Genomic organization of a human 5β-reductase and its pseudogene and substrate selectivity of the expressed enzyme

The enzyme 5 beta -reductase catalyzes the reduction of the 4-ene of 3-ketosteroids, converting them into 5 beta -dihydro-3-ketosteroids and, thus, could be involved in the metabolism of 4-cholestene-3-one, progesterone, 17 proportional to -hydroxyprogesterone, aldosterone, corticosterone, cortisol, 4-androstenedione, and testosterone. In this study, we report the genomic structure of a human 5 beta -reductase gene, its tissue distribution, the characterization of an intronless pseudogene and the substrate selectivity of the enzyme. The gene coding for the active 5 beta -reductase contains nine exons like most members of the aldo-keto reductase family, but the sequence covered by the gene, more than 42 kb, is much longer than the sequence of other members of this family. There are many large introns, especially introns 3, 4 and 7 that span approx. 7 kb, and intron 1 that contains more than 10 kb. Northern blot analysis showed three band sizes of 1.3, 2.2 and 2.7 kb. The 1.3 and 2.7 kb bands are highly expressed in the liver while weaker 2.2 and 1.3 kb bands have been observed in the testis and colon, respectively. We also identified an intronless gene having 86% homology with the 5 beta -reductase cDNA sequence. Since its sequence contains many stop codons, this gene is most probably a pseudogene. To determine more precisely the substrate selectivity of the enzyme, we established a stable cell line expressing human 5 beta -reductase in transformed embryonic kidney (HEK-293) cells. The transfected cells efficiently catalyze the transformation of progesterone, androstenedione, 17 alpha -hydroxyprogesterone and testosterone. However, they catalyze much less efficiently the transformation of compounds containing an 11 beta -hydroxy group, such as aldosterone, corticosterone and cortisol. In addition to its role in cholesterol catabolism, it is well recognized that 5 beta -reductase inactivates active androgens. Indeed, 5 beta -dihydrotestosterone (5 beta -DHT), the product of the reduction of testosterone by 5 beta -reductase, is not active while its 5 proportional to -isomer (DHT) is the most potent natural androgen. Recent findings show that 5 beta -pregnanes are active ligands in the induction of CYP3A through the orphan receptor hPAR. Our results thus open an opportunity for studying the new role of 5 beta -reductase in the formation of a new type of active steroids. (C) 2001 Elsevier Science B.V. All rights reserved.