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Helothermine, a protein from the venom of the Mexican beaded lizard (Heloderma horridum horridum), was found to inhibit [3H]ryanodine binding to cardiac and skeletal sarcoplasmic reticulum, to block cardiac and skeletal ryanodine receptor channels incorporated into planar bilayers, and to block Ca(2+)-induced Ca2+ release triggered by photolysis of nitr-5 in saponin-permeabilized trabeculae from rat ventricle. Cloning of the helothermine cDNA revealed that the protein is composed of 223 amino acids with a molecular mass of 25,376 daltons, and apparently is stabilized by eight disulfide bridges. The peptide sequence showed significant homology with a family of cysteine-rich secretory proteins found in the male genital tract and in salivary glands. The interaction of helothermine and ryanodine receptors should serve to define functional domains within the channel structure involved in the control of Ca2+ release from sarcoplasmic reticulum.

The soluble venom from the Brazilian scorpion Tityus serrulatus Lutz and Mello was fractionated on Sephadex G-50 followed by ion exchange chromatography in carboxymethyl-cellulose, resolving at least 10 different toxic components. In this communication the purification and chemical characterization of four major components is reported. The toxins were found to be single polypeptide chains with molecular weights of approximately 7.000, 61-63 amino acid residues and four disulphide bridges. Amino terminal sequence determination of these toxins after reduction and alkylation revealed three unique sequences up until residue fourty-one. For toxin 11-11 (14) and toxin III-10 the N-terminal sequence is:

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