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The 13 amino acid toxic peptide from the marine snail Conus geographus, conotoxin GI, blocks the acetylcholine receptor at the neuromuscular junction. In this report, we describe a method for analyzing disulfide bonding in nanomole amounts of small cystine-rich peptides. The procedure involves partial reduction and a double-label alkylation of cysteine residues. Using this method, we show that the natural conotoxin GI has a (2-7, 3-13) disulfide configuration. The structure of conotoxin GI has been confirmed by chemical synthesis. The preparation and purification of molecularly homogeneous, iodinated derivatives of this toxin are also described. All derivatives, including the [diiodohistidine,diiodotyrosine]conotoxin GI, retained at least half of the biological activity of unmodified toxin. Since the tetraiodinated toxin, which is greater than 25% by weight iodine, retains considerable toxicity, unmodified histidine and tyrosine residues in conotoxin GI are not crucial for biological activity.

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SYNOPSIS. Various protozoa and lower metazoa were exposed to filtrates of Staphylococcus aureus B (formerly called Type S‐6) (2) and non‐enterotoxigenic filtrates of staphylococci to study the toxicity of staphylococcal enterotoxin to these organisms. No reaction specific for enterotoxin was observed either in those culture filtrates from toxin‐producing strains or in solutions of purified enterotoxin. Non‐specific reactions were obtained with various growth media and a “potassium inhibited” filtrate containing 0.5% K2HPO4.