Abstract 413: Myeloperoxidase-mediated Methionine Oxidation Renders Apolipoprotein A-I Amyloidogenic

Abstract

Background Elevated plasma levels of apolipoprotein A-I (apoA-I) correlate with cardiovascular health, while dysfunctional apoA-I promotes atherosclerosis. The high concentration of myeloperoxidase (MPO) produced by activated macrophages in the atherosclerotic lesions generates an oxidative environment that impacts apoA-I and reduces its antiatherogenic functions. Recent clinical studies revealed that amyloid deposition in the atherosclerotic plaques increases with aging. Interestingly, apoA-I was identified as a central component of these atherosclerosis-related amyloids. Hypothesis In this study, we tested the hypothesis that MPO-mediated oxidation of apoA-I can produce protein variants that are structurally destabilized and amyloidogenic. Methods Plasma apoA-I purified from normal individuals was exposed to chemical (hydrogen peroxide) and enzymatic (MPO-H 2 O 2 -chloride system) oxidation. Oxidized apoA-I was incubated at 37 °C and pH 6.0 to induce fibril formation. Results While no fibril formation was detected in intact apoA-I, both chemically and enzymatically oxidized apoA-I produced fibrillar amyloids after 12 h incubation at 37 °C. A large molar excess (~1000:1) of H 2 O 2 was needed to generate apoA-I fibrils, whereas exposure of apoA-I to a 1:1 oxidant:apoA-I molar ratio in the presence of MPO was sufficient to confer amyloidogenic properties to the protein. Analysis by gel electrophoresis, CD spectroscopy and mass spectrometry of soluble oxidized protein and of isolated fibrils indicate that no significant protein degradation occurred upon oxidation (both chemical and enzymatic) or fibril formation. The amyloid fibrils were composed of full-length apoA-I with all three native methionines converted to methionine sulfoxide. Conclusions In contrast to hereditary amyloidosis wherein specific mutations of apoA-I cause protein destabilization and amyloid deposition, oxidative conditions similar to those promoted by local inflammation in atherosclerosis are sufficient to transform full-length wild-type apoA-I into an amyloidogenic protein. Thus MPO-mediated oxidation may be implicated in the mechanism that leads to amyloid fibril deposition in the atherosclerotic plaques in vivo .