Prostanoids with cyclopentenone structure (cyP) display a potent anti-inflammatory and antiproliferative activity. CyP are reactive compounds, which may modulate cellular functions by multiple mechanisms, including the direct covalent modification of cysteine residues by Michael addition. This interaction displays selectivity since only a subset of cellular proteins is modified by cyP. Several factors have been proposed to influence the selectivity and/or extent of cyP addition to proteins, including determinants related to protein and cyP structure, and levels of cellular thiols, such as glutathione (GSH). Here we have explored the ability of biotinylated cyP analogs to modify several recombinant proteins in vitro, and the influence of GSH in these effects. We have observed that protein modification by cyP is protein- and cyP-selective. Under our conditions, biotinylated 15-deoxy-Delta(12,14)-PGJ(2) (15d-PGJ(2)-B) was more efficient than biotinylated PGA(1) (PGA(1)-B) at forming adducts with components of the transcription factors NF-kappaB and activator protein-1 (AP-1). However, both biotinylated cyP were nearly equipotent at modifying human GSTP1-1. Interestingly, the presence of GSH differentially modulated the formation of protein-cyP adducts. Under our conditions, GSH reduced the incorporation of cyP into GST, but improved their binding to p50, more intensely in the case of PGA(1)-B. These results evidence the importance of GSH-cyP and/or GSH-protein interactions for the selectivity of protein modification by cyP and suggest a complex role for GSH that may be related to its ability to prevent protein oxidation or induce conformational alterations. This may shed light on the factors involved in the pleiotropic effects of electrophiles with therapeutic potential.