Myelin Subfractions in Developing Rat Brain: Characterization and Sulphatide Metabolism

Centrifugation of isolated myelin on discontinuous sucrose gradients resulted in a separation into three bands and a pellet. The three bands were morphologically identical to myelin, whereas the pellet consisted primarily of vesicular membranes. These four fractions differed from one another in their lipid‐to‐protein ratios and in molar ratios of cholesterol:phospholipid:galactolipid. All of the fractions contained proteins typical of myelin, although the proportions of the proteins varied, with the pellet being the lowest in basic protein and proteolipid protein. High activity of 2′,3′‐cyclic nucleotidase and low activity of cerebroside sulphotransferase further distinguished these fractions from the microsomal fraction. Distribution of radioactive sulphatide in the subfractions at 15 min after intracranial injection of radioactive sulphate indicated that newly‐labelled sulphatide first appeared in the lipid‐poor fractions, followed by the lipid‐rich fractions; results of pulse‐chase experiments also suggested this relationship. Several days or weeks after the injection of radioactive sulphate, most of the radioactive sulphatide was in the lipid‐rich fractions.