Differences in Lysine Adduction by Acrolein and Methyl Vinyl Ketone: Implications for Cytotoxicity in Cultured Hepatocytes

Abstract

Acrolein is a highly toxic environmental pollutant that readily alkylates the epsilon-amino group of lysine residues in proteins. In model systems, such chemistry involves sequential addition of two acrolein molecules to a given nitrogen, forming bis-Michael-adducted species that undergo aldol condensation and dehydration to form Nepsilon-(3-formyl-3,4-dehydropiperidino)lysine. Whether this ability to form cyclic adducts participates in the toxicity of acrolein is unknown. To address this issue, we compared the chemistry of protein adduction by acrolein to that of its close structural analogue methyl vinyl ketone, expecting that the alpha-methyl group would hinder the intramolecular cyclization of any bis-adducted species formed by methyl vinyl ketone. Both acrolein and methyl vinyl ketone displayed comparable protein carbonylating activity during in vitro studies with the model protein bovine serum albumin, confirming the alpha,beta,-unsaturated bond of both compounds is an efficient Michael acceptor for protein nucleophiles. However, differences in adduction chemistry became apparent during the use of electrospray ionization-MS to monitor reaction products in a lysine-containing peptide after modification by each compound. For example, although a Schiff base adduct was detected following reaction of the peptide with acrolein, an analogous species was not formed by methyl vinyl ketone. Furthermore, while ions corresponding to mono- and bis-Michael adducts were detected at the N-terminus and lysine residues following peptide modification by both carbonyls, only acrolein modification generated ions attributable to cyclic adducts. Despite these differences in adduction chemistry, in mouse hepatocytes, the two compounds exhibited very comparable abilities to induce rapid, concentration-dependent cell death as well as protein carbonylation. These findings suggest that the acute toxicity of short-chain alpha,beta-unsaturated carbonyl compounds involves their ability to form acyclic Michael addition adducts rather than Schiff conjugates or heterocyclic adducts.