Glutamine synthetase (GS) in the liver is restricted to a small perivenous hepatocyte population and plays an important role in the scavenging of
ammonia that has escaped the periportal
urea-synthesizing compartment. We examined the effect of a single
intraperitoneal injection of
lipopolysaccharide (LPS) in vivo on
glutamine synthesis in rat liver. LPS injection induced expression of
inducible nitric oxide synthase, which was maximal after 6 to 12 hours but returned toward control levels within 24 hours. Twenty-four hours after LPS injection, an approximately fivefold increase in
tyrosine-nitrated
proteins in liver was found, and GS
protein expression was decreased by approximately 20%, whereas GS activity was lowered by 40% to 50%. GS was found to be
tyrosine-nitrated in response to LPS, and immunodepletion of
tyrosine-nitrated
proteins decreased GS
protein by approximately 50% but had no effect on GS activity. Together with the finding via mass spectrometry that
peroxynitrite-induced inactivation of purified GS is associated with nitration of the active site
tyrosine residue, our data suggest that
tyrosine nitration critically contributes to inactivation of the
enzyme. In line with GS inactivation,
glutamine synthesis from
ammonia (0.3 mmol/L) in perfused livers from 24-hour LPS-treated rats was decreased by approximately 50%, whereas
urea synthesis was not significantly affected. In conclusion, LPS impairs hepatic
ammonia detoxification by both downregulation of GS and its inactivation because of
tyrosine nitration. The resulting defect of perivenous scavenger cell function with regard to
ammonia elimination may contribute to
sepsis-induced development of
hyperammonemia in patients who have
cirrhosis.