Effect of sodium molybdate on the thyroxine-binding affinity of transport cytosol proteins.

The presence of sodium molybdate during tissue homogenization is known to increase the number of cytosol binding sites for glucocorticoids, progesterone, androgens and oestrogens. We wondered whether a phenomenon similar to this stabilization of steroid receptors would also occur in thyroxine-binding cytosol protein. We found that the presence of sodium molybdate (10 mmol/l) in rat adenohypophyseal cytosol increased its thyroxine-binding capacity by up to 96%. In the case of binding protein cytosol minus molybdate, Ka = 5.5 X 10(9) l.mol-1, whereas for cytosol plus molybdate Ka(1) = 6.0 X 10(9) l.mol-1 and Ka(2) = 3.0 X 10(10) l.mol-1. Cytosol prepared without molybdate did not contain a binding protein class with a higher Ka. The effect is stereo-specific and the LT4 bond is not displaced by DT4.