The distribution of protein-bound N-acetylneuraminic acid in subcellular fractions of rat brain.

Protein-bound N-acetylneuraminic acid and hexosamine, including the sialomucopolysaccharides, occur mainly in the least dense particles sedimented in the microsomal fraction from rat whole brain. Particles rich in protein-bound N-acetylneuraminic acid and hexosamine are also found in the subcellular fraction separated as a layer between 0.8m- and 1.2m-sucrose after centrifuging the crude mitochondrial preparation in a density gradient. This distribution is similar to that of the gangliosides and suggests an association of all of these substances in the same subcellular structures. It is postulated that the sialomucopolysaccharides, as well as the gangliosides, are components of cell membranes. Evidence is presented that indicates that there are quantitative differences between distribution of the gangliosides on the one hand, and protein-bound N-acetylneuraminic acid and hexosamine on the other. The ratio of protein-bound N-acetylneuraminic acid (and hexosamine) to gangliosidic N-acetylneuraminic acid (and hexosamine) present in individual subcellular fractions obtained by density-gradient centrifugation tends to increase with increasing particle density. Exposure of the crude mitochondrial fraction to osmotic ;shock' before density-gradient centrifugation causes a shift of the protein-bound N-acetylneuraminic acid and gangliosides to the less dense fractions. In some experiments, a selective shift of the protein-bound N-acetylneuraminic acid was observed.