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 iNOS. 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, α-hydroxy-β-oxalosuccinic acid), an inhibitor of both aconitase and NADP-dependent isocitrate dehydrogenase, inhibited nitrite production and iNOS protein expression in lipopolysaccharide (LPS)-activated J774 macrophages, without affecting iNOS mRNA content. Furthermore, injection of OMA precursors to LPS-stimulated rats also decreased nitrite production and iNOS expression in isolated peritoneal macrophages. Interestingly, α-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, iNOS protein levels, evaluated by Western blot analysis and 35S-metabolic labelling, were decreased, suggesting that OMA reduces iNOS biosynthesis and induces an increase in the degradation rate of iNOS protein. Moreover, we showed that OMA inhibits the activity of the iNOS from lung of LPS-treated rats by enzymatic assay. Our results, demonstrating that OMA acts regulating synthesis, catalytic activity and degradation of iNOS, suggest that this compound might have a potential role in reducing the NO overproduction occurring in some pathological conditions.