Oxidative modifications of amino acids in proteins can serve to regulate enzyme activity. This emerging field of redox regulation is related to other cellular signaling pathways, however, neither the chemical mechanisms in the cellular environment nor the affected metabolic and physiological changes are well understood. From data on endotoxin action in vascular tissue and reports on thiol modifications and tyrosine nitrations a unified scheme with five key components is proposed, governed solely by variations in the fluxes of nitrogen monoxide (NO) and superoxide (O(2)(-)). Crucial to the interactions is the formation of peroxynitrite which at concentrations of 10(-9)-10(-6)M elicits events like activation of prostanoid formation, metal catalyzed nitrations and two electron oxidations at cysteines and methionines. As a new concept we postulate that peroxynitrite formed in situ from NO and O(2)(-) is in rapid equilibrium with excess NO to form a nitrosating species that transfers NO(+). The resulting S-nitrosations occur prior to oxidative peroxynitrite action and seem to be involved in the down-regulation of reductive pathways. As the flux of O(2)(-) exceeds the one of NO, cellular damage develops induced by one-electron oxidations caused by nitrogen dioxide and by the Fenton reaction.