Nicotine and cotinine effects on 3 alpha hydroxysteroid dehydrogenase in canine prostate

We have recently observed that cigarette smoking affects plasma androgen concentrations. The effects of nicotine and cotinine, two products of cigarette smoking, on testosterone metabolism were determined. The activity of delta 4 steroid 5α-reductase, which converts testosterone to 5α-dihydro-testosterone (DHT) was measured in isolated dog prostate nuclei using testosterone (0-200nM) as substrate and NADPH as cofactor. Activity of 3α-hydroxysteroid dehydrogenase (HSD), which converts DHT to 3α-androstanediol (3α-diol) and is a reversible enzyme, was measured in isolated dog prostate microsomes with DHT (0-20M) as substrate and NADPH as cofactor. When microsomal fractions were incubated for 1 hour with and without nicotine (0-50M) and cotinine (0-100M), enzyme activity of HSD was significantly suppressed (p < 0.001). The Vmax was not affected significantly (p > 0.60) and Km increased with increasing concentrations of nicotine and cotinine (p < 0.05). Both nicotine and cotinine are competitive inhibitors of HSD in dog prostate microsomes with Ki's of 61 and 89M, respectively. The apparent 5α-reductase activity was unaffected by nicotine and cotinine. The inhibitors produced a marked effect on activity of HSD when used in concentrations achieved in humans who smoke cigarettes. The results suggest that nicotine and cotinine are competitive inhibitors of the HSD, an important enzyme involved in the metabolism of DHT and produce an accumulation of DHT. These products of cigarette smoking could alter androgen action in tissue such as skin and prostate.