NITRIC-OXIDE IS A MEDIATOR OF THE DECREASE IN CYTOCHROME-P450-DEPENDENT METABOLISM CAUSED BY IMMUNOSTIMULANTS

Bacterial lipopolysaccharide (LPS) and a diverse array of other immuno stimulants and cytokines suppress the metabolism of endogenous and exo genous substances by reducing activity of the hepatic cytochrome P450 mixed-function oxidase system. Although this effect of immunostimulant s was first described almost 40 yr ago, the mechanism is obscure. Immu nostimulants are now known to cause NO overproduction by cells via ind uction of nitric oxide synthase. We have investigated whether NO overp roduction is involved suppressing hepatic metabolism by LPS. In vitro treatment of hepatic microsomes with NO, produced by chemical decompos ition of 3-morpholinosydnonimine or by nitric oxide synthase, substant ially suppressed cytochrome P450-dependent oxygenation reactions. This effect of NO was seen with hepatic microsomes prepared from two speci es (rat and chicken) and after exposure to chemicals that induce disti nct molecular isoforms of cytochromes P450 (beta-naphthoflavone, 3-met hylcholanthrene, and phenobarbital). Spectral studies indicate that NO reacts in vitro with both Fe2+- and Fe3+-hemes in microsomal cytochro mes P450. In vivo, LPS diminished the phenobarbital-induced dealkylati on of 7-pentoxyresorufin by rat liver microsomes and reduced the appar ent P450 content as measured by CO binding. These LPS effects were ass ociated with induction of NO synthesis; LPS-induced NO synthesis showe d a strong positive correlation with the severity of cytochrome P450 i nhibition. The decrease in both hepatic microsomal P450 activity and C O binding caused by LPS was largely prevented by the selective NO synt hase inhibitor N(omega)-nitro-L-arginine methyl ester. Our findings im plicate NO overproduction as a major factor mediating the suppression of hepatic metabolism by immunostimulants such as LPS.