Physicochemical properties of the cytoplasmic triiodothyronine binding protein from tadpole liver and tail fin.

The sedimentation velocity, gel filtration properties and pattern of elution from ion exchange gels of the cytoplasmic triiodothyronine (T3) binding protein from Rana catesbeiana liver and tail fin cytosol were determined. The T3 binding protein in liver cytosol had a sedimentation coefficient of 4.4S on sucrose gradients and a Stokes radius from Sephadex gel filtration of 38.2 +/- 4.2 A. From these two values a molecular weight of 71,700 +/- 4,100 and a frictional ratio of 1.28 +/- 0.05 for the T3 binding protein from tadpole liver has been calculated. The liver T3 binding protein was eluted from DEAE--Sephadex gels at a salt concentration of 0.233 +/- 0.017 M NaCl. The sedimentation coefficient, Stokes radius and salt requirement for elution from DEAE--Sephadex of the T3 binding protein from tail fin cytosol were essentially identical (4.4S, 35.2 +/- 4.2 A, nd 0.213 +/- 0.015 M NaCl respectively. The calculated molecular weight is 66,100 +/- 7,900 and the frictional ratio is 1.21 +/- 0.10 for for the tail fin T3 binding protein. The great similarity in the physicochemical properties of the T3 binding protein from the liver and tail fin implies that the T3 binding protein in each tissue is similar if not identical. The possible reason for the differences in the dissociation constants previously reported for the binding of T3 in the two tissues is discussed.