Suppression of flavin-containing monooxygenase by overproduced nitric oxide in rat liver

Effects of excessive nitric oxide (NO) produced in vivo by an i.p. injectio n of bacterial lipopolysaccharide (LPS) on hepatic microsomal drug oxidatio n catalyzed by flavin-containing monooxygenase (FMO) were determined. At 6 and 24 h after the LPS injection, liver microsomes were isolated and FMO ac tivities were determined by using FMO substrates like thiobenzamide, trimet hylamine, N,N-dimethylaniline, and imipramine. Liver microsomal FMO activit ies of LPS-treated rats were decreased significantly for all these substrat es, Microsomal content of FMO1 (the major form in rat liver) in LPS-treated rats as determined by immunoblotting, was severely decreased as well. In s upport of this, hepatic content of FMO1 mRNA was decreased by 43.6 to 67.3% . However, the hepatic content of inducible NO synthase (iNOS) mRNA was inc reased by 2.6- to 5.4-fold and the plasma nitrite/nitrate concentration was increased by about 30-fold in the LPS-treated rats. When this overproducti on of NO in the LPS-treated rats was inhibited in vivo by a single or repea t doses of either a general NOS inhibitor N-G-nitro-L-arginine or a specifi c iNOS inhibitor aminoguanidine, the FMO1 mRNA levels were not severely dep ressed (70-85% of the control level). Attendant with the reduction of plasm a nitrite/nitrate concentration by single and repeated doses of NOS inhibit ors, activity and content of FMO1 in liver microsomes isolated from these N OS inhibitor cotreated rats were restored partially tin single-dose inhibit ors) or completely (in repeat doses). In contrast to these NO-mediated in v ivo suppressive effects on the mRNA and enzyme contents of FMO1 as well as the FMO activity, the NO generated in vitro from sodium nitroprusside did n ot inhibit the FMO activities present in microsomes of rat and rabbit liver as well as those present in rabbit kidney and lung. Combined, the excessiv e NO produced in vivo (caused by the LPS-dependent induction of (NOS) suppr esses the FMO1 mRNA and enzyme contents as well as the FMO activities witho ut any direct in vitro effect on the activities of premade FMO enzyme. Thes e findings suggest that NO is an important mediator involved in the suppres sion of FMO1 activity in vivo. Thus, together with the previously reported suppression on the cytochrome P-450 activities, the overproduced NO in the liver caused by induction of iNOS under conditions of endotoxemia or sepsis suppresses FMO and appears to be responsible for the decreased drug oxidat ion function observed generally under conditions of systemic bacterial or v iral infections.