Kinetics of Dissociation of 17β-Hydroxysteroids from the Steroid Binding β-Globulin of Human Plasma

The dissociation of various tritium-labeled 17β-hydroxysteroids from the steroid binding β-globulin1 (SBβG) was studied by ammonium sulfate precipitation of the steroid-SBβG complex at different intervals after the addition of an excess of unlabeled testosterone or dihydrotestosterone.2 At 0 C the dissociation rate constant found for dihydrotestosterone was 0.009 min−1, 0.031 min−1 for 5α-androstane-3α,17β-diol, 0.054 min−1 for testosterone, 0.088 min−1 for androst-5-ene-3β,17β-diol and 0.21 min−1 for 17β-estradiol. At 37 C the dissociation of dihydrotestosterone and of testosterone was about 40 times faster than at 0 C. The between person variation of the dissociation rate constant of testosterone at 0 C [0.054±0.0028 (sd) min−1; N =13] and of dihydrotestosterone at 20 C [0.059±0.0029 (sd) min−1; N =12] was small. No significant differences in dissociation were found between plasma samples of men, plasma samples of normal, hirsute or pregnant women, and umbilical cord blood plasma.