Snake Venoms

430 Pages·2015·9.406 MB·English

by  P. Gopalakrishnakone, Hidetoshi Inagaki, Ashis K. Mukherjee, Tarek R. Rahmy, Carl-Wilhelm Vogel (eds.)

Structural and Functional Diversity
of Snake Sarafotoxins
Yazine Mahjoub, Stéphanie Malaquin, and Frédéric Ducancel
Contents
Introduction....................................................................................... 2
Sources,Structure,andToxicity.................................................................. 3
ShortSarafotoxinsfromA.engaddensis..................................................... 3
LongSarafotoxinsfromA.microlepidotamicrolepidota.................................... 4
LongSarafotoxinsfromA.irregularis....................................................... 6
Toxicity........................................................................................ 7
PharmacologicalPropertiesofSRTXs........................................................... 7
EndothelinReceptors......................................................................... 7
AffinityandAgonistPropertyofShortandLongSRTXs................................... 8
MechanismofAction............................................................................. 8
EffectofBindingtoET-A.................................................................... 8
EffectofBindingtoET-B.................................................................... 9
BiologicalActivity:DifferencesBetweenShortandLongSarafotoxins....................... 9
ConclusionandFutureDirections................................................................ 10
References........................................................................................ 11
Abstract
Sarafotoxins(SRTXs)areextractedsolelyfromthevenomofsnakesbelongingto
theAtractaspisgenus,aburrowingaspfromAfricaandMiddleEast.Theysharea
high-sequencehomology with endothelins,potent vasoconstrictorsproducedby
Y.Mahjoub(*)
Pôled’Anesthésie-Réanimation,CHUAmiens,Unitéderéanimationchirurgicale,Amiens,France
UnitéINSERMU1088,Amiens,France
e-mail:[email protected]
S.Malaquin
Pôled’Anesthésie-Réanimation,CHUAmiens,Unitéderéanimationchirurgicale,Amiens,France
F.Ducancel
CEA,iMETI,JointUnitIMVA,CEAFontenay-aux-Roses,Fontenay-aux-Roses,France
#SpringerScience+BusinessMediaDordrecht2016 1
P.Gopalakrishnakoneetal.(eds.),SnakeVenoms,Toxinology,
DOI10.1007/978-94-007-6648-8_1-1
2 Y.Mahjoubetal.
mammalianendothelialcellsandimplicatedinvascular toneregulationandcell
growth.SRTXsandendothelinshaveacommoncoreof21aminoacidsandtwo
conserveddisulfidebridgesbetweencysteines+1/+15and+3/+11whichconsti-
tuteauniqueandtypical“signature”amongnaturalbioactivepeptides:Cys -X-
1
Cys ...Cys -X-X-X-Cys .SRTXsbindtoendothelinreceptorsET-AandET-B
3 11 15
situated on the membrane of numerous cells especially endothelial cells and
smooth muscle cells. Recently long SRTXs have been discovered in the venom
of A. microlepidota microlepidota and A. irregularis. These long SRTXs have
three orfour additional amino acids on their C-terminal domain. The affinity of
longSRTXsforendothelinreceptorsisdramaticallydecreasedcomparedtoshort
SRTXs.Nevertheless,theystillhaveahightoxicityastestedonmice.Combining
invasiveandechocardiographicprocedures,recentexperimentsshowedthatshort
andlongSRTXshaveverydifferentinvivohemodynamiceffects.ShortSRTXs
impair leftventricular functionwhilelong SRTXs impairrightventricular func-
tion and increase airway pressures with no effect on left ventricular function.
Furtherexperimentsareneededtoexplainthisdiscrepancy.Thehypothesesofa
new type ofendothelin receptors (non-A non-B) cannot be discarded. Thus,the
C-terminusextensionseemstoplayamajorroleindefiningtheinvivoeffectof
thesepeptides.
