The myeloperoxidase system of human phagocytes generates N-epsilon-(carboxymethyl)lysine on proteins: a mechanism for producing advances glycation end products at sites of inflammation

Reactive aldehydes derived from reducing sugars and peroxidation of lipids covalently modify proteins and may contribute to oxidative tissue damage. W e recently described another mechanism for generating reactive aldehydes fr om free alpha-amino acids. The pathway begins with myeloperoxidase, a heme enzyme secreted by activated neutrophils. Conversion of alpha-amino acids t o aldehydes requires hypochlorous acid (HOCl), formed from H2O2 and chlorid e by myeloperoxidase. When L-serine is the substrate, HOCl generates high y ields of glycolaldehyde. We now demonstrate that a model protein, ribonucle ase A (RNase A), exposed to free L-serine and HOCl exhibits the biochemical hallmarks of advanced glycation end (AGE) products - browning, increased f luorescence, and cross-linking. Furthermore, N-epsilon-(carboxymethyl)lysin e (CML), a chemically well-characterized AGE product, was generated on RNas e A when it was exposed to reagent HOCl-serine, the myeloperoxidase-H2O2-ch loride system plus L-serine, or activated human neutrophils plus L-serine. CML production by neutrophils was inhibited by the H2O2 scavenger catalase and the heme poison azide, implicating myeloperoxidase in the cell-mediated reaction. CML was also generated on RNase A by a myeloperoxidase-dependent pathway when neutrophils were activated in a mixture of amino acids. Under these conditions, we observed both L-serine-dependent and L-serine-indepen dent pathways of CML formation. The in vivo production of glycolaldehyde an d other reactive aldehydes by myeloperoxidase may thus play an important pa thogenic role by generating AGE products and damaging tissues at sites of i nflammation.