THE RELATION OF THE DUAL THYROXINE VITAMIN-D-BINDING PROTEIN (TBP DBP) OF EMYDID TURTLES TO VITAMIN-D-BINDING PROTEINS OF OTHER VERTEBRATES/

The relation of a dual binding protein, involved in the transport of b oth thyroxine (T-4) and vitamin D-3, in the blood of emydid turtles to blood proteins in other vertebrates was examined immunologically. Bin ding studies with 25-OH-[H-3]cholecalciferol (D-3) confirmed the prese nce 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 polya crylamide gel electrophoresis (PAGE) showed that these binding protein s exhibited variable electrophoretic mobilities and in some species mu ltiple DBPs were observed. Western blot analysis of these gels and add itional tests with SDS-PAGE showed that an antiserum against the dual T-4/D-3-binding protein (TBP/DBP) from the turtle Trachemys scripta cr oss-reacted with the DBPs of diverse turtles, one crocodilian (Alligat or), and birds; however, there was little or no cross-reaction with pl asma from squamate reptiles (three snakes, four lizards), one crocodil ian (Osteolaemus), amphibians (two anurans, one urodere), or mammals ( six eutherians, one metatherian). The crossreacting proteins (DBPs) al l exhibit a similar M(r) (approximate to 60 K). Adsorption of plasma b y 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 effectiv ely eliminated D-3 binding); there was only weak cross-reaction with D BP in a lizard and the second crocodilian (50% reduction in D-3 bindin g) 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 fr om 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. (C) 1994 Academic Pre ss, Inc.