Urobilinogen-i is a major derivative of bilirubin in bile of homozygous Gunn rats

Abstract

Gunn rats lack bilirubin UDP-glycosyltransferases, but diazo-negative derivatives of bilirubin have been described in their bile. In order to investigate this alternative disposal of bilirubin, crude bile samples from Gunn and Wistar rats were directly analysed by h.p.l.c. Besides bilirubin (in Gunn rats) or its glycosides (in Wistar rats), two major compounds were detected. A yellow one corresponded to the previously documented vitamin B-2 and was equally prominent in Gunn rats or Wistar-rat bile. The other compound was colourless, but on standing in contact with air it was spontaneously oxidized to a pinkish-yellow pigment. It was far more prominent in Gunn-rat bile. Analysis of bile obtained after intravenous injection of [14C]bilirubin to Gunn rats demonstrated that this compound was highly labelled. Freezing and thawing of the bile resulted in the formation of a series of diazo-negative derivatives, demonstrating that the original compound was quite labile. Spectral (adsorption and fluorescent) and chromatographic (h.p.l.c., t.l.c. and paper chromatography) analysis of the oxidized form of the labelled compound allowed its identification as urobilin-i. The colourless compound secreted in bile was urobilinogen-i. Administration of neomycin and bacitracin to Gunn rats or gut resection suppressed the biliary excretion of urobilinogen and thus confirmed its intestinal origin. Urobilinogen seems thus to represent the major bilirubin derivative present in Gunn-rat bile. Its breakdown products might represent the so-far-unidentified diazo-negative polar bilirubin derivatives. Since only a small amount of bilirubin is present in Gunn-rat bile, the urobilinogen formed in the intestinal lumen seems to be derived from bilirubin reaching the gut via routes other than the biliary one.

Functionally Overlapping Roles of Abcg2 (Bcrp1) and Abcc2 (Mrp2) in the Elimination of Methotrexate and Its Main Toxic Metabolite 7-Hydroxymethotrexate In vivo

“…However, if Abcg2 transports unconjugated bilirubin, it clearly has a low efficiency because the amount excreted into bile is very small, also in wild-type mice. Even in Gunn rats, which have elevated serum unconjugated bilirubin due to the absence of the conjugating enzyme UGT1A1, very little is excreted into bile (33). An alternative explanation for the observed changes is decreased unconjugated bilirubin uptake into the liver of Abcg2-deficient mice.…”

Novel Kinetic Insights into Treatment of Unconjugated Hyperbilirubinemia: Phototherapy and Orlistat Treatment in Gunn Rats

“…The increased biliary excretion of derivatives in the phototherapy group is most likely the consequence of enterohepatic recirculation of urobilinogens and other metabolites due to enhanced supply of substrate in the intestinal lumen. Previously, Schmid et al (10) and Kotal et al (9,24) showed that labeled UCB derivatives in Gunn rat bile are mostly urobilinogen and other diazo-negative polar products. Interestingly, the fractional flux of derivatives undergoing enterohepatic circulation is increased during phototherapy, compared with controls.…”

New concepts in bilirubin chemistry, transport and metabolism: Report of the second international bilirubin workshop, April 9-11, 1992, trieste, Italy

“…Peter Kotul (Prague, Czechoslovakia) then presented conclusive evidence that diffusion of UCB into the intestine is an important, and possibly the major, route of UCB removal in the uninduced Gunn rat (153). The total intestinal content of bile pigments (UCB plus urobilinogen) in two strains of Gunn j j rats and a double-mutant rat strain (Gunn plus TR-) was similar to that in normal Wistar rats, despite the biliary excretion of only 2% to 15% as much bilirubin in the jaundiced strains as compared with controls (154). In the jaundiced strains, concentrations of UCB through-out the intestine were uniformly higher than in bile (2 to 5 times), despite dilution and reductive metabolism of biliary UCB during its transit through the bowel (153).…”

“…In the jaundiced strains, concentrations of UCB through-out the intestine were uniformly higher than in bile (2 to 5 times), despite dilution and reductive metabolism of biliary UCB during its transit through the bowel (153). After intravenous administration of 14C-UCB to Gunn rats with a bile fistula, 30% to 50% of the administered label was detected in the intestinal content (135,1531, and urobilinogen of intestinal origin was the major UCB derivative excreted in the bile (154). In Gunnjj rats, both administration of antibiotics to eliminate conversion of UCB to urobilinogen by intestinal bacteria (153,154) and fasting to decrease fecal output and thus diminish elimination of UCB by this route (1551, led to increases in intestinal UCB content, although plasma UCB levels increased only with fasting.…”

“…After intravenous administration of 14C-UCB to Gunn rats with a bile fistula, 30% to 50% of the administered label was detected in the intestinal content (135,1531, and urobilinogen of intestinal origin was the major UCB derivative excreted in the bile (154). In Gunnjj rats, both administration of antibiotics to eliminate conversion of UCB to urobilinogen by intestinal bacteria (153,154) and fasting to decrease fecal output and thus diminish elimination of UCB by this route (1551, led to increases in intestinal UCB content, although plasma UCB levels increased only with fasting. It was concluded that enhanced reflux into the plasma of UCB accumulated in the intestine was the major cause of the increased hyperbilirubinemia that occurred during fasting in Gunn rats (155).…”