Oxalomalate affects the inducible nitric oxide synthase expression and activity

Inducible nitric oxide synthase (iNOS) is an homodimeric enzyme which produces large amounts of nitric oxide (NO) in response to inflammatory stimuli. Several factors affect the synthesis and catalytic activity of NOS. Particularly, dimerization of NOS monomers is promoted by heme, whereas an intracellular depletion of heme and/or L-arginine considerably decreases NOS resistance to proteolysis. In this study, we found that oxalomalate (OMA, oxalomalic acid, alpha-hydroxy-beta-oxalosuccinic acid), an inhibitor of both aconitase and NADP-dependent isocitrate dehydrogenase, inhibited nitrite production and NOS protein expression in lipopolysaccharide (LPS)-activated J774 macrophages, without affecting NOS mRNA content. Furthermore, injection of OMA precursors to LPS-stimulated rats also decreased nitrite production and NOS expression in isolated peritoneal macrophages. Interestingly, alpha-ketoglutarate or succinyl-CoA administration reversed OMA effect on NO production, thus correlating NO biosynthesis with the anabolic capacity of Krebs cycle. When protein synthesis was blocked by cycloheximide in LPS-activated J774 cells treated with OMA, NOS protein levels, evaluated by Western blot analysis and 35 S-metabolic labelling, were decreased, suggesting that OMA reduces NOS biosynthesis and induces an increase in the degradation rate of NOS protein. Moreover, we showed that OMA inhibits the activity of the NOS from lung of LPS-treated rats by enzymatic assay. Our results, demonstrating that OMA acts regulating synthesis, catalytic activity and degradation of NOS, suggest that this compound might have a potential role in reducing the NO overproduction occurring in some pathological conditions. (c) 2007 Elsevier Inc. All rights reserved.