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Binding affinities of thyroxine-binding proteins in turtle plasma
1Department of Integrative Biology, University of California, Berkeley 94720.
General and Comparative Endocrinology
|April 1, 1993
Summary
Thyroxine (T4) binding in turtles differs from humans, with turtle T4 binding protein (TBP) showing lower affinity than human thyroxine-binding globulin (TBG). Albumin plays a minor role in turtle T4 binding.
Area of Science:
- Comparative endocrinology
- Biochemistry
- Herpetology
Background:
- Thyroid hormones, like thyroxine (T4), are crucial for vertebrate physiology.
- T4 transport in blood is primarily mediated by specific binding proteins.
- Understanding T4 binding in reptiles like turtles (Trachemys scripta, Chelydra serpentina) is important for comparative physiology.
Purpose of the Study:
- To compare the binding affinities (Ka) of thyroxine (T4) in turtle plasma and purified proteins with human plasma.
- To elucidate the roles of T4 binding protein (TBP) and albumin in T4 transport in slider and snapper turtles.
Main Methods:
- Equilibrium dialysis was used to measure binding affinities.
- Purification of T4 binding protein (TBP) and albumin from turtle plasma.
- Affinity chromatography was employed to strip TBP from plasma.
Main Results:
- T. scripta TBP exhibited a fivefold lower T4 binding affinity than human TBG.
- TBP-stripped turtle plasma and purified albumin showed significantly reduced T4 binding affinity.
- Albumin binding affinities were similar across species, but low albumin concentrations in turtles limit its role in T4 transport.
Conclusions:
- T4 binding protein (TBP) is the primary determinant of T4 binding in slider turtles, unlike humans where TBG dominates.
- Low albumin levels in turtles mean TBP accounts for a larger proportion of T4 binding.
- Variations in TBP and T4 levels can significantly impact free T4 concentrations in turtles.