To test whether Trx1(CSAAA) would also undergo authentic interactions under conditions more typical of an extracellular environment, we allowed Trx1(CSAAA) to react with human plasma proteins. To avoid nonspecific absorbance to high-abundance serum proteins, we applied Trx1(CSAAA) to a o30 kDa plasma ultrafiltrate, leading to the capture of a small number of proteins, as visualized by colloidal Coomassie staining ( Figure 1D ). Using LC-MS/MS, the principal interaction partner from the plasma ultrafiltrate was identified as peroxiredoxin-2 (Prx2), a well-established target protein of Trx1, recently found to be present in human plasma (Chen et al, 2004) . These experiments provided proof-of-principle evidence that kinetic trapping is capable of capturing and identifying proven target proteins of human Trx1 from both intra-and extracellular environments.