Modeling gas phase nitric oxide release in lung epithelial cells

Pharmacokinetic-Pharmacodynamic Model for the Effect of l-Arginine on Endothelial Function in Patients with Moderately Severe Falciparum Malaria

Impaired organ perfusion in severe falciparum malaria arises from microvascular sequestration of parasitized cells and endothelial dysfunction. Endothelial dysfunction in malaria is secondary to impaired nitric oxide (NO) bioavailability, in part due to decreased plasma concentrations of l-arginine, the substrate for endothelial cell NO synthase. We quantified the time course of the effects of adjunctive l-arginine treatment on endothelial function in 73 patients with moderately severe falciparum malaria derived from previous studies. Three groups of 10 different patients received 3 g, 6 g, or 12 g of l-arginine as a half-hour infusion. The remaining 43 received saline placebo. A pharmacokinetic-pharmacodynamic (PKPD) model was developed to describe the time course of changes in exhaled NO concentrations and reactive hyperemia-peripheral arterial tonometry (RH-PAT) index values describing endothelial function and then used to explore optimal dosing regimens for l-arginine. A PK model describing arginine concentrations in patients with moderately severe malaria was extended with two pharmacodynamic biomeasures, the intermediary biochemical step (NO production) and endothelial function (RH-PAT index). A linear model described the relationship between arginine concentrations and exhaled NO. NO concentrations were linearly related to RH-PAT index. Simulations of dosing schedules using this PKPD model predicted that the time within therapeutic range would increase with increasing arginine dose. However, simulations demonstrated that regimens of continuous infusion over longer periods would prolong the time within the therapeutic range even more. The optimal dosing regimen for l-arginine is likely to be administration schedule dependent. Further studies are necessary to characterize the effects of such continuous infusions of l-arginine on NO and microvascular reactivity in severe malaria.

Competitive metabolism of L-arginine: arginase as a therapeutic target in asthma

Exhaled breath nitric oxide (NO) is an accepted asthma biomarker. Lung concentrations of NO and its amino acid precursor, L-arginine, are regulated by the relative expressions of the NO synthase (NOS) and arginase isoforms. Increased expression of arginase I and NOS2 occurs in murine models of allergic asthma and in biopsies of asthmatic airways. Although clinical trials involving the inhibition of NO-producing enzymes have shown mixed results, small molecule arginase inhibitors have shown potential as a therapeutic intervention in animal and cell culture models. Their transition to clinical trials is hampered by concerns regarding their safety and potential toxicity. In this review, we discuss the paradigm of arginase and NOS competition for their substrate L-arginine in the asthmatic airway. We address the functional role of L-arginine in inflammation and the potential role of arginase inhibitors as therapeutics.

Impact of substrate protonation and tautomerization states on interactions with the active site of arginase I

Human arginase is a binuclear manganese metalloenzyme that participates in the urea cycle. Arginase catalyzes the hydrolysis of L-arginine into L-ornithine and urea and is linked to several disorders such as asthma and cancer. Currently, the protonation and tautomerization state of the substrate when bound to the active site, which contains two manganese ions, is not known. Knowledge of the charge-dependent behavior of arginine in the arginase I environment would be of utility toward understanding the catalytic mechanism and designing inhibitors of this enzyme. The arginine(+/0) species, including all possible neutral tautomers, were modeled using an aminoimidazole analog as template. All-atom molecular dynamics simulations were then performed on each of the charged and neutral species. In addition, a hydroxide ion was included in selected simulations to test its importance. Results show that the positively charged state of arginine is stable in the active site of arginase I, with that stabilization facilitated by the presence of hydroxide. Glu277 is indicated to play a role in stabilizing arginine in the active site and facilitating its ability to assume a catalytically competent conformation in the presence of hydroxide. The reported interactions and modeled arginine-bound arginase I structures can be used as a tool for structure-based inhibitor design, as experimental data on the structure of the substrate-enzyme complex is lacking.