Description:
<jats:title>Abstract</jats:title><jats:p>It has been shown that progesterone, estrone, and estradiol are present in significant amounts in the human benign prostatic hyperplasia (BPH). Therefore it was of interest to determine the inhibitory effect of these steroids on the 5α‐reductase and 3α(β)‐hydroxysteroiddehydrogenase (HSDH) activities, the enzymes responsible for the conversion of testosterone to 5α‐dihydrotestosterone (DHT), and DHT to 3α(β)‐androstanediols, respectively. The enzyme inhibition was analyzed in vitro by measuring the 5α‐reductase in the presence of either progesterone, estrone, or estradiol, using testosterone as substrate. The DHT was used as substrate for HSDH. The metabolites were quantified by t.l.c. The main results were as follows: (1) Concerning 5α‐reductase in BPH, the mean inhibitor constants (K<jats:sub>1</jats:sub>; μM) of progesterone, estrone, and estradiol were 0.11, 15.5, and 5.1, respectively. (2) Analyzing epithelium and stroma of BPH separately, the inhibition of 5α‐reductase resulted in nearly identical K<jats:sub>1</jats:sub>′S. (3) Concerning HSDH in BPH, the mean K<jats:sub>1</jats:sub>s of progesterone, estrone, and estradiol were 169, 63, and 192, respectively. (4) Analyzing epithelium and stroma of BPH separately, the inhibition of HSDH led to nearly identical K<jats:sub>1</jats:sub>′s. (5) The kinetic parameters (K<jats:sub>M</jats:sub>, V<jats:sub>max</jats:sub>) of the 5α‐reduction of progesterone and testosterone were nearly identical. These results led to the suggestion that the endogenous concentrations of progesterone, estrone, and estradiol have no significant inhibitory effect on the 5α‐reductase and HSDH in vivo. Furthermore, the nearly identical inhibitor constants found for both enzymes in epithelium and stroma of BPH indicate that in both compartments the 5α‐reductase and HSDH are qualitatively identical.</jats:p>