Rates of dissociation of steroid and thyroid hormones from human serum albumin

A rapid filtration assay employing dextran-coated charcoal as acceptor particles for free hormone was used to measure rates of dissociation of steroid and thyroid hormones from human serum albumin. Modification of a previously described assay allowed measurements at 1-s intervals. Nevertheless, this still permitted only minimum estimates of the dissociation rate constants. The hormones studied were thyroxine, 3,5,3′-triidothyronine, cortisol, corticosterone, testosterone, dihydrotestosterone, estradiol, progesterone, and aldosterone. The apparent dissociation rate constant of the thyroxine-albumin complex at 37°C was 1.3±0.2 s −1 (t 1 2 , 0.5 s) . The apparent dissociation rate constants of the other hormone-albumin complexes at 37°C generally exceeded 2 s −1 ( t 1 2