SUBSTRATE-SPECIFICITY, GENE STRUCTURE, AND TISSUE-SPECIFIC DISTRIBUTION OF MULTIPLE HUMAN 3-ALPHA-HYDROXYSTEROID DEHYDROGENASES

We have expressed in Escherichia coli functionally active proteins enc oded by two human cDNAs that were isolated previously by using rat 3 a lpha(-)hydroxysteroid dehydrogenase cDNA as the probe, The expressed p roteins catalyzed the interconversion between 5 alpha-dihydrotestoster one and 5 alpha-androstane-3 alpha, 17 beta-diol. Therefore, we name t hese two enzymes type I and type II 3 alpha-hydroxysteroid dehydrogena ses. The type I enzyme has a high affinity for dihydrotestosterone, wh ereas the type II enzyme has a low affinity for the substrate, The tis sue-specific distribution of these two enzymes was determined by rever se transcription polymerase chain reaction using gene-specific oligonu cleotide primers, The mRNA transcript of the type I enzyme was found o nly in the Liver, whereas that of the type II enzyme appeared in the b rain, kidney, liver, lung, placenta, and testis, The structure and seq uence of the genes encoding these two 3 alpha-hydroxysteroid dehydroge nases were determined by analysis of genomic clones that were isolated from a lambda EMBL3 SP6/T7 library, The genes coding for the type I a nd type II enzymes were found to span approximately 20 and 16 kilobase pairs, respectively, and to consist of 9 exons of the same sizes and boundaries, The exons range in size from 77 to 223 base pairs (bp), wh ereas the introns range in size from 375 bp to approximately 6 kilobas e pairs. The type I gene contains a TATA box that is located 27 bp ups tream of multiple transcription start sites, In contrast, the type II gene contains two tandem AP2 sequences juxtaposed to a single transcri ption start site.