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Definition
Scyliorhinins I (SCY I) and II (SCY II) are fish tachykinins that were first isolated fromII (SCY II)Dogfish gut (Scyliorhinus caniculus) 1.

Related Peptides
On comparison of the amino acid sequences of these peptides with those of other tachykinins, SCY I was found to show structural similarity to physalaemin, and SCY II to neurokinin A, neurokinin B and kassinin 1.

Discovery
SCY I and SCY II are fish tachykinins that were first isolated from (SCY II). Dogfish gut (Scyliorhinus caniculus). In 1986, Conlon et al., isolated, purified and sequenced SCY I and II 1.

Structural Characteristics
SCY I and SCY II belong to the tachykinin family of polypeptides and share the C-terminal amino acid sequence Phe-X-Gly-Leu-Met-NH2. The C terminus is considered to be of primary importance in the interaction of the peptide with its receptor 2. SCY I (Ala-Lys-Phe-Asp-Lys-Phe-Tyr-Gly-Leu-Met-NH2) is a 10-amino acid residues peptide that is immunoreactive with antiserum against the C-terminal region of substance P (SP). On the other hand, SCY II (Ser-Pro-Ser-Asn-Ser-Lys-Cys-Pro-Asp-Gly-Pro-Asp-Cys-Phe-Val-Gly-Leu-Met-NH2) is an 18-amino acid residues peptide that is immunoreactive with antiserum against the C-terminal region of neurokinin A 1.

Mode of Action
The peptide SCY I displays equipotent high affinity and agonistic potency towards both NK-1 and NK-2 tachykinin receptors. SCY II, on the other hand, is a selective agonist of the NK-3 tachykinin receptor 3.

Functions
SCY I and SCY II act directly on tachykinin receptors located on smooth muscle cells and thus cause the excitatory response in the carp intestinal bulb 1. The fish SCY capable of altering the vascular resistances of both the systemic and gastrointestinal circuits, a property which can be useful to the animal during different physiological conditions, such as feeding or exercise. Besides SP, SCY I is the only substance found that consistently reduces the resistance of gut blood vessels in fish. This regulatory function of tachykinin peptides on the circulatory system is also found in mammals, which suggests a conserved role throughout the evolution 1.

References
1.Kitazawa T (1991). Excitatory responses to scyliorhinins I and II in smooth muscle strips isolated from the carp intestinal bulb (Cyprinus carpio). Naunyn-Schmiedeberg's Arch Pharmacol., 343(5):525-531.
2.Kagstrom J, Axelsson M, Holmgren S (1994). Cardiovascular Responses to Scyliorhinin I and II in the rainbow trout, Oncorhynchus mykiss, in vivo and in vitro. J. exp. Biol., 191(1):155-166.
3.Rodziewicz S, Qi XF Krzysztof Rolka K (1998). Conformational studies of tachykinin peptides using NMR spectroscopy. Letters in Peptide Science, 5:429-432.

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