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Proteins of the Brain Extracellular Fluid: Evidence for Release of S‐100 Protein

Abstract: Extracellular protein fractions were obtained (1) by mild, isotonic irrigation of freshly perfused brain tissue; (2) by collection of proteins released into super‐fusing medium by physiologically viable slices of rat hippocampus; and (3) by sampling the CSF of anesthetized rats. Analysis of the S‐100 protein content of these fractions gave values of 2.8, 4.2, and 1.8 μg S‐100/mg protein, respectively. These values were three‐ to sixfold higher than the S‐100 content of the soluble cytoplasmic protein fractions from the same tissue. This several‐fold higher S‐100 content of the extracellular protein fractions relative to the intracellular cytoplasmic protein fractions indicates that S‐100 is selectively released into the extracellular spaces of the brain. We suggest that the biological function of this CNS protein may involve intercellular transfer.