Pyrraline Ether Crosslinks as a Basis for Protein Crosslinking by the Advanced Maillard Reaction in Aging and Diabetes

Pyrraline (ϵ-2-(formyl-5-hydroxymethyl-pyrrol-1-yl)-L-norleucine) is an advanced Maillard reaction product derived from the reaction of glucose with lysine amino group on proteins. Its presence in plasma and tissue proteins has been established by immunological and chromatographic methods. The purified preparation of pyrraline obtained from the reaction of glucose with lysine when stored at room temperature or at refrigeration turned pink in color, suggesting spontaneous formation of degradation products. These products were analyzed by high-performance liquid chromatography and one of the products was isolated to purity. The structure of the compound was established to be a dipyrraline formed by an ether bond between two pyrraline molecules. This finding led us to investigate the reactivity of pyrraline with thiol and hydroxy amino acids. The hydroxy amino acids were in general nonreactive, except hydroxy lysine and hydroxy proline which formed minor condensation products. While the reaction of cysteine resulted in the formation of two distinct thioethers, the reaction of glutathione resulted in the formation of two major unidentified compounds which gradually degraded upon incubation. These data suggest that pyrraline formedin vivocan further react with other amino acids on proteins to form crosslinks, which may explain in part increased protein crosslinking associated with aging and diabetes.