Comparative survey of blood thyroxine binding proteins in turtles

Abstract The nature of plasma thyroxine (T 4 ) binding activity was surveyed in turtles; binding to [ 125 I]T 4 was measured on polyacrylamide gel electrophoresis—PAGE—and on minicolumns of Sephadex G‐25. An electrophoretically distinct T 4 binding protein was identified in all 8 species of Pseudemys studied and in 3 other genera ( Chrysemys, Deirochelys , and Emyoidea ) of the same family, Emydidae. Levels of this binding activity were highly variable among individuals, but they consistently showed a similar low relative mobility (R f ) compared to albumin, and a relatively low capacity was indicated by displacement with unlabeled T 4 . Two emydids ( Terrapene , Clemmys ) showed a similar slow migrating binding peak, but binding activity was low and not as easily displaced by unlabeled T 4 . T 4 binding to albumins was minimal in most of these emydid species, even when binding to the higher affinity, low capacity component was low or displaced by unlabeled T 4 (2.5 μg/ml). In contrast, there was no clear evidence for a similar high affinity, low capacity binding protein in any of the other 19 species representing 13 genera of 8 families from two suborders. In these species, binding activity on Sephadex G‐25 was typically low and binding on PAGE was associated largely with albumin; binding levels for albumins were highly variable. In several nonemydids (from distant lineages), binding activity on Sephadex was elevated and PAGE showed a second binding protein distinct from albumin, but it had high capacity (not readily saturable). Thus, an evolutionary divergence in T 4 transport proteins is suggested within Chelonia.