Purification and Preliminary Biochemical Characterisation of the Human and Rat Forms of the Central Nervous System‐Specific Molecule, F3‐87‐8

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Abstract
The F3‐87‐8 monoclonal antibody recognises a phylogenetically conserved antigenic determinant found exclusively in the mammalian CNS. We used this monoclonal antibody as the major purification step for obtaining pure F3‐87‐8‐bearing molecules from rat and human brains for biochemical analysis. In both rat and man, the F3–87–8 molecule is a heavily glycosylated protein, consisting of 47.6 and 47.0% carbohydrate by weight, respectively. In both species, it occurs as a doublet on polyacrylamide gel electrophoresis in sodium dodecyl sulphate, the Mr of the human form being 130,000 and 100,000. In the rat, the Mr of the doublet is slightly but consistently lower than in man, and the higher‐Mr band is more pronounced. The amino acid composition of the rat and human forms is virtually identical, with a high content of serine and threonine. Significant differences are seen in carbohydrate composition, the rat form containing more sialic acid and neutral sugar and less hexosamine than the human form. β‐Elimination studies, in conjunction with carbohydrate analysis, suggest the presence of approximately 40 O‐linked and 10–15 N‐linked oligosaccharides per polypeptide chain of 500 amino acids, the N‐linked chains predominantly of the high‐mannose type. This makes it likely that the molecule adopts an extended rather than a coiled conformation on the membrane.