Improved measurement of low-density-lipoprotein susceptibility to copper-induced oxidation: application of a short procedure for isolating low-density lipoprotein.

Low-density-lipoprotein (LDL) oxidation may provide the crucial link between plasma LDL and atherosclerotic-lesion formation. Oxidation can be induced in vitro by incubating LDL with cells or metal ions and can be measured by continuously monitoring conjugated-diene absorbance at 234 nm. Measurement of LDL oxidizability was improved by performing the assay with 0.05 g of LDL-protein per liter of phosphate buffer containing 1 mumol of EDTA, by initiating oxidation by adding CuCl2 (5 mumol/L) at 30 degrees C, and by using a short-run ultracentrifugation method for isolating LDL, which reduced the time needed for obtaining purified LDL and thus reduced in vitro oxidation. LDL apolipoprotein analysis and oxidizability determination showed that this method is better than the longer sequential-isolation procedure. Adding butylated hydroxytoluene (BHT) to plasma as an antioxidant unpredictably increased the LDL oxidation lag time, making BHT unsuitable as an antioxidant. Adding EDTA appeared to be sufficient to prevent in vitro oxidation. Additionally, the diene production correlated highly with the concentration of thiobarbituric acid-reactive substances (r = 0.97). No relation between the vitamin E content of LDL and the oxidation lag time was found.