Direct determination of the linoleate/oleate ratio in serum cholesterol esters by liquid chromatography

We describe a convenient method for the direct determination of the serum cholesterol linoleate/cholesterol oleate (L/O) ratio by reversed-phase "high-performance" liquid chromatography. After removal of phospholipids by silicic acid chromatography, a serum extract is analyzed on a 5-micrometers particle size Ultrasphere-ODS column, eluted isocratically with acetonitrile/isopropanol (30/70, by vol). Detection is at 200 nm. Cholesterol palmitoleate interferes with the measurement when the analysis is based on peak area, but not when peak height is used. The overall precision of L/O measurements by this method was very similar to that observed with a gas-liquid chromatographic procedure, in which the cholesterol esters are first isolated and transesterified to the methyl esters. In both cases, the within-run CV for six replicate analyses was less than 2%. Analysis of 53 human serum samples by both methods yielded very similar L/O ratios. A plot of the data (our method = y) vs the usual gas-liquid chromatographic procedure gave a correlation coefficient of 0.988 and a regression equation of y = 1.03x + 0.013. Furthermore, direct analysis of serum cholesterol ester L/O ratios by our liquid-chromatographic method is simpler, quicker, and more readily adaptable to automation.