Further evidence for a specific ganglioside fraction closely associated with myelin

1. 1. Myelin was isolated From gray and white matter separately, and naturally the yield of myelin from white matter was much higher. The white matter myelin from rat brain contains most of the gangliosides found in myelin from whole rat brain, even though the gray matter myelin contains twice as much ganglioside per unit weight as white matter myelin. 2. 2. Myelin isolated from either gray or white matter of adult bovine or rat brain had the characteristic myelin ganglioside pattern found previously for myelin prepared from adult rat whole brain; i.e. a predominance of the normal major monosialoganglioside G₄ (G_M1). 3. 3. Myelin was prepared from the mixed homogenate of unlabeled adult rat brains and neonatal rat brains previously injected with D-[1-¹⁴C]glucosamine. Only 0.2-0.3% of the neonatal brain ganglioside was recovered in the myelin, and more than 99.6% of G₄ (G_M1) in myelin derived from the adult brains. 4. 4. The fatty acid compositions of the major monosialoganglioside G₄ (G_M1) of whole gray matter, white matter and myelin were identical, with stearic acid comprising 90% of the total fatty acids. 5. 5. These findings show that the gangliosides in myelin from whole brain arise predominantly from white matter and are not due to random contamination by non-myelin tissue elements, particularly synaptic membranes. This study indicates the presence of a specific ganglioside fraction localized within the myelin sheath itself, or possibly in an axonal membrane closely associated with myelin.