—Frog myelin basic protein, when subjected to ion‐exchange chromatography at alkaline pH, underwent conversion to a higher molecular weight form. Treatment of the latter with 2‐mercaptoethanol regenerated the monomeric basic protein. Amino acid analysis of the monomer and the higher molecular weight species after performic acid oxidation demonstrated the presence of approximately 1 mol of cysteic acid per mol of protein of molecular weight 19,700. Treatment of the monomer with the mild oxidizing agent azodicarboxylic acid bis dimethyl amide resulted in its partial conversion to the higher molecular weight form. These studies demonstrate that the frog myelin basic protein, unlike those of all other species hitherto examined, contains a single cysteinyl residue in its polypeptide chain.

Immnunization of guinea pigs with homologous myelin basic protein induces antibodies that differ in their ability to bind specific peptide fragments of the protein. Antiserums with differing specificities made it possible to demonstrate at least three mutually exclusive antigenic sites in the protein molecule. One of these sites is located between residues 44 and 89, another between 90 and 116, and the third between 117 and 170.

Bovine and guinea pig myelin basic proteins were cleaved with pepsin at pH 3.0 or pH 6.0 (enzyme/substrate, 1:500, w/w), and the peptides were isolated and identified. At pH 3.0 cleavage of the bovine protein occurred principally at three sites: Phe-Phe (88-89), Phe-Phe (42-43), and Leu-Asp (36-37). Minor cleavages occurred at Leu-Ser (110-111), Phe-Ser (113-114), and Ile-Phe (152-153). A study of the time course of the hydrolysis showed that the reaction was biphasic; nearly all of the protein was cleaved at Phe-Phe (88-89) before significant cleavages at other sites occurred. At pH 6.0 cleavage of the bovine protein occurred almost exclusively at a single site, the Phe-Phe bond at position 88-89, resulting in bisection of the protein. Treatment of the guinea pig protein with pepsin under the same conditions resulted in the production of peptides which were identical with those of the bovine protein in chromatographic and electrophoretic properties and in N-terminal and C-terminal residues but which differed slightly in amino acid composition.

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