Rates of dissociation of sex steroid hormones from human sex hormone-binding globulin: A reassessment

Abstract

A rapid filtration assay employing dextran-coated charcoal as acceptor particles for free hormone was used to measure the rates of dissociation of dihydrotestosterone (DHT), testosterone (T), and estradiol (E 2) from their binding proteins in human serum at 37°C. Because measurements were begun after each hormone had fully (>99%) dissociated from albumin, the observed rates of dissociation correspond to the rates of dissociation of the sex hormone-binding globulin (SHBG)-hormone complexes. The dissociation rate constants of the hormone-SHBG complexes were determined to be 0.016±0.001, 0.056±0.002, and 0.083±0.003 s −1 for DHT, T, and E 2, respectively, corresponding to half-times of dissociation ( t 1 2 ) of 43, 12 and 8.4 s, respectively. The physiological significance of these findings can best be appreciated by comparing these t 1 2 s with the capillary and sinusoidal transit times of various tissues (<1 s to ∼ 10 s).