Inhibition of GTP binding to Rac2 by peroxynitrite: potential role for tyrosine modification

Peroxynitrite is a potent oxidant generated by the reaction of nitric oxide (*NO) and superoxide anion (O2*-), and both can be produced in inflammatory tissues. In the present studies, we analyzed the effects of peroxynitrite treatment on the GTP-binding activity of Rac2, a low molecular weight GTP-binding protein important in regulating a number of cellular functions. Using a fluorescent analog of GTP (methylanthraniloyl guanosine triphosphate or mant-GTP) as a reporter group, we found that treatment of Rac2 with peroxynitrite inhibited the binding of mant-GTP to Rac2 in a dose-dependent manner. Peroxynitrite was also able to react directly with free mant-GTP, resulting in a significant decrease in mant-GTP fluorescence; however, the mechanism of peroxynitrite-mediated damage to mant-GTP was different than with Rac2. In the case of mant-GTP, protection from peroxynitrite-mediated oxidation was observed in the presence of the free radical scavengers, mannitol and DMTU. In contrast, DMTU was unable to prevent peroxynitrite-mediated inhibition of mant-GTP binding to Rac2. Instead, our data demonstrates a role for peroxynitrite-mediated tyrosine modification in the inhibition of mant-GTP binding to Rac2, and we were able to demonstrate the formation of a significant level of nitrotyrosine formation in Rac2 exposed to peroxynitrite. Thus, our studies support the premise that oxidative modification of key cellular proteins, such as Rac2, plays an important role in the cytotoxic effects observed for peroxynitrite and other reactive oxidants.