title: Non-enzymic hydrolysis of bilirubin mono- and diglucuronide to unconjugated bilirubin in model and native bile systems. Potential role in the formation of gallstones.
abstract: Pigment gallstones contain considerable amounts of unconjugated bilirubin (UCB) in the form of calcium bilirubinate and/or bilirubin polymers. Since more than 98% of bile pigments are excreted as conjugates of bilirubin, the source of this UCB needs to be identified. By using a rapid h.p.l.c. method, we compared the non-enzymic hydrolysis of bilirubin monoglucuronide (BMG) and bilirubin diglucuronide (BDG) to UCB in model bile and in native guinea-pig bile. Model biles containing 50 microM solutions of pure BMG and BDG were individually incubated in 25 mM-sodium taurocholate (NaTC) and 0.4 M-imidazole/5 mM-ascorbate buffer (TC-BUF) at 37 degrees C. Over an 8 h period, BMG hydrolysis produced 4-6 times more UCB than BDG hydrolysis. At pH 7.4, 25% of the BMG was converted into UCB, whereas only 4.5% of BDG was converted into UCB. Hydrolysis rates for both BMG and BDG followed the pH order 7.8 greater than 7.6 approximately equal to 7.4 greater than 7.1 Incubation with Ca2+ (6.2 mM) at pH 7.4 in TC-BUF resulted in precipitated bile pigment which, at 100 X magnification, appeared similar to precipitates seen in the bile of patients with pigment gallstones. At pH 7.4, lecithin (crude phosphatidylcholine) (4.2 mM) was a potent inhibitor of hydrolysis of BMG and BDG. The addition of a concentration of cholesterol equimolar with that of lecithin eliminated this inhibitory effect. Guinea-pig gallbladder bile incubated with glucaro-1,4-lactone (an inhibitor of beta-glucuronidase) underwent hydrolysis similar to the model bile systems. The non-enzymic hydrolysis of bile pigments, especially BMG, may be an important mechanism of bile-pigment precipitation and, ultimately, of gallstone formation.
mesh_terms: D000818:Animals; D001646:Bile / Q000187:drug effects / Q000378:metabolism*; D001663:Bilirubin / Q000031:analogs & derivatives / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology; D002769:Cholelithiasis / Q000378:metabolism*; D002784:Cholesterol / Q000494:pharmacology; D002851:Chromatography, High Pressure Liquid; D006168:Guinea Pigs; D006868:Hydrolysis; D008954:Models, Biological; D010713:Phosphatidylcholines / Q000494:pharmacology
chemical_list: D010713:Phosphatidylcholines; C019270:bilirubin diglucuronide; C016266:bilirubin glucuronate; D002784:Cholesterol; D001663:Bilirubin; D002118:Calcium