Inazu A, Sato K, Nakayama T, Deyashiki Y, Hara A, Nozawa Y
Biochemistry Laboratory, Gifu Pharmaceutical University, Japan.
Biochem J. 1994 Jan 1;297 ( Pt 1)(Pt 1):195-200. doi: 10.1042/bj2970195.
Tetrahymena pyriformis was found to exhibit high NADPH-dependent 20-oxosteroid reductase activity that converted 17 alpha-hydroxyprogesterone into 17 alpha,20 alpha-dihydroxypregn-4-en-3-one. The enzyme was purified 400-fold from the cytosolic fraction. The purified enzyme with a specific activity of 6.4 mumol/min per mg of protein had an isoelectric point of 4.9 and M(r) of 68,000, and was composed of two subunits of equal size. The N-terminal sequence was determined to be LAKTVPLNDGTNFPIFGG. The enzyme reduced pregnanes and pregnanes possessing a 17 alpha-hydroxy group to a greater extent than those without the hydroxy group, and oxidized 20 alpha-hydroxy groups of the steroids in the presence of NADP+. The Km values for 17 alpha-hydroxyprogesterone and 17 alpha-hydroxypregnenolone were 2.9 and 3.4 microM respectively. Although the enzyme was inactive towards androgens and oestrogens with 3- or 17-oxo groups, it reduced several nonsteroidal carbonyl compounds and oxidized trans-benzene dihydrodiol. The enzyme activity was inhibited by synthetic oestrogens, barbiturates, aldose reductase inhibitors and quercitrin. Thus, this enzyme is a novel form of 20 alpha-hydroxysteroid dehydrogenase (EC 1.1.1.149) which structurally and functionally differs from the mammalian and bacterial enzymes.
人们发现梨形四膜虫表现出较高的依赖烟酰胺腺嘌呤二核苷酸磷酸（NADPH）的20-氧代类固醇还原酶活性，该酶可将17α-羟基孕酮转化为17α,20α-二羟基孕-4-烯-3-酮。该酶从胞质部分纯化了400倍。纯化后的酶比活性为每毫克蛋白质6.4微摩尔/分钟，其等电点为4.9，相对分子质量为68,000，由两个大小相等的亚基组成。其N端序列确定为LAKTVPLNDGTNFPIFGG。该酶对具有17α-羟基的孕烷和孕甾烷的还原程度大于对没有羟基的孕烷和孕甾烷的还原程度，并且在NADP+存在的情况下可氧化类固醇的20α-羟基。17α-羟基孕酮和17α-羟基孕烯醇酮的米氏常数（Km）分别为2.9和3.4微摩尔/升。尽管该酶对具有3-氧代或17-氧代基团的雄激素和雌激素无活性，但它可还原几种非甾体羰基化合物并氧化反式苯二氢二醇。该酶活性受到合成雌激素、巴比妥类药物、醛糖还原酶抑制剂和槲皮苷的抑制。因此，这种酶是20α-羟基类固醇脱氢酶（EC 1.1.1.149）的一种新形式，在结构和功能上与哺乳动物和细菌的酶不同。
