Comparative survey of blood thyroxine binding proteins in turtles

✍ Scribed by Licht, Paul ;Denver, Robert J. ;Herrera, Brenda E.

• Publisher: John Wiley and Sons
• Year: 1991
• Tongue: English
• Weight: 873 KB
• Volume: 259
• Category: Article
• ISSN: 0022-104X
• DOI: 10.1002/jez.1402590107

✦ Synopsis

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.