To identify the novel mechanism by which nitric oxide (NO) suppresses flavin-containing monooxygenase (FMO) activity in endotoxemic rat livers, NO-overproducing conditions were induced in primary cultured rat hepatocytes by treatment with a mixture (LCM) of lipopolysaccharide and proinflammatory cytokines (IL-1beta, TNF-alpha, and IFN-gamma), or by the addition of a pure NO donor, spermine-NONOate. mRNA levels of the major hepatic form, FMO1, decreased via a cGMP-independent destabilizing effect of NO rather than by decreased transcription. The decrease in the mRNA levels caused by LCM-induced inducible NO synthase (iNOS) was completely blocked by co-treatment with aminoguanidine, a selective NOS inhibitor. Furthermore, spermine-NONOate, but not the cGMP analog, 8-bromo-cGMP, dose- and time-dependently attenuated FMO1 mRNA stability in actinomycin-D-pretreated cells, resulting in decreases in protein levels and biochemical activity. These results suggest that NO acts directly in a cGMP-independent mechanism by decreasing the half-life of FMO1 mRNA, thereby inducing impairment of FMO-related functions in endotoxemia. (C) 2004 Elsevier Inc. All rights reserved.