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A rapid equilibrium random sequential bi-bi mechanism for human placental glutathione S-transferase

1989; Elsevier BV; Volume: 998; Issue: 1 Linguagem: Inglês

10.1016/0167-4838(89)90111-8

1878-1454

Kathryn M. Ivanetich, Richard D. Goold,

Genomics, phytochemicals, and oxidative stress

Double-reciprocal plots of initial-rate data for the conjugation of 1-chloro-2,4-dinitrobenzene (CDNB) and GSH by human placental GSH S-transferase π were linear for both substrates. Computer modelling of the initial-rate data using nonlinear least-squares regression analysis favoured a rapid equilibrium random sequential bi-bi mechanism, over a steady-state random sequential mechanism or a steady-state or rapid equilibrium ordered mechanism. KGSH was calculated as 0.125 ± 0.006 mM, KCDNB was 0.87 ± 0.07 mM and α was 2.1 ± 0.3 for the rapid equilibrium random model. The product, S-(2,4-dinitrophenyl)glutathione, was a competitive inhibitor with respect to GSH, and a mixed-type inhibitor toward CDNB (KP = 18 ± 3 μM). The observed pattern of inhibition is consistent with a rapid equilibrium random mechanism, with a dead-end enzyme · CDNB · product complex, but inconsistent with the inhibition patterns of other bireactant mechanisms. Since rat liver GSH S-transferase 3−3 acts via a steady-state random sequential mechanism [1], while human placental GSH S-transferase and perhaps also rat liver GSH S-transferase 1−1 [2] exhibit rapid equilibrium random mechanisms, we conclude that the kinetic mechanism of the GSH S-transferases is isoenzyme-dependent.