THE INTERACTION OF THYROXINE WITH PLASMA PROTEINS: LOCALIZATION OF THYROXINE-BINDING PROTEIN IN COHN FRACTIONS OF PLASMA 12

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Abstract
Interactions of thyroxine with plasma proteins were studied by means of zone electro-phoresis, using I131-labeled thyroxine. In serum, endogenous and exogenous thyroxine were in ready exchange. Thyroxine was distributed among plasma proteins according to a reversible binding equilibrium between the specific thyroxine-binding protein (TBP) and secondary carriers of lesser affinity. Displacement onto secondary carriers (principally albumin) occurs with increasing thyroxine concentrations. A technique was devised for assaying the relative content of TBP in protein mixtures, and to a study of fractions of plasma obtained by Method 6 of Cohn. Fractions IV-6 and IV-9 constitute the most abundant sources of demonstrable TBP.