Uesawa Yoshihiro, Staines Adam G, Lockley David, Mohri Kiminori, Burchell Brian
Department of Pharmaceutics, Clinical Pharmaceutics Laboratory, Meiji Pharmaceutical University, 2-522-1 Noshio, Kiyose, Tokyo, 204-8588, Japan.
Arch Toxicol. 2007 Mar;81(3):163-8. doi: 10.1007/s00204-006-0138-5. Epub 2006 Aug 5.
Dulcin (DL), now banned, was once a widely used artificial sweetener. DL possesses an ureido group that is metabolized by direct glucuronidation in rabbit liver microsomes. Dulcin N-glucuronide (DNG) is the only type of ureido N-glucuronide known to date; ureido glucuronidation in humans has not been previously reported. Accordingly, the glucuronidation of DL was studied using human liver microsomes (HLM) and expressed human UDP-glucuronosyltransferase (UGT) enzymes. The average K (m) and V (max) values from nine HLM samples were 2.10 mM and 0.156 nmol/mg/min, respectively. Of the six human UGT isoforms screened for their ability to glucuronidate DL, only UGT1A1 and UGT1A9 showed activity. The apparent K (m) values using UGT1A1 and UGT1A9 were 5.06 and 6.99 mM, and the apparent V (max) values were 0.0461 and 0.106 nmol/min/mg, respectively. Phenolphthalein, a substrate for UGT1A9, inhibited DL glucuronidation in HLM competitively (K (i) = 0.356 mM), but bilirubin, a substrate for UGT1A1, did not. These results suggest that UGT1A9 is a key enzyme catalyzing the glucuronidation of DL.
甜蜜素（DL）现已被禁用，它曾是一种广泛使用的人工甜味剂。DL含有一个脲基，在兔肝微粒体中通过直接葡萄糖醛酸化作用进行代谢。甜蜜素N - 葡萄糖醛酸苷（DNG）是迄今为止已知的唯一一种脲基N - 葡萄糖醛酸苷；此前尚未有关于人类脲基葡萄糖醛酸化的报道。因此，我们使用人肝微粒体（HLM）和表达的人尿苷二磷酸葡萄糖醛酸基转移酶（UGT）来研究DL的葡萄糖醛酸化作用。九个HLM样本的平均米氏常数（K（m））和最大反应速度（V（max））值分别为2.10 mM和0.156 nmol/mg/min。在筛选的六种能够使DL葡萄糖醛酸化的人UGT同工型中，只有UGT1A1和UGT1A9表现出活性。使用UGT1A1和UGT1A9时的表观米氏常数分别为5.06和6.99 mM，表观最大反应速度分别为0.0461和0.106 nmol/min/mg。酚酞是UGT1A9的底物，它在HLM中竞争性抑制DL的葡萄糖醛酸化作用（抑制常数K（i） = 0.356 mM），但胆红素作为UGT1A1的底物则没有这种作用。这些结果表明，UGT1A9是催化DL葡萄糖醛酸化的关键酶。
