Blanckaert N, Gollan J, Schmid R
J Clin Invest. 1980 Jun;65(6):1332-42. doi: 10.1172/JCI109797.
Although it is well established that bilirubin monoglucuronide is formed in the liver from bilirubin by a microsomal bilirubin uridine diphosphate (UDP)-glucuronosyltransferase, the subcellular site of conversion of monoglucuronide to diglucuronide and the molecular mechanism involved in diglucuronide synthesis have not been identified. Based on in vitro studies, it has been proposed that two fundamentally different enzyme systems may be involved in diglucuronide synthesis in rat liver: (a) a microsomal UDP-glucuronosyltransferase system requiring UDP-glucuronic acid as sugar donor or (b) a transglucuronidation mechanism that involves transfer of a glucuronosyl residue from one monoglucuronide molecule to another, catalyzed by a liver plasma membrane enzyme. To clarify the mechanism by which bilirubin monoglucuronide is converted in vivo to diglucuronide, three different experimental approaches were used. First, normal rats were injected with either equal amounts of bilirubin-IIIalpha [(14)C]monoglucuronide and unlabeled bilirubin-XIIIalpha monoglucuronide, or bilirubin-XIIIalpha [(14)C]monoglucuronide and unlabeled bilirubin-IIIalpha monoglucuronide. Analysis of radiolabeled diglucuronide excreted in bile showed that [(14)C]glucuronosyl residues were not transferred between monoglucuronide molecules. Second, in normal rats infused intravenously with dual-labeled [(3)H]bilirubin [(14)C]monoglucuronide, no transfer or exchange of the [(14)C]glucuronosyl group between injected and endogenously produced bilirubin monoglucuronide could be detected in the excreted bilirubin diglucuronide. Third, in homozygous Gunn rats, injected (14)C-labeled or unlabeled bilirubin mono- or diglucuronides were excreted in bile unchanged (except that diglucuronide was hydrolyzed to a minor degree). This indicates that Gunn rats, which lack bilirubin UDP-glucuronosyltransferase activity, are unable to convert injected monoglucuronide to diglucuronide. Collectively, these findings establish that a transglucuronidation mechanism is not operational in vivo and support the concept that bilirubin diglucuronide is formed by a microsomal UDP-glucuronosyltransferase system.
尽管人们已经充分认识到，胆红素单葡萄糖醛酸酯是在肝脏中由胆红素通过微粒体胆红素尿苷二磷酸（UDP）-葡萄糖醛酸基转移酶形成的，但单葡萄糖醛酸酯转化为双葡萄糖醛酸酯的亚细胞位点以及双葡萄糖醛酸酯合成所涉及的分子机制尚未明确。基于体外研究，有人提出大鼠肝脏中双葡萄糖醛酸酯合成可能涉及两种根本不同的酶系统：（a）一种需要UDP-葡萄糖醛酸作为糖供体的微粒体UDP-葡萄糖醛酸基转移酶系统，或（b）一种转葡萄糖醛酸化机制，该机制涉及由肝细胞膜酶催化将一个葡萄糖醛酸基残基从一个单葡萄糖醛酸酯分子转移到另一个单葡萄糖醛酸酯分子。为了阐明胆红素单葡萄糖醛酸酯在体内转化为双葡萄糖醛酸酯的机制，采用了三种不同的实验方法。首先，给正常大鼠注射等量的胆红素-IIIα[（14）C]单葡萄糖醛酸酯和未标记的胆红素-XIIIα单葡萄糖醛酸酯，或胆红素-XIIIα[（14）C]单葡萄糖醛酸酯和未标记的胆红素-IIIα单葡萄糖醛酸酯。对胆汁中排泄的放射性标记双葡萄糖醛酸酯的分析表明，[（14）C]葡萄糖醛酸基残基未在单葡萄糖醛酸酯分子之间转移。其次，在静脉内注入双标记的[（3）H]胆红素[（14）C]单葡萄糖醛酸酯的正常大鼠中，在排泄的胆红素双葡萄糖醛酸酯中未检测到注入的和内源性产生的胆红素单葡萄糖醛酸酯之间的[（14）C]葡萄糖醛酸基团的转移或交换。第三，在纯合子Gunn大鼠中，注入的（14）C标记或未标记的胆红素单葡萄糖醛酸酯或双葡萄糖醛酸酯在胆汁中未发生变化地排泄出来（双葡萄糖醛酸酯有轻微程度的水解除外）。这表明缺乏胆红素UDP-葡萄糖醛酸基转移酶活性的Gunn大鼠无法将注入的单葡萄糖醛酸酯转化为双葡萄糖醛酸酯。总的来说，这些发现表明转葡萄糖醛酸化机制在体内不起作用，并支持胆红素双葡萄糖醛酸酯是由微粒体UDP-葡萄糖醛酸基转移酶系统形成的这一概念。
