Factors affecting events during oxidation of low density lipoprotein: correlation of multiple parameters of oxidation

The present study shows that copper oxidation of LDL is a tightly-ordered process which can be finely controlled by appropriate selection of duration of oxidation and of concentrations of LDL and copper. Oxidation of LDL (0.1-2.0 mg LDL protein/ml) was carried out by copper catalysis (in the ratio of 2.5 microM Cu2+ to 0.1 mg LDL protein/ml) in phosphate-buffered saline, and was monitored by agarose gel electro-phoresis, gas chromatography (GC), anion exchange fast protein liquid chromatography (FPLC), fluorescence spectroscopy and dynamic light scattering. Analysis of the data showed strong cross correlations between many of the parameters of oxidation. Oxidation was more rapid for lower concentrations than for higher concentrations of LDL, despite the same ratio of copper to LDL being employed. Chemical kinetics analysis of the GC data suggested that 7beta-hydroxycholesterol formation occurred as a first order (or pseudo first order) consecutive reaction to the oxidation of linoleate. The first order rate constants for decomposition of linoleate and production of 7beta-hydroxycholesterol correlated closely with the theoretically-calculated times between collision of LDL particles. LDL particle diameter, measured by dynamic light scattering, increased by ca. 50% over 24 h oxidation, suggesting unfolding of apo B-100. Prolonged oxidation of LDL at low concentration suggested that the radical chain reaction was able to propagate, albeit slowly, on cholesterol after all the polyunsaturated fatty acid was consumed. For higher concentrations of LDL, prolonged oxidation resulted in partial aggregation. These findings are applicable to preparing oxidised LDL with different degrees of oxidation, under controlled conditions, for studying its biological properties.