The thyroxine-binding properties of rat and rabbit serum proteins

The thyroxine (T4) binding properties of rat and rabbit serum proteins were studied using a gel equilibration technique, gel filtration and polyacrylamide gel electrophoresis. In both species two different T4 binding molecules were identified in whole serum and in Cohn fraction V preparations. Only one of these binding species demonstrated the characteristics of specific binding, i.e., high affinity for the hormone and binding site saturability. The mean (+/- SD) apparent association constants (ka) at 37 C for the specific binding proteins in whole rat and rabbit serum were 3.5 +/- 0.5 x 10(8)M-1 and 2.8 +/- 0.5 x 10(8)M-1, respectively. The mean T4 binding capacities of these proteins were 4.3 +/- 1.2 x 10(-6)M and 7.4 +/- 1.0 x 10(-6)M for rat and rabbit serum, respectively. Non-specific binding was due to serum albumin. Rat albumin (ka = 6.1 +/- 1.6 x 10(5)M-1, at 37 C) bound T4 significantly more strongly than did rabbit albumin (ka = 2.3 +/- 0.4 x 10(5)M-1, at 37 C) when it was assumed that there was only one T4 binding site/molecule of albumin. The ability of rat albumin to bind more T4 could also be explained by a greater number of T4 binding sites/molecule. Partial separation of the two T4 binding species was attained by gel filtration on Sephadex G200 columns. The specific binding protein of both rat and rabbit serum was eluted slightly later than the corresponding serum albumin. Polyacrylamide gel electrophoresis resulted in the separation of two distinct T4 binding peaks in the rat, but with rabbit serum clear separation of the two T4 binding molecules was not attained. In both species the specific binding protein migrated anodal to serum albumin.