Oxygenation of lipoproteins by mammalian lipoxygenases

Abstract

Oxidative modification converts low-density lipoprotein (LDL) into its atherogenic form and appears to be a necessary precondition for LDL uptake by macrophages during foam cell formation. Cellular lipoxygenases have been implicated in this process. We studied the interaction of purified mammalian lipoxygenases with human LDL in v i m and found that the arachidonate 15-lipoxygenases of rabbit and man are capable of oxygenating lipoproteins as indicated by oxygen uptake and by the formation of thiobarbituric-acid-reactive substances. Furthermore, oxygenated polyenoic fatty acids, such as 13-hydro(pero)xy-9Z, 11 E-octadecadienoic acid and 15-hydro(pero)xy-5,8,11,13 (Z,Z, Z,E)-eicosatetraenoic acid were detected in the lipid compartment of various lipoproteins classes after lipoxygenase treatment. More than 90% of the oxygenated polyenoic fatty acids were found in the ester-lipid fraction, particularly in the cholesterol esters, whereas only small amounts of free hydro(pero)xy polyenoic fatty acids were detected. Lipoxygenase-catalyzed oxygenation of LDL is not restricted to the lipid compartment but also leads to a cooxidative modification of the apoproteins as indicated by changes in the electrophoretic mobility and by the formation of carbonyl derivatives of amino acid side chains. The possible biological significance of lipoxygenase-induced oxidative modification of lipoproteins in the pathogenesis of atherosclerosis is discussed.Atherosclerosis is a multifactoral disease, the pathogenesis of which is not fully understood [l-31. It has been shown by morphological studies that the accumulation of lipid-loaded foam cells in the subendothelial space leads to the formation of fatty streaks, which are generally accepted as the early atherosclerotic lesions [l -41. Foam cells develop from monocyte-derived macrophages [5, 61 or from smooth muscle cells [7] by taking up modified low-density lipoproteins (LDL) via scavenger-receptor(s)-mediated pathways. The occurrence of oxidatively modified proteins in atherosclerotic lesions [8, 91 and the fact that oxidatively modified LDL is rapidly taken up by macrophages [lo] suggested an involvement of oxidative processes in the pathogenesis of atherosclerosis. However, the mechanism of oxidative modification in vivo remains unclear. In-vitro studies with cell-free systems indicated that copper-mediated oxidation converts LDL into its atherogenic form [ll] The oxygenation of lipoproteins by mammalian lipoxygenases has not been studied so far. The soybean lipoxygenase which largely differs from mammalian lipoxygenases with respect to its protein chemical and enzymic properties is capable of oxidizing LDL only in the presence of phospholipase A, which provides free polyenoic fatty acids by cleaving lipoprotein phospholipids [20]. This result is not surprising since the soybean lipoxygenase has been shown to effectively oxygenate ester lipids only in the presence of detergents [21]. However, the rabbit reticulocyte lipoxygenase is capable of oxygenating complex ester lipids such ...

The N-terminal Domain of the Reticulocyte-type 15-Lipoxygenase Is Not Essential for Enzymatic Activity but Contains Determinants for Membrane Binding

“…The N-terminal ␤-Barrel Is Not Required for Membrane Binding-The native 15-LOX effectively interacts with biomembranes and lipoproteins (27,28), and here we confirmed this finding for the His-tagged wild-type enzyme. This enzyme species exhibited a membrane oxygenase activity of 0.066 s Ϫ1 using submitochondrial particles as model membrane.…”

“…In contrast, the reticulocyte-type 15-LOXs are capable of oxygenating complex ester lipids (24,25) even if they are bound in biomembranes (26,27) or lipoproteins (28,29). In fact, the oxygenation of membrane lipids was implicated in membrane degradation and/or remodeling during erythropoiesis (30,31) and LOXinduced oxidation of low density lipoprotein was suggested to be involved in the pathogenesis of atherosclerosis (32).…”

Investigations into Calcium-dependent Membrane Association of 15-Lipoxygenase-1

“…Membrane Binding-It has been reported before that the rabbit 15-LOX is capable of oxygenating biomembranes (27,28) and lipoproteins (29) and that calcium ions are required for effective membrane oxygenation (23). More recently, two clusters of hydrophobic amino acids have been implicated in membrane binding (24).…”

Oxidation of Low Density Lipoproteins Greatly Enhances Their Association with Lipoprotein Lipase Anchored to Endothelial Cell Matrix

“…The process of LDL oxidation is not very well understood, but it appears to occur within the artery wall, facilitated by cellular lipoxygenase-dependent mechanisms or by nonenzymatic pathways. The phospholipid fatty acids of LDL are believed to be oxidatively modified first (25,26), resulting in the formation of phospholipids containing a spectrum of peroxidized fatty acids. Some of these, through chemical decomposition and/or enzymic hydrolysis, are believed to generate reactive aldehydes, which covalently modify the lysine residues of apoB and cause the surface charge to become more negative (27).…”