Hepatic cytochrome P450 is directly inactivated by nitric oxide, not by inflammatory cytokines, in the early phase of endotoxemia.
OBJECTIVE
Although the activity of the liver in metabolizing and eliminating various drugs decreases in endotoxemia, the mechanism remains to be elucidated. The generation of nitric oxide by the inducible type of nitric oxide synthase increases in endotoxemia. Nitric oxide readily reacts with heme proteins such as cytochrome P450 that metabolize various compounds, including steroids and eicosanoids. The purpose of this study was to determine the effect of nitric oxide on the function of hepatic cytochrome P450 in endotoxemic rats.


METHODS
To determine the dynamic aspects of nitric oxide metabolism, hepatic levels of the inducible type of nitric oxide synthase and heme-iron nitrosyl complexes, and plasma levels of nitrite and nitrate were determined in rats before and after intravenous administration of lipopolysaccharide. Changes in the levels of P450 isoforms and testosterone hydroxylation activity in hepatic microsomes were also determined. To evaluate in vivo CYP3A2 activity, midazolam sleep time was measured.


RESULTS
When lipopolysaccharide increased the hepatic inducible type of nitric oxide synthase and plasma levels of nitric oxide metabolites, the intensity of low-spin signal of electron spin resonance responsible for the ferric form of P450 decreased with a concomitant increase in heme-iron nitrosyl complexes in the liver. Lipopolysaccharide-related nitric oxide generation is followed by an early decrease in the levels of cytochrome P450 and of testosterone hydroxylation activity in liver microsomes. Midazolam sleep time was prolonged by lipopolysaccharide. All these early changes were prevented by the inhibitor of nitric oxide synthase, N(G)-iminoethyl-L-ornithine. Moreover, lipopolysaccharide suppressed the gene expression of CYP2C11 and CYP3A2. Decreases in levels of cytochrome P450 and their mRNAs were more pronounced at 24 h after LPS administration, but apparently they are NO-independent.


CONCLUSIONS
These results suggest that lipopolysaccharide-induced modulation of cytochrome P450 may occur via the interplay of two different mechanisms and that, especially in the early phase, nitric oxide-dependent inhibition is more important.
Takemura
S
S
Department of Surgery, Osaka City University Medical School, Osaka, Japan.
Minamiyama
Y
Y
Imaoka
S
S
Funae
Y
Y
Hirohashi
K
K
Inoue
M
M
Kinoshita
H
H
D000818:Animals; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D019446:Endotoxemia / Q000201:enzymology*; D006900:Hydroxylation; D006993:Hypnotics and Sedatives / Q000494:pharmacology; D008070:Lipopolysaccharides; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology*; D008874:Midazolam / Q000494:pharmacology; D009566:Nitrates / Q000097:blood; D009569:Nitric Oxide / Q000378:metabolism / Q000502:physiology*; D019001:Nitric Oxide Synthase / Q000378:metabolism; D052247:Nitric Oxide Synthase Type II; D009573:Nitrites / Q000097:blood; D051381:Rats; D017208:Rats, Wistar; D013250:Steroid Hydroxylases / Q000378:metabolism; D013739:Testosterone / Q000378:metabolism
D065607:Cytochrome P-450 Enzyme Inhibitors; D006993:Hypnotics and Sedatives; D008070:Lipopolysaccharides; D009566:Nitrates; D009573:Nitrites; D009569:Nitric Oxide; D013739:Testosterone; D003577:Cytochrome P-450 Enzyme System; D013250:Steroid Hydroxylases; D019001:Nitric Oxide Synthase; D052247:Nitric Oxide Synthase Type II; C496318:Nos2 protein, rat; C022657:steroid hormone 6-beta-hydroxylase; D008874:Midazolam