Plasma Protein Thyroid Hormone Complexes

Summary

Nine serum protein fractionation experiments using the continuous flow paper electrophoresis technique were performed. Six experiments were performed using the serum collected from individual patients 24 and 48 hours after receiving therapeutic doses of I-131 and 3 using the serum of euthyroid individuals which was incubated in vitro for 1/2 hour at 37°C with I–131 labelled thyroxine prior to fractionation. Protein fractions obtained were identified by conventional paper electrophoresis and analyzed for radioactivity, total protein and cholesterol, the latter being taken as an index of lipid contents. Similar results were obtained for both in vivo and in vitro experiments. Beta globulins were found to be capable of forming thyroid hormone protein complexes to a larger extent than gamma globulins, but the largest amount of radioactive hormone was found as an alpha-2 globulin-thyroid hormone complex. A patient showing a marked abnormality in his plasma proteins showed the highest percent of radioactive hormone as protein thyroid hormone complex with the mobility of albumin. The highest percent of the radioactivity was found in fractions having a low protein concentration, indicating that the existence of a protein thyroid hormone complex does not depend on the abundance of a particular protein but on the number of specific binding sites that a particular protein may have and on thermodynamic factors influencing the association constant of the complex so formed. It was also found that the fractions having the highest cholesterol level and hence higher lipid contents contained little radioactivity, indicating that the bulk of thyroid hormone binding is accomplished by a lipid poor protein.