoresis (PAGE) showed that these binding proteins exhibited variable electrophoretic mobilities and in some species multiple DBPs were observed. Western blot analysis of these gels and additional tests with SDSPAGE showed that an antiserum against the dual T 4/D 3-binding protein (TBP/DBP) from the turtle Trachemys scripta cross-reacted with the DBPs of diverse turtles, one crocodilian ( Alligator), and birds; however, there was little or no cross-reaction with plasma from squamate reptiles (three snakes, four lizards), one crocodilian ( Osteolaemus), amphibians (two anurans, one urodele), or mammals (six eutherians, one metatherian). The cross-reacting proteins (DBPs) all exhibit a similar M r (≈60 K). Adsorption of plasma by TBP/DBP-affinity chromatography confirmed this phylogenetic pattern of cross-reactions between the anti-TBP/DBP serum and functional DBPs in chicken, turtle, and alligator blood (affinity adsorption effectively eliminated D 3 binding); there was only weak cross-reaction with DBP in a lizard and the second crocodilian (50% reduction in D 3 binding) and none with DBP of snakes or mammals (D 3 binding was unaffected). Results suggest that the plasma protein (TBP/DBP) that exhibits dual, high-affinity binding of T 4 and D 3 in one turtle family evolved from the more "primitive" vitamin D-binding protein of stem reptiles; thus, the high-affinity T 4-binding site on this molecule is probably a derived characteristic of DBP in the Emydidae.