Abstract
A peptide with substance P-like immunoreactivity was isolated from extracts of the brains of the pallid sturgeon, Scaphirhynchus albus and the North American paddlefish, Polyodon spathula. The primary structure of the peptide (Lys-Pro-Lys-Pro-His-Gln-Phe-Phe-Gly-Leu-Met.NH2) is the same in both species and contains 2 amino acid substitutions (Arg1→Lys and Gln5→His) compared with human substance P and 1 substitution (Arg3→Lys) compared with substance P from the trout (Teleostei). Scyliorhinin I, a tachykinin previously isolated from an extract of sturgeon intestine, was not detected in either brain extract. A peptide with neurokinin A-like immunoreactivity (Ser-Ser-Ala-Asn-Arg-Gln-Ile-Thr-Gly-Lys10Arg-Gln-Lys-Ile-Asn-Ser-Phe-Val-Gly-Leu20Met.NH2) was isolated from sturgeon brain and contains 10 amino acid substitutions compared with human neuropeptide γ (a specific product of the posttranslational processing of γ-preprotachykinin A) but only 4 substitutions compared with trout neuropeptide γ. It was not possible to obtain the paddlefish neurokinin A-related peptide in pure form. The structural similarity between the sturgeon and the trout tachykinins supports the hypothesis that the Acipenseriformes (sturgeons and paddlefish) represent the sister group of the Neopterygii (gars, bowfin, and teleosts).
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