Detoxification of styrene oxide by human liver glutathione transferase

Cytosolic glutathione transferase (GST) was investigated in four human livers. The profile of GST activity was determined by isoelectric focusing using 1-chloro-2,4-dinitrobenzene as the electrophilic substrate. Three livers contained at least one basic and a near-neutral isoenzyme (GST mu). GST mu was not detectable in the fourth liver. The kinetics of GST with styrene oxide as the electrophilic substrate were studied in the cytosolic fraction, with the pooled fractions from isoelectric focusing containing high activity of GST mu transferase, and with GST mu purified to homogeneity. The cytosol obeyed Michaelis-Menten kinetics when styrene oxide was used as the variable substrate. The average (+/- s.e.m.) of the Vmax and Km were 21.9 +/- 7.9 nmol min-1mg-1 and 4.9 +/- 0.4 mM, respectively. At varying concentrations of glutathione, the enzyme did not obey Michaelis-Menten kinetics. Such kinetics were also observed with the fractions from isoelectric focusing and with the homogeneous GST mu fraction. The Eadie-Hofstee plot showed two phases: one with a low and another with a high Km value. The apparent Km values for the cytosol were 0.035 +/- 0.022 and 0.88 +/- 0.36 mM. The kinetic pattern of purified GST mu is consistent with that found in the cytosol.