Comparison in different species of biliary bilirubin-IX alpha conjugates with the activities of hepatic and renal bilirubin-IX alpha-uridine diphosphate glycosyltransferases

The formation and distribution of bilirubin monoglucuronide and diglucuronide in rat liver slices

1. Bilirubin conjugation in rat liver slices was reassessed by using analysis of ethyl anthranilate azopigments to estimate separately the formation of bilirubin mono- and di-glucuronides. 2. Conjugation in slices resembles the situation in vivo more closely than does microsomal conjugation, in that diglucuronide is formed in appreciable quantity. 3. Both bilirubin mono- and di-glucuronides were present in slices in approximately equal amounts, but the monoglucuronide was the major product found in the incubation medium. 4. These results are discussed in relation to recent theories on the relationship between bilirubin mono- and di-glucuronide formation in vivo.

Bilirubin diglucuronide synthesis by a UDP-glucuronic acid-dependent enzyme system in rat liver microsomes

Incubation of rat liver homogenate or microsomal preparations with bilirubin or bilirubin monoglucuronide with (BMG) resulted in formation of bilirubin diglucuronide (BDG). Both synthesis of BMG and its conversion to BDG were critically dependent on the presence of UDP-glucuronic acid. Pretreatment of the animals with phenobarbital stimulated both reactions. When 33 microM bilirubin was incubated with microsomal preparations from phenobarbital-treated rats, 80-90% of the substrate was converted to bilirubin glucuronides; the reaction products consisted of almost equal amounts of BMG and BDG. When phenobarbital pretreatment was omitted or when the substrate concentration was increased to 164 microM bilirubin, proportionally more BMG and less BDG were formed. Homogenate and microsomes from homozygous Gunn rats neither synthesized BMG nor converted BMG to BDG. These findings in vitro suggest an explanation for the observations in vivo that, in conditions of excess bilirubin load or of genetically decreased bilirubin UDP glucuronosyltransferase (EC 2.4.1.17) activity, proportionally more BMG and less BDG are excreted in bile.

Comparative aspects of bile pigment formation and excretion

Breakdown of haem which is of key importance in most organisms, involves oxidative CO-evolving cleavage of the macrocyclic ring with formation of biliverdin-IX. In two major pathways established so far formation of biliverdin-IX alpha is followed by (a) biliary secretion or (b) reduction to bilirubin-IX alpha, formation of more hydrophilic derivatives (usually glycosidic conjugates) and biliary secretion. The scattered comparative information available indicates marked species variation with regard to the methin-bridge carbon atom removed from haem and the metabolic site of cleavage, the nature of bilirubin conjugates and the developmental sequence of maturation of enzyme activities and transport proteins involved in the chain of events leading from breakdown of haem to the excretion of the final end products.

Twenty-five years of progress in bilirubin metabolism (1952-77)

This review deals with the development of our understanding of the chemistry of bilirubin and its glucuronide derivatives during the years 1952-1977. It examines the relation between haem metabolism and bilirubin formation and our present knowledge of hepatic transport of bilirubin. The heterogeneity of familial hyperbilirubinaemia is discussed.

The fate of bilirubin-IXalpha glucuronide in cholestasis and during storage in vitro. Intramolecular rearrangement to positional isomers of glucuronic acid

1. In aqueous solution above pH7 bilirubin-IXalpha 1-O-acylglucuronide rapidly isomerizes to the non-C-1 glucuronides by sequential migration of the bilirubin acyl group from position 1 to positions 2, 3 and 4 of the sugar moiety. The transformations are enhanced by increasing the pH. Compared with the rates at 37 degrees C the transformations are rather slow at 0 degrees C. Virtually complete inhibition is observed at values below pH6. The isomerization at 25 degrees C and pH 7.4 is not affected by the presence in the solutions of a molar excess of human serum albumin. 2. Isomerization in bile kept at 37 degrees C at pH7.7-7.8 is probably non-enzymic, as the rates of change are similar to those observed under comparable conditions for aqueous solutions of glucuronides of bilirubin-1Xalpha and of azodipyrrole. 3. Analysis without delay of normal biles of man and rats collected at 0 degrees C over a maximum period of 10 min shows that the bilirubin-IXalpha mono- and di-glucuronides consist exclusively of the 1-O-acyl isomers. 4. The mixtures of the four positional isomers of bilirubin-IXalpha glucuronide found in freshly collected biles of man and rats with cholestasis probably originate from initially synthesized 1-O-acylglucuronide by the same mechanism of sequential migration as has been observed in aqueous solutions of conjugated bilirubin-IXalpha.