Keywords
Atractaspis (cid:129) Sarafotoxin (cid:129) Endothelin (cid:129) C-terminal domain (cid:129) Vasoconstriction
activity(cid:129)Rightventricle
Introduction
Snake venom sarafotoxins (SRTXs) and mammalian endothelins (ETs) form a
structurally and functionally related family of potent vasoconstrictor peptides.
Endothelins are synthetized by mammalian endothelial cells and are released in
verysmallamounts(plasmaconcentrationinpicomolarrange).
SRTXs have been discovered in the early 1980s in the venom of Atractaspis
engaddensis, a snake belonging to the Atractaspidae family (Kochva et al. 1982).
The name “Sarafotoxin” comes from the Hebrew name of the snake: Saraf Ein
Geddi. Saraf means dragon and “Ein Geddi” is the name of the oasis where
A. engadensis has been first described. Atractaspididae are a single genus family
of oviparous burrowing asps from Africa and Middle East. They are of medium
length between 70 and 120 cm. These snakes have lateral small fangs that can be
seenevenwhentheirmouthisclosed(Fig.1).Hence,tothecontrarytootherasps
theydonotbitebuttheycanstabtheirpreymouthclosed,thustheyaresometimes
knownasside-stabbers.Theyfeedessentiallyfromsmallreptiles,amphibians,and
rodents. “Bites” from Atractaspis snakes are responsible of pain, local edema and
ischemiaornecrosis,generalweakness,nauseaandvomiting,bradycardia,T-wave
StructuralandFunctionalDiversityofSnakeSarafotoxins 3
Fig.1 HeadofAtractaspisengaddensiswithavisiblefangprotrudingfromclosedmouth
inversion,atrioventricularblock,andmayleadtocardiacarrest(Kurniketal.1999;
Wollebergetal.1988;Leeetal.1986).
Recently, sarafotoxins extracted from other snakes belonging to the
atractaspididae family have been described (Hayashi et al. 2004; Quinton
et al. 2005). These sarafotoxins have a longer C-terminus extension but have the
same 3-D structure as SRTXs from A. engaddensis and endothelins (Mourier
et al. 2012). After presenting the structural diversity of SRTXs, and the effect of
agonist propriety of SRTXs on endothelin receptors, this review will explain the
functionaldiversityrelatedtothestructuraldiversityofSRTXs(shortvs.long)based
onrecentexperimentalinvivostudies.
Sources, Structure, and Toxicity
Short Sarafotoxins from A. engaddensis
In 1988, the primary sequences of three peptides extracted from the venom of A.
engaddensis and named SRTX-a, SRTX-b, and SRTX-c have been described
(Takasaki et al. 1988). The same year, Yanagisawa et al. discovered a potent
vasoconstrictor peptide synthetized by mammalian endothelial cells: endothelin-1
(ET-1)(Yanagisawaetal.1988).Later,twootherisoformsofendothelinshavebeen
discovered(ET-2,ET-3andVIC)(Inoueetal.1989;Saidaetal.1989)whileanother
SRTX called bibrotoxin (Btx) was identified in the venom of Atractaspis bibroni
(Beckeretal.1993).
Endothelins and SRTXs share a high sequence homology (about 60 %). They
haveacommoncoreof21aminoacidsandtwoconserveddisulfidebridgesbetween
cysteines +1/+15 and +3/+11 which constitute a unique and typical “signature”
