Evidence that human prostatic 5 alpha-reductase is located exclusively in the nucleus

The 5 alpha-reductase activities in human prostatic nuclei and microsomes were compared. The activities in both subcellular fractions were identical with respect to pH dependence, heat inactivation and Michaelis constants for NADPH and testosterone. Subcellular distribution studies using DNA and nicotinamide mononucleotide adenyl transferase as nuclear markers showed that the amount of 5 alpha-reductase present in the microsomes was directly proportional to the amount of nuclear contamination. These results indicate that human prostatic tissue contains only one form of 5 alpha-reductase, which is located exclusively in the nucleus. This finding has important implications for the mechanism of steroid action in the prostate.