Isolation of non-myelin plasma membranes unique to white matter

A procedure is described for isolating two membrane fractions from rabbit spinal-cord white matter enriched with 5'-nucleotidase, a nonspecific plasma membrane marker, 2',3'-cyclic nucleotide phosphohydrolase, an oligodendroglial plasma membrane marker, and acetylcholinesterase, an axonal plasma membrane marker. While the two membrane fractions exhibited similar enrichments with respect to cyclic nucleotide phosphohydrolase, enrichments of 5'-nucleotidase and acetylcholinesterase were significantly greater in the heavier membrane fraction. Selected enzyme markers for cyto- and mitochondrial membranes were not detected. Moreover, gray matter did not yield homologous membrane fractions in the gradient when subjected to the identical procedure, indicating that the two membrane fractions were unique to white matter. While electronmicroscopic examination revealed that both membrane fractions were comtaminated with myelin, the heavier fraction was least contaminated and exhibited a fair degree of homogeneity with respect to single membrane vesicular profiles. It was concluded that both membrane fractions were enriched with oligodendroglial and axonal plasma membranes, with the heavier fraction containing significantly more axolemma.