Binding of bilirubin and its structural analogues to hepatic microsomal bilirubin UDP glucuronyltransferase

Journal Article · Tue Sep 22 00:00:00 EDT 1987 · Biochemistry; (United States)

OSTI ID:5562184

Vanstapel, F; Blanckaert, N

Hepatic glucuronidation of the asymmetrical natural bilirubin molecule results in formation of two different positional isomers, bilirubin C-8 monoglucuronide and bilirubin C-12 monoglucuronide. In view of the existence of multiple isoforms of UDPglucuronyltransferase, which is the microsomal enzyme system responsible for bilirubin esterification, the authors performed kinetic analysis of microsomal glucuronidation of bilirubin and a number of its structural congeners to determine whether synthesis of the two monoglucuronide isomers involved two distinct substrate-binding sites or reflected two different modes of binding to a single catalytic site. Both isomers were found in all tested species (man, rat, guinea pig, sheep), but there were marked species differences in the C-8/C-12 ratio of monoglucuronide found in bile or formed by liver microsomes. Correspondence between in vivo and in vitro results for such regioselectivity of glucuronidation was excellent in each species. On the basis of these results of kinetic analysis of bilirubin esterification at variable pigment substrate concentrations and inhibition studies with alternative substrates, the authors postulate that both natural monoglucuronide isomers are synthesized at a single binding site. Possible mechanisms responsible for the markedly regioselective esterification of bilirubin by rat and sheep liver were investigated by study of glucuronidation of selected structural analgoues of the pigment. Collectively, their findings suggest that the molecular from(s) of bilirubin able to engage in catalytically effective binding to UDPglucuronyltransferase does (do) not correspond with intramolecularly hydrogen-bonded conformers and that the nature of the ..beta..-substituents of the outer pyrromethenone rings is a key determinant of glucuronidation rate.

• Research Organization: Univ. of California School of Medicine, San Francisco
• OSTI ID: 5562184
• Journal Information: Biochemistry; (United States), Vol. 26:19
• Country of Publication: United States
• Language: English

Related Subjects

59 BASIC BIOLOGICAL SCIENCES
BILIRUBIN
BIOCHEMISTRY
GLYCOSYL TRANSFERASES
RADIOASSAY
CARBON 14 COMPOUNDS
MICROSOMES
NUCLEOTIDES
AZOLES
CARBOXYLIC ACIDS
CELL CONSTITUENTS
CHEMISTRY
ENZYMES
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
LABELLED COMPOUNDS
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
ORGANOIDS
PIGMENTS
PYRROLES
TRANSFERASES
550201* - Biochemistry- Tracer Techniques