Hauser S C, Ziurys J C, Gollan J L
Gastroenterology. 1986 Aug;91(2):287-96. doi: 10.1016/0016-5085(86)90559-7.
Hepatic bilirubin uridine diphosphate glucuronyl-transferase (UDP-glucuronyltransferase) catalyzes the formation of bilirubin monoglucuronides (BMG, C-8 and C-12 isomers) and bilirubin diglucuronide (BDG) from bilirubin and the cosubstrate, UDP-glucuronic acid. Distinctive patterns of bile pigment excretion occur in different species and in pathologic disorders (e.g., Gilbert's syndrome). In normal human and monkey (Macaca fascicularis) bile, the proportion of BDG exceeds that of BMG and the C-8/C-12 BMG isomer ratio approaches unity. To investigate the mechanisms responsible for the patterns of BDG and BMG isomers in bile, we used a radiochemical assay to analyze the kinetics and regulation of bilirubin UDP-glucuronyltransferase in microsomes prepared from monkey liver. The synthesis of BMG from bilirubin was a higher capacity, lower affinity step (Vmax = 295 pmol/mg protein X min, Km = 24 microM) than BDG synthesis from endogenously formed BMG (Vmax = 170 pmol/mg protein X min, Km = 14 microM). This observation was confirmed when biosynthetically prepared BMG was used as substrate. The rate of formation of BDG relative to BMG was modulated by both bilirubin and UDP-glucuronic acid concentration, whereas the C-8/C-12 BMG isomer ratio was influenced exclusively by UDP-glucuronic acid concentration. The data obtained with increasing UDP-glucuronic acid concentration did not obey conventional single-site kinetics, suggesting the presence of more than one binding site on the enzyme or a membrane transporter for this nucleotide sugar. These findings demonstrate that microsomal bilirubin UDP-glucuronyltransferase has a greater capacity for BMG than BDG synthesis, and thus support the concept that the decreased BDG/BMG ratio in the bile of patients with reduced hepatic enzyme activity (i.e., Gilbert's syndrome and type II Crigler-Najjar disease) reflects the diminished capacity of the enzyme to synthesize BDG from BMG. Bilirubin and UDP-glucuronic acid concentrations both appear to be pivotal in regulating the synthesis of individual bilirubin glucuronides and hence the patterns of bilirubin conjugates excreted in bile.
肝脏胆红素尿苷二磷酸葡萄糖醛酸基转移酶（UDP - 葡萄糖醛酸基转移酶）催化胆红素与共底物UDP - 葡萄糖醛酸形成胆红素单葡萄糖醛酸酯（BMG，C - 8和C - 12异构体）和胆红素双葡萄糖醛酸酯（BDG）。不同物种以及病理疾病（如吉尔伯特综合征）中胆汁色素排泄模式各异。在正常人类和猴（食蟹猴）胆汁中，BDG的比例超过BMG，且C - 8/C - 12 BMG异构体比例接近1。为研究胆汁中BDG和BMG异构体模式的形成机制，我们采用放射化学分析法分析了猴肝制备的微粒体中胆红素UDP - 葡萄糖醛酸基转移酶的动力学和调控情况。由胆红素合成BMG是一个能力较高、亲和力较低的步骤（Vmax = 295 pmol/mg蛋白质×分钟，Km = 24 μM），而由内源性生成的BMG合成BDG的步骤（Vmax = 170 pmol/mg蛋白质×分钟，Km = 14 μM）能力较低。当使用生物合成制备的BMG作为底物时，这一观察结果得到证实。相对于BMG，BDG的形成速率受胆红素和UDP - 葡萄糖醛酸浓度的调节，而C - 8/C - 12 BMG异构体比例仅受UDP - 葡萄糖醛酸浓度影响。随着UDP - 葡萄糖醛酸浓度增加所获得的数据不符合传统的单位点动力学，这表明该核苷酸糖在酶或膜转运蛋白上存在多个结合位点。这些发现表明，微粒体胆红素UDP - 葡萄糖醛酸基转移酶合成BMG的能力大于合成BDG的能力，因此支持这样的观点，即肝酶活性降低的患者（即吉尔伯特综合征和II型克里格勒 - 纳贾尔病）胆汁中BDG/BMG比例降低反映了该酶从BMG合成BDG的能力下降。胆红素和UDP - 葡萄糖醛酸浓度似乎在调节单个胆红素葡萄糖醛酸酯的合成以及胆汁中排泄的胆红素结合物模式方面都起着关键作用。
