The alpha‐amino group of l‐arginine mediates its antioxidant effect

Antioxidant effects may constitute part of the possible antiatherogenic effects of the amino acid l‐arginine. These antioxidant properties were further characterized in a model of lipoprotein oxidation.Oxidation of lipoproteins in unfractionated human serum was continuously monitored by a fluorescent probe. The antioxidant effects of l‐arginine, N‐α‐acetyl‐arginine and vitamin E in combination with l‐arginine were measured after initiation of free radical generation with either copper or 2,2′‐azobis(2‐amidinopropane) hydrochloride (AAPH). The half‐time of the fast propagation rate for copper‐induced lipoprotein oxidation increased after incubation with l‐arginine in a dose‐dependent manner (P N‐α‐acetyl‐arginine did not show such effects. Vitamin E and l‐arginine show different effects on copper‐induced oxidation, the former increasing only lag‐time, the latter increasing only propagation rate, and do not have reciprocal effects. In contrast to copper‐induced oxidation, l‐arginine increased the lag‐time of AAPH‐induced lipoprotein oxidation (P N‐α‐acetyl‐arginine did not show any antioxidant effects.Our experiments provide further evidence that mechanisms other than serving as a substrate for the NO‐synthase could be involved in the antiatherosclerotic effect of l‐arginine. In addition, our experiments clearly show, that the antioxidant effect of l‐arginine is due to a chemical moiety different from that serving as the substrate for NO biosynthesis.