Proline Amide Catalyzes Formation of Toxic Crotonaldehyde from Acetaldehyde Under Physiologically Relevant Conditions

Crotonaldehyde is a human toxin that reacts with nucleophilic groups on DNA and proteins. Putative crotonaldehyde-derived adducts on DNA are reported in cells and patients after ethanol exposure, which implies that crotonaldehyde is formed in cells. Here, we show that proline amide, which is a model of N-terminal proline-containing proteins, catalyzes the aldol condensation of the ethanol metabolite acetaldehyde to crotonaldehyde under physiologically relevant conditions. This reaction is more efficient at neutral pH than under acidic or basic conditions, but is inhibited by competing imidazolidin-4-one formation. Crotonaldehyde formation is also slower than the analogous aldol condensation of propionaldehyde. Comparative studies additionally suggest that proline amide is a more efficient catalyst than other amino acid amides. Overall, the work evidences a biochemically plausible mechanism for intracellular crotonaldehyde formation and implies that proline amide derivatives can catalyze aldol chemistry in humans.