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Definition
An oligopeptide isolated from the skin of Physalaemus fuscumaculatus, a South American frog. It is a typical kinin, resembling substance P in structure and action and has been proposed as a sialagogue, antihypertensive, and vasodilator.

Discovery
Bertaccini et al., in 1965  studied physalaemin and found that this peptide exerts a potent hypotensive action and stimulates extravascular smooth muscle. It is a poor inhibitor of water-intake but potently lowers arterial blood pressure 1.

Takeuchi et al., in 1977 described a tripeptide, separated from α-chymotrypsin-treated physalaemin, with a strong inhibitory effect on the excitability of the tonically autoactive neurone (TAN) 2.  Venturi F et al identified PHLIP-7 and PHLIP-8, two peptides reproducing at least in part the N-terminal aminoacid sequence of physalaemin, in mammalian tissues. These peptides produce alterations of fighting behaviour in the mouse, but are devoid of any behavioural activity in the rat 3.

Structural Characteristics
Amino acids sequence of physalaemin is Pyr-Ala-Asp-Pro-Asn-Lys-Phe-Tyr-Gly-Leu-Met-NH2. The conformational and spatial configuration of the biologically active undecapeptide physalaemin was studied using 350-MHz 1H NMR. The NMR analyses suggested the existence of a strong hydrogen bond between the amide proton of the Phe7 and a carbonyl group in the N-terminal moiety, most likely the Pro4 one. Other bondings were postulated, involving the side-chain amine of Lys6 and the side-chain amide of Asn5 and respectively the side-chain carboxyl of Asp3 and the terminal amide carbonyl of Met-NH2. Thus unlike its shorter peptidic fragments, physalaemin exhibited a stable molecular structure in solution, giving some insight into the conformation required for interaction at the biological receptor of tachykinins 4

An antiserum specific for the NH2-terminal sequence of physalaemin enabled the quantitation and localization of physalamin-like immunoreactivity (PSLI) in mammalian tissues. Data indicate that the mammalian PSLI is different in structure from the amphibian peptide. In contrast to amphibian physalaemin, mammalian PSLI elutes earlier from a C18 alkylsilane resin with increasing concentrations of methanol, and can be separated from physalaemin by thin-layer chromatography 5

Mode of Action
PSLI is found in acid extracts of whole trachea from rat, rabbit, and guinea pig and in the tracheal mucosal layer in the dog, cow, and pig. The concentration determined by radioimmunoassay ranged from 1 to 15 ng/g dry weight of tissue, with rat trachea containing the highest amount. Gel filtration of an extract of rabbit trachea on Bio-Gel P-4 revealed a single peak of immunoreactivity that had an approximate Mr of 1700, similar to that detected in extracts of guinea pig and rabbit stomach. In contrast to amphibian physalaemin, mammalian PSLI 1) has a higher molecular weight, 2) is resistant to α-chymotrypsin or trypsin digestion 5

Structure-activity relationships of sialogogic heptapeptides analogous to physalaemin have been analysed. The C-terminal amide in position 11 was essential for salivation, but not the pyrolidine group or the N-terminal amino acid residues in positions 1 to 4. In 18 heptapeptides, in which L10 or G9 was replaced, three peptides caused salivation but none had significantly increased secretory activities. In 18 hepta-peptides in which Y8 was replaced, four caused salivation but only one (I) had significantly increased secretory activity. In 18 heptapeptides in which F7 was replaced, only Y caused salivation but with significantly reduced secretory activity. In contrast, in 18 heptapeptides in which K6 and N5 were replaced, most caused salivation and some of them had significantly increased secretory activities 6.

Functions
Affinity for brain receptors, physalaemin-like N-terminal fragments of mammalian tissues, PHLIP-7 and PHLIP-8 do not modify drinking behavior and sodium appetite in rats PHLIP-7 and PHLIP-8 have no affinity for brain receptors involved in the behavioural effects evoked by tachykinins in the rat, and that the brain of this animal is lacking in receptors for the N-terminal sequence of tachykinins   4.

Physalaemin fragment, a tripeptide, separated from α-chymotrypsin-treated physalaemin, with a strong inhibitory effect on the excitability of the tonically autoactive neurone (TAN). Several peptides were separated from-chymotrypsin-treated physalaemin by high voltage paper electrophoresis, and inhibition of the excitability of a molluscan giant neurone (tonically autoactive neurone) by the tripeptide, Lys-Phe-Tyr, have been demonstrated.

Ligand specific for the substance P, physalaemin is a high-affinity ligand specific for the substance P receptor in mammals ligand specific for the substance P receptor in mammals, stimulates pepsinogen secretion from dispersed gastric glands prepared from the rabbit stomach 7.

References

1.     Bertaccini G, Cei JM, Erspamer V (1965). The action of physalaemin on the systemic arterial blood pressure of some experimental animals. Br. J. Pharmacol., 25(2):380-391.

2.     Takeuchi H, Morimasa T, Matsumoto M (1977). Inhibitory tripeptide, Lys-Phe-Tyr, as a fragment of physalaemin. Experientia, 33(7):938-939.

3.     Venturi F, Caraffa E, Cucculelli M, Fede M, Fiorelli A(1989). Physalaemin-like N-terminal fragments of mammalian tissues, PHLIP-7 and PHLIP-8 do not modify drinking behavior and sodium appetite in rats. Acta Biomed Ateneo Parmense., 60(1-2):95-101.

4.     Bernier JL (2004). Conformation of physalaemin. European Journal of Biochemistry., 142(2):371-377.

5.     Lazarus LH, DiAugustine RP, Soldato CM (1982). A substance with immunoreactivity to the peptide physalaemin in mammalian respiratory tissue. Exp Lung Res., 3(3-4):329-341.

6.     Gao C, Abe K (2008). Structure-activity relationships of sialogogic heptapeptides analogous to physalaemin. Oral Diseases, 6(3):180-186.

7.     Kasbekar DK, Jensen RT, Gardner JD (1983). Pepsinogen secretion from dispersed glands from rabbit stomach. Am. J. Physiol., 244:392-396.

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