ARGINASE MODULATES NITRIC-OXIDE PRODUCTION IN ACTIVATED MACROPHAGES

In macrophages and many other cell types, L-arginine is used as a subs trate by both nitric oxide synthase (NOS) and arginase to produce nitr ic oxide (NO) and urea, respectively. Because the availability of L-ar ginine is a major determinant for NO synthesis in the activated macrop hage, we hypothesized that NO production may be reduced by arginase vi a depleting the common substrate in this cell type. To test this hypot hesis, we investigated the effect of an arginase inhibitor, L-norvalin e, on NO production in J774A.1 mouse macrophages activated by lipopoly saccharide (LPS, 1.0 mu g/ml) for 22 h. In the absence of LPS, macroph ages produced a low level of NO. In contrast, NO production from these cells was significantly increased in the presence of LPS. Increasing extracellular levels of L-arginine (0.01-0.8 mM) produced a concomitan t increase in NO production of activated macrophages. L-Norvaline (10 mM), which specifically inhibits arginase activity (i.e., reducing ure a production by 50%) without altering NOS activity, enhanced NO produc tion (by 55%) from activated macrophages. The enhancement of NO produc tion by L-norvaline was inversely related to the extracellular level o f L-arginine. A more pronounced increase in NO production was observed at the lower level of extracellular L-arginine, i.e., a 55 vs. 28% in crease for 0.05 and 0.1 mM extracellular L-arginine, respectively. Whe n the L-arginine concentration exceeded 0.5 mM, the L-norvaline effect was abolished. These results indicate that arginase can compete with NOS for their common substrate and thus inhibit NO production. This re gulatory mechanism may be particularly important when the extracellula r supply of L-arginine is limited.