It has been shown that diabetic patients have up to three-fold increases in plasma nitrated tyrosine. We hypothesize that nitration of plasminogen could impair its catalytic properties and be a factor in diabetic thrombogenicity. To test this hypothesis, in this study we addressed the effects of the peroxynitrite donor 3-morpholinosydnonimine (SIN-1) on human streptokinase-induced plasmin activity. Given the link between glycation and oxidation we also explored whether peroxynitrite enhances the effect of fructose (1-5 mmol/l) and glucose (5-50 mmol/l) on plasminogen. We provide evidence that plasminogen, but not antithrombin III, is quickly inactivated by exogenously generated peroxynitrite (0-20 mmol/l SIN-1), in a time-and dose-dependent manner. The effect occurs even when the molar ratio of other plasma proteins and key antioxidants is respected. In our system, peroxynitrite did not enhance the effect of the sugars. Preincubation of the sugars with peroxynitrite also failed to produce any effect. This suggests that in conditions and times approaching the in vivo situation, plasminogen is more susceptible to peroxynitrite damage than to carbonyl damage. Plausibly, nitration of tyrosine should play a critical role in either conformational or functional changes. If proven in ulterior in vivo studies, this factor would provide another mechanism by which nitrosative stress participates in diabetic complications.