among natural bioactive peptides: Cys -X-Cys ...Cys -X-X-X-Cys . Sequence
1 3 11 15
homologybetweenSRTXsfromA.engaddensisandET-1ismorethan60 %,while
4 Y.Mahjoubetal.
SRTX-b and BBTX differ only from one amino acid in position +4. Three-
dimensionalstructuresanalysesofthesepeptidesareconsistentwithasimilaroverall
structure: SRTXs and endothelins adopt a cysteine-stabilized alpha-helical motif
characterizedbyanextendedstructureofthefirstthreeorfourresidues,abeta-turn
structurebetweenposition+5and+8,analpha-helicalconformationofthesegment
Lys -Cys , and the absence of conformation of the C-terminal domain (Mourier
9 15
et al. 2012). Complementary DNA (cDNA) encoding SRTXs in A. engaddensis
comprise1948basepairs(bp)includingoneopenreadingframeof1629pbcoding
for a long pre-pro-polypeptide of 543 amino acids which starts with a methionine
that initiates translation followed by a hydrophobic peptide characteristic of signal
sequence (Ducancel et al. 1993). This long open reading frame deduced for this
precursorhasanoriginalrepetitivestructurecomprisingonesequenceof39amino
acids followed by 11 sequences of 40 residues. Each of these contains a SRTX
sequenceprecededbyaninvariantspacerpeptideof19aminoacids.Thelastcopyof
SRTX is followed by a peptide that differs the previous one both in terms of its
sequence and length (13 amino acids). In all, five different isoforms of SRTXs are
encoded by this single precursor: SRTX-a, SRTX-a1, SRTX-b, SRTX-c, and
SRTX-e. A sixth isoform called SRTX-b1, differing from SRTX-b by one amino
acid at position +7, was also identified during sequencing of an incomplete cDNA
(Ducanceletal.1993).So,sixisoformspresentinthevenomofA.engaddensisare
produced from 2 precursors. This so-called rosary-type organization is unique in
terms of its regularity in precursors of snake toxins with disulfide bridges (Fig. 2).
Thus “three-fingered” toxins, such as neuromuscular toxins, muscarinic toxins,
fasciculins and type A2 phospholipases for which numerous isoforms have been
identified, are all produced from mono-cistronic precursors (Ducancel et al. 1991).
This peculiar organization seems to constitute a simple, economical, and effective
way of amplifying the production of SRTXs in the venom of A. engaddensis
(Ducancel2005).
Long Sarafotoxins from A. microlepidota microlepidota
ThroughexhaustivestudyofthevenomofA.mmicrolepidota,anotherspeciesofthe
genus Atractaspis, a second poly-cistronic precursor encoding SRTXs have been
describedbyHayashietal.(Hayashietal.2004).Usingmassspectrometryanalysis
ofthevenomcompositionandmolecularcloningoftheprecursorsencodingSRTXs,
these authors identified a new family of longer SRTXs, since displaying three
additional residues at their C-terminus extremities. A cDNA fragment containing
seven 144-nucleotide repeats, each coding for a peptide link followed by a SRTX
sequence, hasbeen identified. ThiscDNAfragment displays a poly-cistronic orga-
nization as in the case of precursor encoding SRTXs from A. engaddensis. Five
differentisoformsnamedSRTX-m,SRTX-m1,SRTX-m2,SRTX-m,andSRTX-m4
are encoded by this precursor. These isoforms (except SRTX-m4) successively
present downstream of the invariant tryptophan at position +21, an aspartate,
glutamate,andaproline.Comparisonoftheprimarystructuresofthefiveisoforms
StructuralandFunctionalDiversityofSnakeSarafotoxins 5
Fig. 2 Complete “rosary-type” structure of the precursor of sarafotoxins from A. engaddensis
(DDBJ/EMBL/GenBankAccessionNumberAAA48515).The5’and3’noncodingendsareshown
initalics.Thededucedaminoacidsequence,whichconstitutestheopenreadingframe,isnumbered
totherightofthefigure.Theaminoacidsequencecorrespondingtoeachsarafotoxinishighlighted
inred.Thenameofeachsarafotoxiniswritteninredtotheleftofthefigure
6 Y.Mahjoubetal.
revealsbetween54 %and96 %homology:SRTX-m1and-m2differfromSRTX-m
only by one substitution, when SRTX-m4 is the most distant isoform with eight
substitutions in the +1/+21 sequence, as well as a different C-terminal extension
(Fig.3).NMRstudiesshowedthatdespiteitslongerC-terminalextension,SRTX-m
displaysatypicalSRTX/ETthree-dimensionalstructure.Nevertheless,unlikeshort-
SRTXs and ETs, the conformation of the long C-terminus tail is restricted. NOE
connectivitiesareobservedbetweentheendofthealpha-helixandthebeginningof
theC-terminustail.Asaresult,theresiduesH16toV19arelooselyloopedbackonto
hydrophobicpatch,restrictinglargeamplitudemotion(Mourieretal.2012).
