Formation and Stability of Pyrrole Adducts in the Reaction of Levuglandin E2 with Proteins

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Abstract
Levuglandin (LG) E2 is rapidly sequestered by covalent binding with proteins. The reaction of LGE2 with a protein in neutral aqueous solution exhibits two phases. A metastable adduct rapidly accumulates initially. In the second phase, a protein-bound pyrrole is generated. Pyrrole formation and stability were monitored with an immunoassay using antibodies that were raised against a stable isostere. That LG-derived pyrroles are the major products (> 76%) of the LGE2-protein reaction is suggested by the level of antibody binding found for LG-protein adducts compared with that found for a pyrrole derived from LGE2 and 6-amino-1-hexanol. Because the initial metastable LG-protein adduct is a reactive electrophile, it can be trapped with amines, such as glycine, to give stable ternary adducts that do not cross-react with the antibodies. Although highly alkylated pyrroles are chemically sensitive compounds, the protein-bound LG-derived pyrrole appears to be stable in aqueous solution at pH 7.4. Thus, it shows no decrease in immunoreactivity over several weeks. This discovery leads to the expectation that such pyrroles will accumulate in vivo, especially in proteins that do not turn over rapidly. Thus, the LG-derived protein-bound pyrrole may be a useful marker of oxidative lipid damage, and an immunoassay for this post-translational protein modification can be exploited as a mild, sensitive method for detecting and quantifying the generation of LGs in chronic inflammatory states.

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