Dynamic assessment of nitration reactions in vivo.

Assessment of protein nitration is commonly used as a footprint for the formation of reactive nitrogen species in vivo. However, one of the major disadvantages of measuring nitrotyrosine in proteins is that nitrated proteins are broken down at variable rates, and the resulting free nitrotyrosine is taken up by cells, metabolized, and excreted. We have discovered a biochemical pathway in which circulating para-hydroxyphenylacetic acid (PHPA) undergoes nitration to form 3-nitro-4-hydroxyphenylacetic acid (NHPA), which is rapidly excreted in the urine. Using various animal models, we have shown that measurement of urinary NHPA can be used to assess the formation of reactive nitrogen species in vivo.