Bacterial GUSB hydrolyses BDG to BIL

Bilirubin diglucuronide (BDG) is a substrate for microbial β-glucuronidase, which can cleave the glucuronosyl moieties and liberate bilirubin for reabsorption through the basolateral surfaces of the intestines where it can undergo further metabolism or pass directly back into the circulation. This process, known as enterohepatic circulation, can extend the half-life of bilirubin while adding to the total serum bilirubin load (Seyfried et al. 1976). Conjugated bilirubin is excreted in bile through the duodenum, where it can be unconjugated by enteric bacteria (Kim et al. 1995). Many bacterial β-glucuronidases can cleave the glucuronosyl moieties from conjugated bilirubins in the human gut. In vitro assays reveal the C. perfringens species produce beta-glucuronidase enzyme activity that is at least 30-fold higher than other bacterial species (Leung et al. 2001).<br><br>Urobilinogen (D-urobilinogen) is closely related to two other compounds: mesobilirubinogen (I-urobilinogen) and stercobilinogen (L-urobilinogen). Somewhat confusingly, all three compounds are frequently collectively referred to as "urobilinogens".

胆红素二葡萄糖苷（BDG）是微生物β-葡萄糖苷酶的底物，能够裂解葡萄糖苷基团，释放胆红素，使其通过肠道基侧表面重新吸收，在此过程中，胆红素可进一步代谢或直接返回血液循环。此过程称为肠肝循环，能够延长胆红素的半衰期，同时增加血清总胆红素负荷（Seyfried等，1976年）。结合型胆红素通过十二指肠排入胆汁，在此处可被肠道细菌去结合（Kim等，1995年）。许多细菌β-葡萄糖苷酶能够从人体肠道中的结合型胆红素中裂解葡萄糖苷基团。体外实验显示，产气荚膜梭菌（C. perfringens）产生的β-葡萄糖苷酶酶活性至少比其他细菌种属高出30倍（Leung等，2001年）。

尿胆原（D-尿胆原）与两种其他化合物密切相关：中胆红素原（I-尿胆原）和粪胆原（L-尿胆原）。有些令人困惑的是，这三种化合物常常被统称为“尿胆原”。