Figure 1. Total plasma GST activity in the GSTM1-positive and -null groups.

The total plasma enzyme activity (mean ± SEM, in mU.mg⁻¹.min⁻¹) was found to be similar in both groups, though GSTM1 isoenzyme activity should be undetectable in the GSTM1-null individuals. The similar level of total plasma GST activity indicates the presence of a counter-balance mechanism.