who showedthat rat liver cytosolic fractions lose glutaredoxin activitywhen treated with peroxynitrite. Considering the potentialrole of S-glutathionylation in signaling, inactivation ofglutaredoxins by reactive nitrogen species deserves detailedstudy.We report here that peroxynitrite among the nitric oxidecongeners is a very effective inactivator of purified Grx1 andGrx2. Inactivated glutaredoxins are not reactivated by gluta-thione. Glutathione, however, protects glutaredoxins fromperoxynitrite-induced inactivation.