Specific hemin catalyzed low density lipoprotein ocidation reactions: implications for metabolic and inflammatory diseases

Oxidation of LDL and lipids by transition metal catalyzed processes is a key factor in atherogenesis. Clinical and experimental evidence suggests that these processes involve binding of hemin, a product of hemoglobin degradation, to descrete binding sites of LDL thus forming centers for redox cycling and repeated radical production. The mechanisms for these observations remain unclear. In the present study, we found that hemin binds rapidly to low density lipoprotein subfractions (LDL1, 1.019-1.044 kg/L; LDL2, 1.044-1.063 kg/L) with binding rates in the nM range. Spectrosphotometric and fluorescence experinemts indicated that the amphiphilic hemin molecule is buried in the lipoprotein surface monolayer with the carboxylic groups in contact with positively charged surface regions. In vitro, in the presence of H2O2, hemin oxidizes both LDL1 and LDL2 with formation of specific products of oxidation of positively charged protein amino acid residues (gamma-glutamyl semialdehyde and alpha-aminoadipic semialdehyde) and phospholipids (1-palmitoyl-2-oxovaleroyl-sn-glycero-3-phosphorylcholine). The formation of these products was more than twofold higher (p<0.01) when compared with oxidation systems containing free iron or copper. Hemin catalyzed LDL oxidation was inhibited by the iron-chelating agents 1,2-Dimethyl-3-hydroxypyrid-4-one and N,N-bis(2-hydroxybenzyl)ethylenediamine-N,N-diacetic acid, respectively. In vivo, a significantly increased formation of all three specific oxidation products could be confirmed in circulating plasma LDL1 and LDL2 particles obtained from patients with impaired glucose tolerance (p<0.05), Type 2 diabetes mellitus (p<0.05), and rheumatoid arthritis (p<0.01) when compared with healthy controls. The rsults provide further evidence on pathophysiological relevance of hemin catalyzed LDL oxidation in metabolic an inflammatory diseases.