Serine hydrolases activate/inactivate trisubstituted nitrosoureas in dependence on intra- and extracellular enzyme location: an SCE study in Chinese hamster V79-E cells

Trisubstituted nitrosoureas are very stable in aqueous systems.But they are potent genotoxins in Chinese hamster V79-E cells,if no exogenous metabolizing system is added, and the mechanismof their genotoxic and carcinogenic activity has been largelyunknown. This investigation shows that the sister-chromatid-exchange(SCE)-inducing capacity of 1,3-dimethyl-3-phenyl-1-nitrosourea(DMPNU) is eliminated by adding dlisopropylfluorophospbate (DFP)or porcine liver carboxylesterase to the incubation system.These effects are caused by two different mechanisms: (i) DFPinhibits endogenous amidases existing in V79-E cells, thus preventingthe intracellular decomposition, which means an activation;and (ii) exogenous carboxylesterase cleaves DMPNU extra cellularly,and the genotoxic decomposition product is obviously too short-livedto reach a critical intracellular target. A second trisubstitutednitrosourea, 3,3-diethyl-1-methyl-1-nitrosourea (DEMNU), whichis mainly activated by monooxygenases, but in the absence ofan exogenous metabolizing system also induces SCEs in V79-Ecells, was studied in the same way. It was found that the ‘direct’genotoxicity of DEMNU may be inhibited by DFP as well, but carboxylesterasedecomposes this trialklynitrosourea with a much lower efficiencythan DMPNU suggesting a low substrate affinity. The SCE-inducingcapacity of both compounds is strongly influenced by the presenceof calf serum in the culture medium. The nature of the serumfactor is still unknown. Pathways for the amidase catalysisof DMPNU and for the activation of DEMNU by monooxygenases andamidases are proposed and discussed with respect to the topicalor systemic carcinogenicity of these agents.