title: Reversible inhibition of rat hepatic glutathione S-transferase 1-2 by bilirubin.
abstract: The inhibition of rat hepatic glutathione (GSH) S-transferase 1-2 by bilirubin exhibited pseudo first-order kinetics with k(obs) values of 0.0214 +/- 0.0005 and 0.040 +/- 0.008 sec-1 at 4 and 8 microM bilirubin, when followed to 72 and 84% completion respectively. These correspond to calculated second-order rate constants of 5.3 +/- 0.1 x 10(3) and 5.0 +/- 1.0 x 10(3)/M.sec. The extent of inhibition of the transferase increased with bilirubin concentration, with half-maximal inhibition at 4 microM bilirubin. Inhibition was reversed by 10-fold dilution of bilirubin or by increasing the pH from 6.0 to 7.4. Premixing 0.2 to 0.5 microM albumin, hemoglobin or aldolase with bilirubin prevented inhibition of GSH S-transferase 1-2. Protection by these proteins occurred at a selected high concentration (0.2 to 0.4 microM) at which they reduced free bilirubin to concentrations (less than 0.5 microM) that did not inhibit isoenzyme 1-2 significantly. No protection was afforded by a selected low protein concentration (0.001 to 0.01 microM) which did not strikingly reduce bilirubin levels in solution. We conclude that bilirubin inhibition of GSH S-transferase 1-2 appears to be a second-order process; the reaction is clearly first-order with respect to GSH S-transferase and appears also to be first-order with respect to bilirubin. It is proposed that (a) inhibition of GSH S-transferase 1-2 results from slow, reversible bilirubin binding, and (b) added proteins appear to prevent GSH S-transferase inhibition by binding high molar ratios of bilirubin.
authors.affiliation: Biomolecular Resource Center, University of California Medical School, San Francisco 94143-0541.
authors.forename: K M
authors.initials: KM
authors.lastname: Ivanetich
authors.forename: A E
authors.initials: AE
authors.lastname: Thumser
authors.forename: S E
authors.initials: SE
authors.lastname: Phillips
authors.forename: C N
authors.initials: CN
authors.lastname: Sikakana
mesh_terms: D000418:Albumins / Q000494:pharmacology; D000818:Animals; D001663:Bilirubin / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D005634:Fructose-Bisphosphate Aldolase / Q000494:pharmacology; D005982:Glutathione Transferase / Q000037:antagonists & inhibitors*; D006454:Hemoglobins / Q000494:pharmacology; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D051381:Rats
chemical_list: D000418:Albumins; D006454:Hemoglobins; D005982:Glutathione Transferase; D005634:Fructose-Bisphosphate Aldolase; D001663:Bilirubin