Chowdhury J R, Chowdhury N R, Wu G, Shouval R, Arias I M
Hepatology. 1981 Nov-Dec;1(6):622-7. doi: 10.1002/hep.1840010610.
Bilirubin diglucuronide, the major pigment in human bile is formed in two steps. Bilirubin is converted to bilirubin monoglucuronide by transfer of the glucuronosyl moiety of uridine diphosphoglucuronic acid catalyzed by the microsomal enzyme, uridine diphosphoglucuronate glucuraonosyl transferase (UDP glucuronyl transferase, EC 2.4.1.17). Bilirubin monoglucuaronide is converted to bilirubin diglucuronide in vitro by two enzymatic mechanisms: (a) UDP glucuronyl transferase-mediated transfer of a second mole of glucuronic acid form UDP-glucuronic acid to bilirubin monoglucuronide; (b) dismutation of 2 moles of bilirubin monoglucuronide to 1 mole of bilirubin diglucuronide and 1 mole of unconjugated bilirubin, catalyzed by bilirubin monoglucuronide dismutase (bilirubin glucuronoside glucuronosyl transferase EC 2.4.1.95). Assay methods for the three enzymatic mechanisms in human liver homogenate by high-pressure liquid chromatography analysis of underivatized bilirubin tetrapyrroles have been developed. UDP glucuronyl transferase was activated in five human liver homogenates with digitonin, Triton X-100, or UDP-N-acetylglucosamine. Greatest activation was observed with Triton X-100. The pH optimum for conversion of bilirubin to bilirubin monoglucuronide was 7.4, and UDP glucuronyl transferase activity was 625 +/- 51 nmoles per 20 min per gm liver. At high initial bilirubin concentrations (342 microM), the product of UDP glucuronyl transferase assay with bilirubin as substrate was predominantly bilirubin monoglucuronide. At lower initial bilirubin concentrations (6.5 to 34 microM), up to 15% bilirubin diglucuronide was formed. Glucuronyl transferase-mediated UDP glucuronic acid-dependent conversion of bilirubin monoglucuronide to diglucuronide was assayed using UDP-14-C-glucuronic acid. The pH optimum was 7.4, and the rate was 21 +/- 7 nmoles per gm liver per 20 min. The rate of bilirubin diglucuronide formation by enzymatic dismutation of bilirubin monoglucuronide was 470 +/- 112 nmoles per gm liver per min. The pH optimum was 6.6. The products of enzymatic dismutation were of the IX alpha configuration.
胆红素双葡萄糖醛酸酯是人类胆汁中的主要色素，其形成分两步。胆红素通过微粒体酶尿苷二磷酸葡萄糖醛酸葡萄糖醛酸基转移酶（UDP葡萄糖醛酸基转移酶，EC 2.4.1.17）催化，将尿苷二磷酸葡萄糖醛酸的葡萄糖醛酸基部分转移，转化为胆红素单葡萄糖醛酸酯。胆红素单葡萄糖醛酸酯在体外通过两种酶促机制转化为胆红素双葡萄糖醛酸酯：（a）UDP葡萄糖醛酸基转移酶介导的第二摩尔葡萄糖醛酸从UDP - 葡萄糖醛酸转移至胆红素单葡萄糖醛酸酯；（b）由胆红素单葡萄糖醛酸酯歧化酶（胆红素葡萄糖苷葡萄糖醛酸基转移酶EC 2.4.1.95）催化，2摩尔胆红素单葡萄糖醛酸酯歧化为1摩尔胆红素双葡萄糖醛酸酯和1摩尔未结合胆红素。已开发出通过对未衍生化胆红素四吡咯进行高压液相色谱分析来检测人肝匀浆中这三种酶促机制的方法。用洋地黄皂苷、Triton X - 100或UDP - N - 乙酰葡糖胺激活了五个人肝匀浆中的UDP葡萄糖醛酸基转移酶。用Triton X - 100观察到最大激活效果。胆红素转化为胆红素单葡萄糖醛酸酯的最适pH为7.4，UDP葡萄糖醛酸基转移酶活性为每克肝脏每20分钟625±51纳摩尔。在高初始胆红素浓度（342微摩尔）下，以胆红素为底物进行UDP葡萄糖醛酸基转移酶测定的产物主要是胆红素单葡萄糖醛酸酯。在较低初始胆红素浓度（6.5至34微摩尔）下，形成了高达15%的胆红素双葡萄糖醛酸酯。使用UDP - 14 - C - 葡萄糖醛酸测定了葡萄糖醛酸基转移酶介导的依赖UDP葡萄糖醛酸的胆红素单葡萄糖醛酸酯向双葡萄糖醛酸酯的转化。最适pH为7.4，速率为每克肝脏每20分钟21±7纳摩尔。胆红素单葡萄糖醛酸酯通过酶促歧化形成胆红素双葡萄糖醛酸酯的速率为每克肝脏每分钟470±112纳摩尔。最适pH为6.6。酶促歧化的产物为IXα构型。
