Selective oxidation of methionine residues in apolipoprotein A-I and its potential biological consequences

The earliest stages of HDL oxidation are accompanied by the oxidation of specific Met residues in apolipoprotein AI and AII and the formation of Met sulfoxides (Met(O)) has been proposed to play a significant role in the reduction and hence detoxification of lipid hydroperoxides associated with HDL. Oxidation of HDL may generally decrease the anti-atherogenic properties of this lipoprotein, although both, the inhibition and the enhancement of cholesterol removal from cells has been reported for different types of oxidation. In light of these findings we have investigated the secondary structure, lipid affinity, LCAT activation and cholesterol-efflux promoting properties of native and selectively oxidized apo A-I(apo A-I+32, containing Met(O) at Metl12 and Metl48) in purified or reconstituted forms. Data obtained by circular dichroism revealed that selective oxidation of Met residues 112 and 148 does not alter alpha helicity of the protein in solution, indicating that this oxidation is not sufficient to influence significantly this type of secondary structure of apo A-I in its 'lipid-free' form. The lipid affinity of native apo A-I and apo A-I+32 was determined as the rate of clearance of DMPC multilamellar to small unilamellar vesicles. Compared with the native protein, apo A-I+32 induced a 2-3 fold faster rate of clearance, suggesting that the increased hydrophilicity due Met(O) increased the rate for protein-lipid interactions. Met residues 112 and 148 reside in the hydrophobic faces of helices 5 and 7, and both these regions have been suggested to be important for both, LCAT activation and cholesterol efflux. Kinetic experiments have revealed that the affinity for LCAT is comparable for HDL reconstituted with either apo A-I or apo A-I+32. Efflux of [3H]-cholesterol from lipid-laden human monocytederived macrophages to isolated apolipoproteins was enhanced for apo A-I+32 compared with apo A-I, consistent with the DMPC clearance data. Together these findings suggest that selective oxidation of Met residues of apo A-I may enhance rather than diminish known antiatherogenic activities of the apolipoprotein. Thus, our results are consistent with the overall hypothesis that detoxification of lipid hydroperoxides by Met residues of apo A-I is potentially antiatherogenic