The
glutathione transferase (GST)
isoenzyme profile was determined in two human tumor cell lines, U1690 derived from a
small cell lung cancer and U1810 derived from a
non-small cell lung cancer. U1810 cells are 3.2-fold more resistant to
1,3-bis(2-chloroethyl)-1-nitrosourea (
BCNU) than are U1690 cells, a finding ascribable in part to the expression of O6-alkylguanine-DNA
alkyltransferase activity in the U1810 cells. GST P1-1 and GST A1-1 were determined quantitatively by
enzyme-linked immunoassay and were found to be 1.3- and 15-fold higher in the cytosol fraction of U1690 cells as compared to U1810 cells, respectively. The higher
BCNU resistance in U1810 cells can, therefore, not be correlated with the expression of these
isoenzymes. However,
sodium dodecyl sulfate/
polyacrylamide gel electrophoresis in combination with immunoblot analysis demonstrated a class Mu GST, which was identified as GST M3-3 on the basis of electrophoretic mobility and cross-reaction with anti-rat GST 3-3
antibodies. This
isoenzyme was detectable in U1810 cells but not in U1690 cells. Studies with purified human GST A1-1, GST M1-1, GST M3-3, and GST P1-1 demonstrated that GST M3-3, but not the other
isoenzymes, catalyzed the denitrosation of
BCNU. Such inactivation of
BCNU has previously been demonstrated with rat class Mu
GSTs (M. T. Smith et al.,
Cancer Res., 49: 2621-2625, 1989) but not with any human GST. These findings suggest that GST M3-3 contributes to
BCNU resistance in the U1810 cells.