Ethanolamine and choline phospholipids in nascent very-low-density lipoprotein particles.
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OBJECTIVE: To establish methods to examine the polar lipids of triglyceride-rich lipoproteins (TRLs) and to examine postprandial changes in glycerophosphocholine (PC) and glycerophosphoethanolamine (PE) polar lipids and in the ratio of diacyl to alkenylacyl components. The membrane phospholipids of TRLs have received little attention. PC and PE constitute major fractions and both comprise diacyl- and alkenylacyl-phospholipids. There has been recent interest in possible antioxidant properties of alkenylacyl (plasmalogen) PE. DESIGN: Analysis of PE and PC fractions in blood samples taken from the subjects while fasting and at four 2-h intervals after eating a 4749-kJ breakfast. PARTICIPANTS: Five healthy subjects. OUTCOME MEASURES: Levels of PE and PC fractions isolated from blood samples by ultracentrifugation (to isolate the d < 1.006 fraction of plasma) and reversed-phase chromatography. RESULTS: The concentration of triglyceride in particles of density less than 1.006 g/cm2 in the samples increased rapidly, peaking 4 h after the meal at 0.51 mmol/L above the fasting level. Apolipoprotein B-48 was maximal in the sample taken 2 h after eating and below 1%, as a percentage of apolipoproteins B-48 and B-100, in all other samples. The concentration of PC exceeded that of PE by an order of magnitude, but there was a proportionately greater increase in PE postprandially. PE contained a rapidly cleared alkenylacyl fraction, which was maximal (at a threefold increase over the baseline level) in the sample taken 4 h after eating. CONCLUSIONS: As blood triglyceride levels rise and fall postprandially, the polar lipid composition of the TRLs also changes; these changes are more marked in the PE than in the PC fraction. The fact that the level of the PE fraction peaked later (at 4 h after eating) than that of B-48 apolipoprotein (which peaked at 2 h after eating) suggests that the peak in the PE fraction had a hepatic (very-low-density lipoprotein) origin. Alkenylacyl (plasmalogen) phospholipid accompanied both PE and PC fractions but was about 10 times greater in PE. This result is significant if alkenylacyl PE does indeed function as an antioxidant.
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