Lack of antioxidant activity of the antiatherogenic compound L-arginine

L-Arginine, the physiologic substrate of nitric oxide synthase, has antiatherogenic properties in animal models of atherosclerosis, and improves endothelial function in hypercholesterolemic humans. Some of these effects may be mediated by increased production of nitric oxide; however, some investigators have postulated a direct antioxidant action related to its aminoguanidine moiety. We aimed therefore, to assess the antioxidant properties of L-arginine. The antioxidant properties of 200 microM L-arginine. 200 microM D-arginine and 200 microM L-glutamate were compared with the powerful antioxidant ascorbate and a control (phosphate-buffered saline). Compounds were tested using four in vitro methods: (1) the Esterbauer technique (which tests the ability of the compounds to scavenge free radicals or chelate transition metals); (2) the effect on the autoxidation of ascorbate; (3) anti-tocopherol mediated peroxidation (which tests the compound's ability to synergize with alpha-tocopherol to prevent mild chemically induced LDL oxidation); and (4) the ability of the compounds to attenuate alpha-tocopherol radical in micellular emulsions (TRAA). The above methods were repeated using the metabolites of the test compounds after incubation with human endothelial cells. Ex vivo studies were then carried out by measuring levels of lipid peroxide production (using HPLC with UV and chemiluminescence detection) in three healthy volunteers before and 2 h after a single 7-g oral dose of L-arginine. By the Esterbauer technique, L-arginine increased lag time by 45% compared to control, as did D-arginine by 50%; L-glutamate had no effect and ascorbate increased lag time by 325%. Neither L-arginine, D-arginine or L-glutamate had significant effects on the autoxidation of ascorbate or anti-tocopherol mediated peroxidation. By the TRAA method, L-arginine had a small effect on preventing the decay of tocopherol. The results were similar for the studies of the compound's metabolites. In ex vivo studies, no changes were seen in lipid peroxide levels following acute dosage with L-arginine. L-Arginine has only weak and non-specific antioxidant effects, suggesting that its major cardioprotective benefits occur through other mechanisms, such as via the nitric oxide pathway.