The biological relevance of tyrosine nitration is a subject of much interest, because extensive evidence supports formation of 3-nitrotyrosine in vivo under a variety of different pathological conditions. Several reagents are likely to be responsible for nitration in vivo, among others peroxynitrite and nitrite in the presence of H(2)O(2)/peroxidases. In this work we show that also metmyoglobin and methemoglobin can nitrate free tyrosine in the presence of nitrite and H(2)O(2). The results of these studies are simulated rather well by using a scheme that comprehends all the possible reactions that can take place in the system. Thus, a good understanding of the factors that determine the yields is achieved. Finally, we demonstrate that the system metMb/H(2)O(2)/NO(2)(-) can also lead to the nitration of tryptophan and produces, in particular, 6-, 4-, and 5-nitrotryptophan.