A method to determine the carbamoylating potential of 1-(2-chloroethyl)-1-nitrosoureas

1-(2-Chloroethyl)-1-nitrosoureas (CNUs) are alkylating agents that also possess carbamoylating activity, depending on the chemical nature of the substituent at N-3. Although effects on a variety of enzymes, including inhibition of glutathione reductase (GS-R) have been attributed to carbamoylation, the biological significance is still not well understood. This deficiency is due at least in part to the analytical method that has been used to measure carbamoylation:in-vitro reaction with the omega-amino group of lysine. Reaction of CNUs with glutathione (GSH) offers a better estimation of carbamoylating potential in vitro. The decrease in free thiol groups during incubation of GSH with various CNUs can be followed using the thiol reagent 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB). With this test, carbamoylating potential relative to that of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) as 100% was 94% for 1-(2-chloroethyl)-3-(4-methylcyclohexyl)-1-nitrosourea (MeCCNU), 86% for 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU), 16% for 1-(2-chloroethyl)-3-(2-hydroxyethyl)-1-nitrosourea (HECNU) and 6% for chlorozotocin (CLZ). Various carbamoylated and alkylated GSH derivatives, such as S-(2-chloroethylcarbamoyl)-, S-(2-hydroxyethylcarbamoyl)-, S-(cyclohexylcarbamoyl)-, S-(4-methylcyclohexylcarbamoyl)- and S-(2-hydroxyethyl)glutathione, are formed on incubation of GSH with CNUs. High-performance liquid chromatography (HPLC) revealed that, in comparison to carbamoylated compounds, alkylated GSH derivatives are formed in only low yields (less than 3%). Formation of carbamoylated products during incubation correlated with the decrease in free thiol groups. Concentrations achieving 50% GS-R inhibition in vitro were 0.16 mM for BCNU and 1.9 mM for HECNU.(ABSTRACT TRUNCATED AT 250 WORDS)