Long Sarafotoxins from A. irregularis
InadditiontothepreviousworkdoneonA.m.m.,amolecularstudyonaspecimenof
A. irregularis, another species of the genus Atractaspis, has been carried out
(Quintonetal.2005).Directanalysisbynanospray-Fourier-Transformioncyclotron
resonance of the crude venom of this snake revealed about 60 distinct molecular
mass in the range 0.6–15.0 kDa, half of which is between 2.0 and 3.5 kDa, which
correspondstopeptides between20and 30amino acids, acharacteristic lengthfor
SRTXs. Inparallel,theprecursorsofA.irregularis SRTXswereclonedbyhomol-
ogy,anddenovosequencingbymassspectrometryofthereducedcrudevenomwas
carriedout.ThepresenceofaSRTXsequenceattheC-terminusofthe118-amino-
SRTX-b C S C K D M T D K E C L Y F C H Q D V I W
SRTX-c C T C N D M T D E E C L N F C H Q D V I W
Btx C S C A D M T D K E C L Y F C H Q D V I W
SRTX-m C S C N D I N D K E C M Y F C H Q D V I W D E P
SRTX-i3 C S C T D M S D L E C M N F C H K D V I W V N R N
ET-1 C S C S S L M D K E C V Y F C H L D I I W
ET-2 C S C S S W L D K E C V Y F C H L D I I W
ET-3 C T C F T Y K D K E C V Y Y C H L D I I W
VIC C S C N S W L D K E C V Y F C H L D I I W
D
K
M C S C
T
SRTX-b
D
K
D C L Y F C H Q D V I W
Fig.3 Aminoacidsequencesofendothelins(ET1,2,3andVIC),short-sarafotoxins(SRTX-b,-c
andBtx)andlong-SRTXs(SRTX-mand-i3).Cysteinesresponsibleforinvariantdisulfidebridges
areinyellow.Allpeptidespossesstwodisulfidebonds:betweenC1andC15andbetweenC3and
C11(asshownforSRTX-b).Conservedaminoacidsareingray.Btxbibrotoxin
StructuralandFunctionalDiversityofSnakeSarafotoxins 7
acid residue precursor protein clearly establishes that it corresponds to an
A. irregularis SRTX precursor. However, in the case of A. irregularis, only one
copyofmatureSRTXemergesfromeachprecursorwhenseveraltandemcopiesof
mature SRTXs are encoded per molecule of the rosary-type precursors in the two
previously studied Atractaspididae snakes. In total, three isoforms of SRTX
emerged from this analysis: SRTX-i1, SRTX-i2, and SRTX-i3. These SRTXs are
25aminoacidlongpeptides(Fig.3)withfouradditionalresiduesattheirC-terminus
domain. SRTX-i1 and SRTX-i3 (the most abundant isoform) differ only by one
amino acid: a valine for SRTX-i3 and an isoleucine for SRTX-i1 at position +22
followingtheconservedtryptophaneatposition+21(Quintonetal.2005).
NMR structure characterization showed that SRTX-i3 have the typical three-
dimensional structure of SRTXs and endothelins. Unlike SRTX-m, the longer
C-terminus extension of SRTX-i3 is found flexible and disordered without any
interaction with the folded N-terminus CSH motif. This is due to the absence of
NOE connectivities between these two regions and in agreement with the small
dispersion of amide proton chemical shifts for residues H16 to N25 (Mourier
etal.2012).
Toxicity
SRTX-b from A. engaddensis is among the most lethal toxins ever described. It
causes death in mice within minutes. The mean lethal 50 (LD ) doses of SRTX-b
50
aresimilartothatofET-1andareabout15ug/kg(Kochvaetal.1993).SRTX-cis
about30timeslesstoxicwithaLD of300ug/g.ToxicityofSRTX-miscloseto
50
thatofSRTX-bwithanLD of32ug/gwhileSRTX-i3isslightlylesswithaLD
50 50
of115ug/g(Hayashietal.2004;Mourieretal.2012).
