IN-VITRO INFLUENCE OF PLASMA STEROID-BINDING PROTEINS ON ANDROGEN METABOLISM IN HUMAN-LEUKOCYTES

The purpose of this study was to investigate the influence of plasma s teroid-binding proteins on androgen metabolism in intact leukocytes pr epared from normal male and female blood samples. Leukocyte preparatio ns were incubated for 24 h at 37 degrees C with either labeled or unla beled testosterone (T), 5 alpha-dihydrotestosterone (5 alpha-DHT), and androstenedione (A). After extraction, the formed labeled metabolites were first identified by high performance liquid chromatography, then , using unlabeled substrates, metabolite concentrations were measured by specific radioimmunoassays. The conversion ratios of substrate to m etabolite were calculated for each preparation using either labeled or unlabeled substrates. In the absence of steroid-binding proteins, the mean conversion ratios of T to A, A to T, T to 5 alpha-DHT, and 5 alp ha-DHT to 3 alpha-androstanediol (3 alpha-D) were, in males and female s, respectively, 5.6% and 6.1% (n = 11), 5.6% and 5.6% (n = 5), 2.8% a nd 2.2% (n = 11), 43.1% and 40.0% (n = 5), these sex differences being non-significant. The presence of increasing amounts of plasma, purifi ed albumin or sex hormone binding-globulin (SHBG) in the incubation me dia reduced metabolite formation dose-dependently. However, a 1000-fol d greater concentration of albumin than of SHBG was necessary for 50% inhibition of androgen metabolism by leukocytes, showing SHBG to have the main protective effect. Moreover, in the presence of various conce ntrations of T or 5 alpha-DHT, and of albumin or SHBG, metabolite form ation was positively and highly significantly correlated with the conc entration of protein-unbound (free) substrate measured by equilibrium dialysis (r = 0.964 for conversion of T to A and r = 0.998 for convers ion of 5 alpha-DHT to 3 alpha-D) but weakly correlated with total subs trate concentration (r = 0.375 for total T and r = 0.669 for total 5 a lpha-DHT). Additionally, leukocyte metabolism of 5 alpha-DHT was signi ficantly enhanced when free 5 alpha-DHT levels increased in the presen ce of 17 beta-estradiol (which displaced 5 alpha-DHT from the SHBG-bin ding sites). We conclude that the metabolism of androgens by human leu kocytes is mainly regulated by SHBG level, in both men and women. Alth ough the rate of androgen metabolism by leukocytes was not determined in this study, reduced serum SHBG levels with unchanged albumin levels or drugs capable of displacing androgens from serum SHBG can be expec ted to increase androgen metabolism in human leukocytes.