In Hepatocytes the Regulation of NOS-2 Activity at Physiological L-arginine Levels Suggests a Close Link to the Urea Cycle

Nitric Oxide. 2006 Jun;14(4):300-8. doi: 10.1016/j.niox.2005.11.009. Epub 2006 Jan 10.


High-output synthesis of nitric oxide (NO) by the inducible isoform of NO-synthases (NOS-2) plays an important role in hepatic pathophysiological processes and may contribute to both organ protection and organ destruction during inflammatory reactions. As they compete for the same substrate, L-arginine, an interdependence of NOS-2 and arginase-1 has been repeatedly observed in cells where arginase-1 is cytokine-inducible. However, in hepatocytes, arginases are constitutively expressed and thus, their impact on hepatic NOS-2-derived NO synthesis as well as the influence of L-arginine influx via cationic amino acid transporters during inflammatory reactions are still under debate. Freshly isolated rat hepatocytes were cultured for 24h in the presence of various L-arginine concentrations with or without cytokine addition and nitrite and urea accumulation in culture supernatants was measured. We find that both, cytokine-induced NOS-2 and arginase activities strongly depend on extracellular L-arginine concentrations. When we competed for L-arginine influx via the cationic amino acid transporters by addition of L-lysine, we find a 60-70% inhibition of arginase activity without significant loss of NOS-2 activity. Addition of L-valine, as an arginase inhibitor, leads to a 25% increase in NO formation and an 80-90% decrease in arginase activity. Interestingly, product inhibition of arginase and competitive inhibition of CATs through the addition of L-ornithine leads to a highly significant increase in hepatocytic NOS-2 activity with a concomitant and complete abolishment of its dependence on extracellular L-arginine concentrations. In conclusion, hepatocytic NOS-2 activity shows a surprising pattern of dependence on exogenous L-arginine concentrations. Inhibition and competition experiments suggest a relatively tight link of NOS-2 and urea cycle activities. These data stress the hypothesis of a metabolon-like organization of the urea cycle together with NOS-2 in hepatocytes as excess L-ornithine will be metabolized to l-arginine and thereby increases NO production.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Transport Systems, Basic / metabolism
  • Animals
  • Arginase / antagonists & inhibitors
  • Arginase / metabolism*
  • Arginine / metabolism
  • Arginine / pharmacology*
  • Base Sequence
  • Cells, Cultured
  • Cytokines / pharmacology
  • Dose-Response Relationship, Drug
  • Enzyme Inhibitors / pharmacology
  • Hepatocytes / drug effects*
  • Hepatocytes / enzymology
  • Hepatocytes / metabolism
  • Inflammation / pathology
  • Lysine / pharmacology
  • Male
  • Nitric Oxide / metabolism
  • Nitric Oxide Synthase Type II / metabolism*
  • Ornithine / pharmacology
  • Rats
  • Rats, Wistar
  • Urea / metabolism*
  • Valine / pharmacology


  • Amino Acid Transport Systems, Basic
  • Cytokines
  • Enzyme Inhibitors
  • Nitric Oxide
  • Urea
  • Arginine
  • Ornithine
  • Nitric Oxide Synthase Type II
  • Arginase
  • Valine
  • Lysine