Og. Khatsenko et al., NITRIC-OXIDE IS A MEDIATOR OF THE DECREASE IN CYTOCHROME-P450-DEPENDENT METABOLISM CAUSED BY IMMUNOSTIMULANTS, Proceedings of the National Academy of Sciences of the United Statesof America, 90(23), 1993, pp. 11147-11151
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.