Piscivorin is a component of snake venom

Snake venom

Snake venom is highly modified saliva that is produced by special glands of certain species of snakes. The glands which secrete the zootoxin are a modification of the parotid salivary gland of other vertebrates, and are usually situated on each side of the head below and behind the eye,...

 secreted by the Eastern Cottonmouth

Agkistrodon piscivorus

Agkistrodon piscivorus is a venomous snake, a species of pit viper, found in the southeastern United States. Adults are large and capable of delivering a painful and potentially fatal bite. When antagonized they will stand their ground by coiling their bodies and displaying their fangs...

 (Agkistrodon piscivorus piscivorus). It is a member of the cysteine-rich secretory protein

Cysteine-rich secretory protein

Cysteine-rich secretory proteins, often abbreviated as CRISPs, are a group of glycoproteins found exclusively in vertebrates. They are a subgroup of the CRISP, antigen 5 and Pr-1 protein superfamily and are substantially implicated in the functioning of the mammalian reproductive system...

 (CRISP) family, which blocks voltage-dependent calcium channels.

Etymology

The name of piscivorin comes from the snake species name piscivorus, which is derived from the Latin words pisces and vorare, meaning 'fish' and 'to devour' respectively.

Source

Piscivorin is produced in the venom glands of the Eastern Cottonmouth

Agkistrodon piscivorus

Agkistrodon piscivorus is a venomous snake, a species of pit viper, found in the southeastern United States. Adults are large and capable of delivering a painful and potentially fatal bite. When antagonized they will stand their ground by coiling their bodies and displaying their fangs...

 snake (Agkistrodon piscivorus piscivorus), which populates the Eastern United States. Typically, crude venom from the Eastern Cottonmouth contains approximately 1.25% of piscivorin.

Biochemistry

Piscivorin belongs to the cysteine-rich secretory protein (CRISP) family, which are secreted as single-chain proteins with molecular masses between 20 and 30 kDa. They display significant amino acid sequence homology. Sixteen cysteine residues, forming 8 disulfide bonds, are strictly conserved in CRISPs. Ten of these cysteine residues are clustered into the C-terminal part of the protein.

The molecular mass of piscivorin is 24.842 kDa. The nucleotide sequence of piscivorin cDNA spans 1323 bp, containing an open reading frame of 240 codons.

Piscivorin has the following amino acid sequence.

10	20	30	40	50	60
MIAFIVLPIL	AAVLQQSSGS	VDFDSESPRK	PEIQNQIVDL	HNSLRRSVNP	TASNMLKMEW
70	80	90	100	110	120
YPEAAANAER	WAYRCIESHS	PRNSRVLGGI	KCGENIYMSS	IPIKWTEIIH	AWHGENKNFK
130	140	150	160	170	180
YGIGADPPNA	VIGHFTQIVW	YKSYLVGCAA	AYCPSSEYSY	FYVCQYCPAG	NIIGKIATPY
190	200	210	220	230	240
KSGPPCGDCP	SACVNGLCTN	PCTKEDKYTN	CKSLVQQYGC	QDKQMQSECS	AICFCQNKII

Target and mode of action

Piscivorin reduces high potassium-evoked smooth muscle contraction, but does not inhibit caffeine

Caffeine

Caffeine is a bitter, white crystalline xanthine alkaloid that acts as a stimulant drug. Caffeine is found in varying quantities in the seeds, leaves, and fruit of some plants, where it acts as a natural pesticide that paralyzes and kills certain insects feeding on the plants...

-stimulated contraction of smooth muscle. Since caffeine normally causes contraction through the release of Ca²⁺ from the sarcoplasmic reticulum, this differential effect indicates that piscivorin is an L-type calcium channel

L-type calcium channel

The L-type calcium channel is a type of voltage-dependent calcium channel. "L" stands for long-lasting referring to the length of activation. Like the others of this class, the α1 subunit is the one that determines most of the channel's properties....

 blocker. At a concentration of 1 μM, its effect on depolarization-induced smooth muscle contraction is weaker than of the related CRISP family toxins ablomin

Ablomin

Ablomin is a toxin present in the venom of the Japanese Mamushi snake, which blocks L-type voltage-gated calcium channels.- Etymology :The protein ablomin is a component of the venom of the Japanese Mamushi snake, Gloydius blomhoffi...

, triflin

Triflin

Triflin is a cysteine-rich secretory protein , which is excreted by the venom gland of the Habu snake . Triflin reduces high potassium-induced smooth muscle contraction, suggesting a blocking effect on L-type calcium channels.- Source :Triflin is a toxin derived from snake venom...

 or latisemin

Latisemin

Latisemin is a cysteine-rich secretory protein that can be isolated from the venom of the Black-banded sea krait, a sea snake indigineous to the warmer waters of the western Pacific Ocean...

. A sequence comparison of piscivorin and other CRISP family proteins suggests that the Glu186 residue is the crucial site for the blocking of the calcium channels.

Unlike some other CRISP family proteins, piscivorin does not block cyclic nucleotide-gated channels.