The chemistry of acetaldehyde-protein adducts

Acetaldehyde reacts with a variety of proteins to form both stable and unstable adducts. Unstable adducts are readily reversible and are largely represented by Schiff bases. Stable adducts are essentially irreversible products that are characterized by their resistance to various treatments. The structures of stable adducts have not been established, but it appears that the epsilon-amino group of certain lysines and the amino group of N-terminal amino acids participate in aldehyde binding. Stable adducts can form in the absence of a reducing agent, such as sodium cyanoborohydride, and are distinctly different than the ethylated amino groups formed under reductive conditions. Upon prolonged incubation, stable binding of acetaldehyde results in the formation of fluorescent products, and Schiff bases serve as intermediates of these advanced stable products. It is likely that several acetaldehyde molecules and perhaps at least two amino groups participate in the formation of these stable fluorescent products. Such a reaction mechanism could account for both inter-and intra-molecular cross-linking of proteins. Furthermore, it is likely that stable adducts are represented by multiple products whose structures may vary, depending upon the particular target protein. A reaction mechanism involving an aldol condensation and generation of a crotonaldehyde Schiff base is proposed as an essential step in stable adduct formation.