Although both direct and
glutathione S-transferase (GST)-catalyzed interactions between many electrophiles and GSH generally result in inactivation of the former, there are several reports of compounds whose electrophilic, alkylating, and cytotoxic activities are potentiated by GSH. This study investigates the effects of direct in vitro interaction between GSH and
BCNU at physiological pH (7.2) and temperature (37 degrees C) and how this affects the cytotoxic and
DNA cross-linking activity of
1,3-bis(2-chloroethyl)-1-nitrosourea (
BCNU) in target human malignant
brain tumor cells. The kinetics and dose-response relationship of this interaction were determined by measuring residual GSH and residual
BCNU-cytotoxicity in aGSH/
BCNU mixture over a 45-min period and at varying
BCNU concentrations. The results demonstrate that reaction of
BCNU with four times its molar concentration of GSH for 45 min significantly inactivates
BCNU, as expressed by a 32% decrease in induction of cellular
DNA cross-linking, a 21% increase in
DNA synthesis, and a 15% increase in clonogenic survival of human
brain tumor cells compared to incubates of
BCNU alone. Equine liver (EL)-GST increased the inactivation of
BCNU only slightly (insignificant at p = 0.05). These results suggest that, in contrast to agents such as the alkyl-N-nitro-N'-
nitrosoguanidines which become more potent
alkylators after reacting with GSH, the 2-chloroethylnitrosoureas (CENUs) undergo inactivation by GSH. We propose that such interactions between GSH and the CENUs may constitute an important aspect of
CENU metabolism and provide a potential means by which
brain tumor cells can circumvent
CENU toxicity and exhibit resistance to this class of agents.