Characterization of two subcellular fractions isolated from myelinated axons

Myelin and a heavy membrane fraction (1.0/1.2 fraction) were isolated from rabbit white matter by a slight modification of the procedure for bovine CNS. The specific activities of acetylcholinesterase and Na+, K+-ATPase were higher in the 1.0/1.2 fraction than in myelin. In contrast, the cerebroside content and 2'3'-cyclic nucleotide 3'-phosphohydrolase activity in the 1.0/1.2 fraction were 4.5 and 3.4 times lower than in myelin. Total lipids accounted for only 30% of the 1.0/1.2 fracton's dry weight; for myelin, they represented 70%. Polacrylamide gel electrophoresis showed the presence of many high molecular weight proteins and glycoproteins in the 1.0/1.2 fraction but myelin components were practically missing. Cytochrome c oxidase and NADPH-cytochrome c reductase activities suggested about 15% contamination in the 1.0/1.2 fraction but less than 5% for myelin. In electron micrographs of the 1.0/1.2 fraction, there were many membraneous profiles that varied in size, some mitochondrial fragments, and only a few lamellar whorls of compact myelin. The results suggest that the 1.0/1.2 fraction is different from other myelin-related fractions and is probably enriched in axolemma.