Increased serum N-hydroxy-L-arginine in patients with rheumatoid arthritis and systemic lupus erythematosus as an index of an increased nitric oxide synthase activity

Objectives—To determine the feasibility of monitoring the serum concentration of N-hydroxy-L-arginine (L-NHA) as an index of an increased nitric oxide (NO) synthase activity in patients with rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) compared with nitrate (NO3 ), the major circulating metabolite of NO whose concentration is influenced by dietary intake. Methods—The serum concentrations of L-NHA, L-arginine (L-Arg), and NO3 − were determined in 33 patients with RA, 25 patients with SLE and, 29 healthy subjects. Results—Serum L-NHA was significantly increased in RA patients with high disease activity (287% of control, p<0.01), but not with low disease activity (115%), as well as in patients with SLE (173%, p<0.01). In contrast, serum NO3 − did not diVer significantly between either group of patients and the respective control group. Conclusion—NO synthase activity or expression, or both, is upregulated in RA patients with high disease activity and in patients with SLE. Serum L-NHA seems to be a more specific and reliable index of an increased activity of this enzyme in patients with acute or chronic inflammatory diseases than NO3 . (Ann Rheum Dis 1997;56:330–332) There is now substantial evidence suggesting a role for nitric oxide (NO) in the pathogenesis of rheumathoid arthritis (RA). Increased production of nitrite (NO2 ) and nitrate (NO3 ), as an index of NO formation, has been reported in several animal studies of adjuvant induced arthritis, an experimental immunopathy that is believed to share many features with human RA. 2 Moreover, suppression of NO generation by in vivo administration of NO synthase inhibitors, such as N-monomethyl-Larginine (L-NMA), attenuates or inhibits the development of the disease, whereas supplementation with the NO precursor L-arginine (L-Arg) results in an exacerbation of the inflammatory process. 4 Increased concentrations of NO2 − or NO3 , or both, and nitrotyrosine have also been detected in the serum and synovial fluid 6 8 or urine of patients with RA. In this context, however, it is important to note that the single determination of the concentration of NO2 − does not provide a reliable index of endogenous formation of NO because of the rapid conversion into NO3 , the predominant circulating metabolite of NO in the body. Moreover, the concentration of both compounds is strongly influenced by dietary intake and hence derived to a large extent from NO synthase independent sources. In endotoxin treated rats, the NO synthase metabolite N-hydroxy-L-arginine (L-NHA) is released from NO producing cells and accumulates in the circulating blood, presumably because of a competition of the amino acid with the circulating L-Arg for the y amino acid transport system in vascular cells, hence limiting its metabolism. As there is no source other than NO synthase known to produce L-NHA in the body, we have proposed that monitoring the serum concentration of this amino acid may represent a specific and thus more reliable index of an increased NO synthase activity than serum NO2 − or NO3 , or both, in patients with acute or chronic inflammatory diseases. To substantiate this hypothesis, we have determined the concentration of L-NHA and L-Arg by high performance liquid chromatography (HPLC) analysis in serum samples from patients with RA and systemic lupus erythematosus (SLE) as well as in healthy subjects. In addition, we have determined the concentration of NO3 − in these serum samples by using the nitrate reductase assay.