title: Forward and reverse catalysis and product sequestration by human glutathione S-transferases in the reaction of GSH with dietary aralkyl isothiocyanates.
abstract: The reversible reaction of GSH with two dietary anticarcinogens, benzyl isothiocyanate (BITC) and phenethyl isothiocyanate (PEITC), has been studied in the absence and presence of human glutathione S-transferases (GSTs). The spontaneous reaction at pH 7.4 and 37 degrees C yielded values for k2 of 17.9 and 6.0 M-1.s-1 for GSH conjugation of BITC and PEITC respectively (forward reaction), and k1 values of 6.9 x 10(-4) and 2.4 x 10(-4) s-1 for dissociation of the respective GSH conjugates, BITC-SG and PEITC-SG (reverse reaction). GSTs A1-1, A2-2, M1a-1a and P1-1 catalysed both the forward and reverse reactions with specific activities (mumol/min per mg at 30 microM isothiocyanate or GSH conjugate) ranging from 23.1 for the GSH conjugation of BITC by GST P1-1 to 0.03 for the dissociation of BITC-SG by GST A1-1. When present at similar concentration to substrates (12 microM), GSTs A1-1 and A2-2 but not GST M1a-1a shifted the equilibrium in favour of BITC-SG or PEITC-SG. Kinetic studies confirmed that GST A1-1 interacted selectively with the GSH conjugates in the micromolar range (Km 6.9 microM, Ki 4.3 microM), whereas GST M1a-1a interacted with BITC-SG and PEITC-SG with approx. 5-fold lower affinity. In conclusion, GSTs are true catalysts; at high intracellular concentration they also sequester GSH conjugates, promoting GSH conjugation, whereas trace extracellular GSTs promote dissociation of effluxed organic isothiocyanate-GSH conjugates.
authors.affiliation: Department of Biochemistry and Molecular Biology, University College London, U.K.
authors.forename: D J
authors.initials: DJ
authors.lastname: Meyer
authors.forename: D J
authors.initials: DJ
authors.lastname: Crease
authors.forename: B
authors.initials: B
authors.lastname: Ketterer
mesh_terms: D016588:Anticarcinogenic Agents / Q000008:administration & dosage / Q000378:metabolism*; D002384:Catalysis; D004032:Diet; D005978:Glutathione / Q000378:metabolism*; D005982:Glutathione Transferase / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D017879:Isothiocyanates / Q000008:administration & dosage / Q000378:metabolism*; D007700:Kinetics
chemical_list: D016588:Anticarcinogenic Agents; D017879:Isothiocyanates; C058305:phenethyl isothiocyanate; C031403:benzyl isothiocyanate; D005982:Glutathione Transferase; D005978:Glutathione