Protein heterogeneity in rat CNS myelin subfractions

Microsomal fraction-free myelin from forebrain and spinal cord of young and mature rats, when subjected to hypo-osmotic shock and slow speed centrifugation, yielded a myelin pellet and a supernatant fraction (SN 4). Fraction SN 4 consisted of small vesicular profiles in which the major myelin proteins were reduced whereas high molecular weight material such as Wolfgram protein, myelin-associated glycoprotein and CNP were substantially increased over myelin. A close correlation of the SN 4 fraction to the myelin-like fraction of Davison and coworkers was suggested. The myelin pellets were subfractioned on zonal sucrose gradients to yield bell-shaped particle distributions. Besides shifts in densities of the maxima between myelin of young and mature forebrain and spinal cord, a decrease was observed from the light to the heavy gradient end in basic proteins, and an increase in Wolfgram protein and other high molecular weight proteins. Proteolipid protein took an intermediate position. Light fractions from adult spinal cord displayed CNP activities below those of the total homogenate. This result, together with the very high CNP activities in fraction SN 4 casts some doubt on CNP being a marker for compact myelin; rather it appears that CNP is a marker for the process of myelin formation.