The Origins of Unconjugated Bilirubin in Bile

In order to investigate the origin of unconjugated bilirubin in bile, beta-glucuronidase activity in rat and human bile was determined at various pH. beta-Glucuronidase in rat and human bile had their optimum pH at 5.5 when phenolphthalein glucuronide and delta 1-azopigment were used as substrate, and at 6.0 when bile itself was incubated. In human and rat bile the hydrolysis was suppressed to a minimum at each physiologic pH. However, human bile shows remarkable hydrolysis in alkaline pH (7.5--8.0). On the other hand, when delta 1-azopigment was incubated in various buffers, several per cent of delta 1-azopigment were hydrolyzed non-enzymatically in neutral to alkaline pH. Thus, it was suggested that enzymatic and nonenzymatic hydrolysis contributes to the existence of unconjugated bilirubin in bile.