Thyroxine Binding by Human Serum Albumin*

Thyroxine and triiodothyronine are transported in the blood as noncovalent complexes with certain serum proteins. This has been shown by many investigators and was the subject of an extensive recent review by Robbins and Rall (1). The binding proteins are an a-globulin (called thyroxine-binding globulin), a prealbumin, and serum albumin. The first appears to be the most significant binder of the thyroxine normally present under physiological conditions. Of these three, only albumin is presently available in a state of reasonable purity, and for this reason we have studied the binding of thyroxine by this protein. The present paper presents our findings with regard to the intensity and the optimal conditions for this interaction. The measurements of the binding are based on the observation by Klebanoff (2) that thyroxine has a stimulatory effect on the oxidation of reduced diphosphopyridine nucleotide by hydrogen peroxide in the presence of peroxidase. This stimulatory effect is obliterated by albumin, and it will be shown that this is a quantitative measure of thyroxine binding by albumin. This is in agreement with the preliminary observation by Klebanoff that plasma proteins can inhibit the thyroxine-potentiated oxidation of diphosphopyridine nucleotide by the peroxidase system, although an ultrafiltrable inhibitor was found to be present as well.