Purification and characterization of a cardioexcitatory neuropeptide from the central ganglia of a bivalve mollusc.

We have purified a cardioexcitatory substance, previously designated peak C, from ganglia of the Sunray Venus clam, Macrocallista nimbosa. Low concentrations (10(-9)-10(-8) M) of this substance not only excite the isolated clam heart, but also produce tonic contractions of the isolated radula protractor muscle of the whelk, Busycon contrarium. These two muscle preparations have therefore been used as a parallel bioassay for peak C. Peak C is inactivated by proteolytic enzymes, has an isoelectric point greater than pH 10 and has an ultraviolet absorption spectrum similar to that of phenylalanine. On thin layer chromatograms, peak C separates into two components; one of these is probably a partially oxidized form produced during purification. Both components react with ninhydrin and with the Sakaguchi reagent for guanidino groups. The amino acid composition of peak C is Phe2.00Met0.81Arg1.12. N-terminal analysis, one round of Edman-dansyl degradation, and tryptic digesting are consistent with the identification of Macrocallista peak C as a tetrapeptide amide: Phe-Met-Arg-Phe-NH2 (FMRFamide).