Lipid Composition of Axolemma-enriched Fractions from Human Brains

The lipid composition was determined for axolemma-enriched fractions and myelin which were isolated via a preparation of purified myelinated axons. The myelin had a lipid composition which was compatible with that previously reported for myelin isolated by alternative procedures. The most dense axolemma-enriched fraction contained 25.3% cholesterol, 25.8% galactolipid (21.3% cerebrosides and 4.8% sulfatides), and 48.9% phospholipid. The major phospholipids were the ethanolamine phospholipid (19.8% of total lipid weight; 49.0% in the plasmalogen form) and choline phospholipids (18.7% of total lipid weight; 16.0% in the plasmalogen form) with lesser amounts of sphingomyelin, phosphatidylserine, and phosphatidylinositol also present; the ganglioside content was 13.9 micrograms of acetylneuraminic acid per mg protein. The less dense axolemma-enriched fraction had a lipid composition which was intermediate between that of myelin and the more dense axolemma-enriched fraction. On the average, less than 2.3% of the total protein in the axolemma-enriched fraction was myelin basic protein. Both axolemma-enriched fractions stained uniformly with Luxol fast blue and demonstrated specific saxitoxin-binding which was enriched 2- to 7-fold over that of the whole white matter homogenate from which the fractions were isolated. The choline and ethanolamine phospholipids in that most dense axolemma-enriched fractions contained a greater percentage of unsaturated fatty acids compared with the comparable phospholipids in myelin. The content of unsaturated fatty acids in these phospholipids of the axolemma-enriched fraction was not as great as that of human CNS synaptic plasma membranes. However, the chain length distribution of these phospholipid fatty acids was similar in myelin, synaptic plasma membrane, and the axolemma-enriched fraction. The distribution of aldehydes derived from the ethanolamine phospholipids of the more dense axolemma-enriched fraction closely resemble the distribution of the comparable aldehydes in the myelin fraction. The possible origin and function of the lipids in the axolemma-enriched fractions are discussed.