Esculetin (6,7-dihydroxycoumarin) and its C-4 derivatives have multiple pharmacologic activities, but the poor metabolic stability of these catechols has severely restricted their application in the clinic. Glucuronidation plays important roles in catechols elimination, although thus far the effects of structural modifications on the metabolic selectivity and the catalytic efficacy of the human UDP-glucuronosyltransferase (UGT) enzymes remain unclear. This study was aimed at exploring the structure-glucuronidation relationship of esculetin and its C-4 derivatives, including 4-methyl esculetin, 4-phenyl esculetin, and 4-hydroxymethyl esculetin as well as 4-acetic acid esculetin. It was achieved by identifying the main human UGTs responsible for the different reactions and by careful characterization of the reactions kinetics. These catechols, with the exception of 4-acetic acid esculetin, are selectively metabolized to the corresponding 7-O-glucuronides. UGT1A6 and UGT1A9 are the two major UGTs involved in the 7-O-glucuronidation of 4-methyl esculetin and esculetin. UGT1A6 was the major contributor for 7-O-glucuronidation of 4-hydroxymethyl esculetin, and UGT1A9 played a significant role in the 7-O-glucuronidation of 4-phenyl esculetin. The results of the kinetic analyses revealed that the Km values of the compounds, in both UGT1A9 and human liver microsomes, decreased with increasing hydrophobicity of the C-4 substitutions. The outcome of this was that C-4 hydrophobic and hydrophilic groups on 6,7-dihydroxycoumarin exhibited contrasting effects on UGT affinity. All of these findings provide helpful guidance for further structural modification of 6,7-dihydroxycoumarins with improved metabolic stability.
埃斯奎林(6,7-二羟基
香豆素)及其 C-4 衍
生物具有多种药理活性,但这些
儿茶酚代谢稳定性差,严重限制了它们在临床上的应用。
葡萄糖醛酸化在消除
儿茶酚的过程中发挥着重要作用,但迄今为止,结构修饰对人类
UDP-
葡萄糖醛酸基转移酶(UGT)的代谢选择性和催化效能的影响仍不清楚。本研究旨在探索
鱼藤素及其 C-4 衍
生物(包括 4-甲基
鱼藤素、4-苯基
鱼藤素、4-羟甲基
鱼藤素和 4-
乙酸鱼藤素)的结构-
葡萄糖醛酸化关系。这是通过确定负责不同反应的主要人类 UGTs 和仔细分析反应动力学来实现的。除 4-
乙酸鱼藤酮外,这些
儿茶酚都会被选择性地代谢为相应的 7-O-
葡萄糖醛酸。UGT1A6 和 UGT1A9 是参与 4-甲基
鱼藤素和
鱼藤素 7-O-
葡萄糖醛酸化反应的两个主要 UGT。UGT1A6 是 4-羟甲基
鱼藤素 7-O-
葡萄糖醛酸化的主要贡献者,而 UGT1A9 则在 4-苯基
鱼藤素的 7-O-
葡萄糖醛酸化中发挥了重要作用。动力学分析结果表明,在 UGT1A9 和人类肝脏微粒体中,化合物的 Km 值随着 C-4 替换疏
水性的增加而降低。结果表明,6,7-二羟基
香豆素上的 C-4 疏
水基和亲
水基对 UGT 亲和力的影响截然不同。所有这些发现都为进一步改造 6,7-二羟基
香豆素的结构、提高其代谢稳定性提供了有益的指导。