Identification of a calcium-binding, brain specific protein in the axoplasm of squid giant axons.

Abstract

Abstract— An acidic protein has been isolated from the optic lobes of two cephalopods, Sepia officinalis and Loligo vulgaris. The protein has been obtained in pure form by fractionation with ammonium sulphate and chromatography on DEAE‐cellulose and Sephadex G 100. Its apparent molecular weight is 13,000–15,000. Glutamic and aspartic acids account for 35 per cent of the amino acid residues. The protein binds Ca2+ ions with an apparent dissociation constant of 2·5 × 10−5 M at physiological concentrations of KCI. Antibodies have been prepared against the protein purified from Sepia officinalis. By the micro‐complement fixation technique it has been shown that the protein is highly concentrated in the nervous system of cephalopods and that the amount in the axoplasm of squid giant axons is eight to nine‐fold higher than in the optic lobes of the same animal.