The Phospholipid and Phospholipid Fatty Acid Composition of Human Serum Lipoprotein Fractions*

Previous studies on human serum phospholipids have dealt with concentration in whole serum (l-3). With the introduction of the concept of lipoproteins (4) and lipoprotein classes (5) it became of interest to study the phospholipids of serum as structural units of these macromolecules. Whereas there is now agreement on the relative proportions of the phospholipid species in whole serum, it has not previously been known how the individual phospholipids are distributed in the lipoprotein classes. Whiie this work was in progress, Phillips (6) reported such a study. His results will be discussed in some detail in the appropriate section. It has been generally assumed that the major phospholipids of serum are phosphatidyl ethanolamine, lecithin, and sphingomyelin (7). However, Phillips (8) reported the isolation and determination of 7% lysolecithin in human serum phospholipids. The minor components are plasmalogens, inositol phosphatides, and phosphatidyl serine. The total of these minor compounds is not believed to exceed 1 or 2’% of the total phospholipids (8). In the course of this work a gas chromatography unit became available in this laboratory. A corollary study of the fatty acid complement of the phospholipids was started and some preliminary results will be presented, as little is known concerning the composition and distribution of serum phospholipid fatty acids. Studies on the fatty acids of phospholipids have been primarily restricted to samples from tissues. Luddy et al. (9) reported values for whole serum phospholipids as a single class, with the use of alkali isomerization techniques, and found 40 To saturated fatty acids and 60% unsaturated. The unsaturated acids were primarily oleic, linoleic, and arachidonic, 35, 15, IS%, respectively, with traces of other unsaturated acid on the order of 1 %, which is the limit of error of their method. The saturated group was not characterized further. James and Lovelock (10) also reported fatty acid analyses on total serum phospholipid fractions by gas chromatography. They found a range of values for sera from 10 subjects. The average values were as follows in %: pahnitic 30, pahnitoleic 2, stearic 20, oleic 20, linoleic 20, and arachidonic 8. Mukherjee et al. (11) reported the total phospholipid fraction fatty acid composition of rat serum by the use of alkali isomerization. They found rat serum phospholipids to be highly saturated, approximately 75%. In spite of these few instances, our knowledge of fatty acids of serum phospholipids is very incomplete. No data are available on the fatty acid composition of