Relation of plasma thyroxine binding to thyroidal activity and determination of thyroxine binding proteins in a turtle, Pseudemys scripta.

The ability of plasma to bind thyroxine (T4) was examined in the turtle, Pseudemys scripta, in relation to variations in thyroidal state associated with age, sex, environment, and surgical and chemical manipulations. Relative plasma binding activity was assessed by use of binding to [125I]T4 on minicolumns of Sephadex G-25 (fine). Hypothyroidism induced by surgical thyroidectomy (Tx) or goitrogen (Methimazole) treatment resulted in a marked depression of plasma binding (50- to 100-fold) in juveniles, and T4 treatment restored binding after 4-6 weeks in long-term Tx animals and increased levels in intact animals. Among intact turtles or those made slightly hypothyroid by partial thyroidectomy, binding was consistently correlated with plasma T4. For example, juvenile turtles kept under continuous light and constant temperature (28 degrees) for 4.5 months showed a pronounced depression of plasma T4 (2.6 +/- 1.1 ng/ml) and binding capacity compared to animals raised under variable conditions (T4 = 69.6 +/- 22 ng/ml) for the last 2 months. Plasma T3 was less than 1 ng/ml in all cases. Binding levels in adult turtles were similar to juveniles, but females had significantly higher binding levels than males which paralleled differences in their plasma T4 (137 +/- 17.4 vs 83.9 +/- 13.8 ng/ml). These variations in binding were independent of total plasma protein and albumin. Plasma T4 binding measured on Sephadex G-25 was reversible and reduced by addition of exogenous T4. The affinity for T3 was 10- to 100-fold less than for T4. When plasma preincubated with [125I]T4 was electrophoresed on polyacrylamide slab gels (7% nonreducing) only a small percentage of radiolabel was associated with albumin and the majority with a slower migrating protein(s). Addition of unlabeled T4 displaced binding from the slower migrating region to the albumin and dye front (unbound). In contrast, plasma from Tx turtles showed only minimal binding and radiolabel was associated primarily with the albumin fraction. Elution of proteins from gels confirmed that only the slower migrating components bound T4 when tested on Sephadex G-25, and Tx animals lacked this binding component. The protein(s) responsible for most of the T4 binding appears to exist in low concentration. Limited comparative studies with human blood showed a similar binding activity on Sephadex G-25, but electrophoretic mobilities of binding proteins were distinct from those in the turtle. Evidence suggests that this binding protein is not prealbumin.(ABSTRACT TRUNCATED AT 400 WORDS)