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.

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