Abstract 808: Myeloperoxidase Catalyzed Protein Carbamylation: A Mechanism Linking Smoking, Inflammation And Atherogenesis
Post-translational modification of proteins through carbamylation, a process thought to predominantly occur in patients with end stage renal disease, is implicated in the “toxemia of uremia”. Cyanate, an electrophilic species in equilibrium with urea, reacts with protein lysine residues to produce ϵ-carbamyl-lysine (homocitrulline, HCit). Herein we report the discovery of an alternative and quantitatively dominant mechanism for cyanate formation and protein carbamylation in vivo, myeloperoxidase (MPO)-catalyzed oxidation of the pseudohalide thiocyanate (SCN−), an anion in blood whose levels are markedly elevated in smokers. Both in vitro studies with human MPO, and in vivo studies with MPO-KO mice and inflammation models demonstrate that protein carbamylation, as monitored by HCit formation, is catalyzed by MPO using physiological concentrations of SCN− as substrate. Immunohistochemical studies confirm co-localization of MPO and protein carbamylation epitopes within human atherosclerotic lesions. MPO-carbamylation of low density lipoprotein (LDL) is shown to promote multiple pro-atherosclerotic activities, including promotion of vascular smooth muscle proliferation and impairment of LDL recognition by the LDL receptor. Among subjects undergoing diagnostic cardiac catheterization (N=600), protein-bound HCit levels dose-dependently predicted increased risk for cardiovascular disease, independent of Framingham Risk Score, estimated GFR (by MDRD), and CRP. While SCN− levels were markedly increased among smokers (P<0.01), elevated SCN− levels did not predict increased CVD risks. In contrast, compared with the lowest quartile, subjects with the highest HCit quartile were 8.1-fold (95%CI 4.1–16.3) more likely to have CAD and 7.0-fold (95%CI 2.9 –16.7) more likely to have PAD. Subjects with the highest HCit quartile were also 5.8-fold (95%CI 1.9 –17.1) more likely to die and 4.7-fold (95%CI 2.2–9.8) more likely to experience a major adverse cardiac event (death, non-fatal myocardial infarction or stroke). MPO-catalyzed carbamylation thus represents a novel biochemical pathway serving as a potential mechanistic link between inflammation, smoking and cardiovascular disease pathogenesis.