Cooke G M, Pothier F, Murphy B D
Department of Physiology, University of Ottawa, Ontario, Canada.
J Steroid Biochem Mol Biol. 1997 Mar;60(5-6):353-9. doi: 10.1016/s0960-0760(96)00223-3.
The enzyme 3-oxo-steroid: NADP+ 4-oxidoreductase (EC 1.3.1.22; 5alpha-reductase) was assayed in testicular microsomes of pigs of 3, 20 and 24 weeks of age. The activity was very low in 3-week-old animals and approximately 10-fold higher in 5- and 6-month-old pigs. The pH optimum was 6.3 in 6-month-old animals, 5.7 in 5-month-old animals, but could not be reliably determined in 3-week-old animals. The kinetic parameters for 5alpha-reductase in testis microsomes from 6-month-old animals were; K((m)(app)), 8.0 micromol/l, V((max)(app)), 6.7 nmoles/90 min/mg protein. Progesterone was a competitive inhibitor of testosterone 5alpha-reduction with an apparent K((i)(app)) of 0.86 micromol/l. However, 4,16-androstadien-3-one (dienone), which undergoes 5alpha-reduction in the biosynthesis of the pheromonally active 16-androstenes, was a comparatively poor inhibitor with a K((i)(app)) of 4.9 micromol/l. Similarly, MK434, which is a selective inhibitor of the human type 2 5alpha-reductase, but which inhibits both types 1 and 2 in the rat, was also a poor competitive inhibitor of testosterone 5alpha-reductase in the pig testis (K((i)(app)), 3.1 micromol/l). It would appear from these studies that the pig testis microsomal 5alpha-reductase corresponds to a type 1 isozyme that is not capable of reducing dienone other than under conditions where the dienone concentration would be in considerable excess of testosterone. It is, therefore, probable that substrate-specific 5alpha-reductases exist in the pig testis for the 5alpha-reduction of testosterone and dienone.
对3周龄、20周龄和24周龄猪的睾丸微粒体中的3-氧代类固醇：NADP⁺ 4-氧化还原酶（EC 1.3.1.22；5α-还原酶）进行了测定。3周龄动物的该酶活性非常低，而5月龄和6月龄猪的活性约高10倍。6月龄动物的最适pH为6.3，5月龄动物为5.7，但3周龄动物的最适pH无法可靠测定。6月龄动物睾丸微粒体中5α-还原酶的动力学参数为：表观米氏常数（K(m)(app)），8.0微摩尔/升；最大表观反应速度（V(max)(app)），6.7纳摩尔/90分钟/毫克蛋白。孕酮是睾酮5α-还原反应的竞争性抑制剂，表观抑制常数（K(i)(app)）为0.86微摩尔/升。然而，在具有信息素活性的16-雄烯类生物合成过程中进行5α-还原的4,16-雄二烯-3-酮（二烯酮）是一种相对较弱的抑制剂，K(i)(app)为4.9微摩尔/升。同样，MK434是人类2型5α-还原酶的选择性抑制剂，但在大鼠中可抑制1型和2型，它也是猪睾丸中睾酮5α-还原反应的较弱竞争性抑制剂（K(i)(app)，3.1微摩尔/升）。从这些研究看来，猪睾丸微粒体5α-还原酶对应于1型同工酶，除了二烯酮浓度大大超过睾酮的情况外，它不能还原二烯酮。因此，猪睾丸中可能存在底物特异性的5α-还原酶，用于睾酮和二烯酮的5α-还原。
