Cholesterol metabolism in myelin and other subcellular fractions of rat brain.

For many years the bulk of myelin in adult brain was believed to be metabolically stable, although some metabolic activity of a small myelin fraction, especially in the gray matter of the brain, was recognized. We have attempted to compare the composition of myelin fractions isolated from two different areas of the brain. No differences in chemical composition were observed. We have also investigated the metabolism of cholesterol in myelin and other subcellular fractions from the two areas. Both young (16-day-old) and adult rats were used. Results show an uptake of radioactive cholesterol by all subcellular fractions of the brain, including myelin, in both young and adult animals, with ultimate uniform distribution of the radioactive sterol and its persistence in all uniformly labeled subcellular fractions of the brain. On the basis of these results we suggest that there is a pool of cholesterol in the brain from which all metabolizing structures, including myelin, draw their cholesterol supplies. There is continuous exchange of cholesterol between the brain pool and the blood. The rate of this exchange may be related to the rate of blood flow through the tissue.