Distribution of 3 alpha-hydroxysteroid dehydrogenase in rat brain and molecular cloning of multiple cDNAs encoding structurally related proteins in humans.

3 alpha-Hydroxysteroid dehydrogenase in the brain is responsible for production of neuroactive tetrahydrosteroids that interact with the major inhibitory gamma-aminobutyric acid receptor complexes. Distribution of 3 alpha-hydroxysteroid dehydrogenase in different regions of the brain in rats was evaluated by activity assay and by Western immunoblotting using a monoclonal antibody against liver 3 alpha-hydroxysteroid dehydrogenase as the probe. The olfactory bulb was found to contain the highest level of 3 alpha-hydroxysteroid dehydrogenase activity, while moderate levels of the enzyme activity were found in other regions such as cerebellum, cerebral cortex, hypothalamus and pituitary. Some activity was found in the rest of the brain such as amygdala, brain stem, caudate putamen, cingulate cortex, hippocampus, midbrain, and thalamus. The protein levels of 3 alpha-hydroxysteroid dehydrogenase in different regions of the brain as detected by Western immunoblotting are comparable to those of the enzyme activity. We used the rat cDNA as the probe to screen a human liver lambda gt11 cDNA library. A total of four different cDNAs were identified and sequenced. One of the cDNAs is identical to that of the human chlordecone reductase cDNA except that our clone contains a much longer 5'-coding sequence than previously reported. The other three cDNAs display high degrees of sequence homology to those of both rat 3 alpha-hydroxysteroid dehydrogenase and human chlordecone reductase. We are currently investigating the functional relationship between the enzymes encoded by these human cDNAs and 3 alpha-hydroxysteroid dehydrogenase.