Protein and enzyme distribution in microsomal and myelin fractions from rat and Jimpy mouse brain.

The protein, glycoprotein and enzyme composition of myelin and myelin-related fraction (SN 4) from rat forebrain was compared with that of microsomal fractions. Acetylcholinesterase was largely confined to the microsomal fractions, wheras 2',3'-cyclic nucleotide-3'-phosphohydrolase (CNP) showed a high specific activity in myelin and SN 4 fractions. Nevertheless, the total specific activities of CNP present in microsomal membranes and in a water-soluble form were not negligible, and suggest that this enzyme has a wide distribution among subcellular particles. A high molecular weight protein was identified in myelin and all the other fractions studied. This protein (X), which co-migrates with the major myelin glycoprotein, was present in myelin and in fractions lacking typical myelin components as well as in fractions from a myelin deficient mutant, the Jimpy mouse. The results suggest that the X protein is probably a contaminant in isolated myelin, although the occurrence of this protein as an intrinsic component of several different membranes cannot be ruled out. Despite substantial overlap in density upon zonal centrifugation between SN 4 and microsomal fractions, the enzyme patterns of the fractions were different.