Protocol for the study of the metabolism of retinyl esters in plasma lipoproteins during postprandial lipemia

An efficient protocol is described for the study of the kinetics of retinyl esters in whole plasma and several lipoprotein fractions following the consumption of an oral fat load containing vitamin A (retinol). To allow for a more complete characterization of the kinetics of retinyl esters in different lipoprotein fractions, a simplified two-step ultracentrifugation procedure is reported for the efficient and reproducible isolation of triglyceride-rich chylomicrons from nonfasting subjects, VLDL-sized lipoprotein particles, and the triglyceride-poor lipoprotein fraction. The present method for the determination of retinyl esters is based on the direct application of the lipid fraction onto a normal phase HPLC column without requiring the lipid extract to be desiccated and resolubilized. All of the commonly occurring esters of retinol elute as a single peak with a retention time of 1.6-1.8 min followed by retinyl acetate (serving as the internal standard) and retinol with retention times of 2-2.5 min and 5-5.5 min, respectively. With this system, a new sample can be processed every 10 min and a complete set of 60 samples from a typical oral fat load can analyzed in one working day with minimal technical interaction. By normalizing to the area under the internal standard to correct for variability in the injected volume, the coefficient of variability for the concentration retinyl esters within a single run is less than 5% and less than 10% between runs.