Conjugation of styrene oxide by the basic and acidic forms of glutathione transferase in the human fetal liver.

Two forms of glutathione transferase were isolated by means of isoelectric focusing of human fetal liver cytosol preparations. The enzyme activity was measured with 1-chloro-2,4-dinitrobenzene as the electrophilic substrate. One peak focused at pH 9-10 (basic form) and the other at pH 4-5 (acidic form). The basic and the acidic forms are representatives of glutathione transferase classes alpha and tau, respectively. These classes constitute two of the three classes defined for cytosolic forms of the enzyme in several mammalian species [Mannervik et al., Proc. natn. Acad. Sci. USA 82: 7202-7206, 1985]. Only the basic fraction isolated from human fetal liver catalyzed the conjugation of styrene oxide with glutathione at a significant rate. The kinetics of this form were studied keeping the concentration of styrene oxide constant (6 mM) and varying the glutathione concentration from 0.05 to 25 mM. The enzyme activity displayed non-Michaelis-Menten kinetics. The basic and acidic forms of glutathione transferase from a fetal liver were purified to homogeneity. Both purified forms catalyzed the conjugation of glutathione with styrene oxide. The kinetics were studied at varying glutathione concentrations and for both forms, it was found to be of a non-Michaelis-Menten type. The results are consistent with previous findings in the cytosolic fraction [Pacifici et al., Biochem. Pharmac. 30: 3367-3371, 1981] and show that the non-Michaelian kinetics observed with glutathione in human fetal liver cytosol are reflections of the intrinsic properties of the basic as well as the acid form of this enzyme and not primarily depending on the simultaneous catalytic action of the two forms.