Pharmacological Properties of SRTXs
Endothelin Receptors
Several data confirmed that SRTXs and ETs acts the same receptors called the
endothelin receptors. Two main subtypes have been described: ET-A and ET-B
(Ducancel2002).Theyaremembersofthehepta-helicalG-proteincoupledreceptor
superfamily and range from 45 to 50 kDa in size. Human ET-A is formed by
426 amino acids and is encoded by a gene located on chromosome four. Human
ET-Bisformedby442aminoacids andisencoded bya genesituated onchromo-
some 13.Theyshare about 60 %homology andarehighlypreservedacross mam-
malian species. Each subtype receptor consists of a long extracellular amino-
terminal domain,sevenloops ofmembrane-spanningdomains,andanintracellular
C-terminal domain. The C-terminal tail and the third cytoplasmic loop contain
several putative phosphorylation sites. The Asp-Arg-Tyr motif in the second cyto-
plasmic loop is highly conserved and is possibly involved in coupling G-proteins.
8 Y.Mahjoubetal.
Moreover, the tyrosine on position +129 situated on the second transmembrane
domain seems to play a major role in receptor selectivity (Krystek et al. 1994).
ET-A has a high affinity for ET-1 and ET-2 and a 100 times less affinity for ET-3.
ET-Bhasthesameaffinityforall3ETisoforms(BartonandYanagisawa2008).
Affinity and Agonist Property of Short and Long SRTXs
TheaffinityofSRTXsfortheendothelinissignificantlydifferentbetweenshortand
long SRTXs. On one hand, endothelins and short SRTXs have high affinities for
ET-B with affinity constants in the picomolar range, and on the other hand, long
SRTXs have low affinities for ET-B receptors with constant affinity in the micro-
molarrange.Hence,thetruncatedformsoflongSRTXShavesimilaraffinitythatof
shortSRTXs.Moreover,theadditionofanextendedtailofalongSRTXtoashort
SRTX is responsible of a drop in affinity. These results are confirmed to a lesser
extentforET-Areceptors(Mourieretal.2012).
Mourier et al. have investigated the agonist property of SRTXs on ET-B recep-
tors. They used ET-B receptors stably expressed at the surface of CHO cells and
measuredthefluorescenceofacalciumdyethatvariesaccordingtotheintracellular
calciumconcentration.TheystudiedSRTX-b,ET-1,andSRTX-cbutalsotruncated
forms of long SRTXs (SRTX-m and SRTX-i3 without their extended C-terminus
tail)andchimericlongSRTXs(SRTX-bwiththeextendedtailofSRTX-morSRTX-
i3). They measured the dose–response curves and highlighted the full agonist
property of all SRTXs but also the high potency of short SRTXs (EC in the
50
10nanomolarrange)andthelowpotencyoflongSRTXs(EC inthemicromolar
50
range)(Mourieretal.2012).
Mechanism of Action
Effect of Binding to ET-A
ET-A subtype receptor is present on the surface of smooth muscle cells but not on
endothelialcells.ET-AbindingstimulatesaphospholipaseClinkedtoaG-protein.
Phospholipase C hydrolyzes phosphatidyl inositol 4,5 biphosphate to form the
water-soluble inositol triphosphate (IP3) and the neutral diacylglycerol (DAG).
IP3 and DAG are second messengers. IP3 induces calcium mobilization from the
sarcoplasmicreticulumleadingtoanincreaseofintracellularcalciumconcentration
that provokes the opening of store-dependent calcium channels (SOC) that further
increase cytosolic calcium. DAG opens receptor-dependent calcium channels
(Horinouchi et al. 2013). The intracellular calcium elevation activates various ions
channels resulting in calcium entry across the plasma membrane and the sustained
increase of intracellular calcium concentration. Intracellular calcium concentration
leadstosmoothmusclecellcontractionandcontributestogrowth-promotingeffects
throughinductionofproto-oncogeneslikec-fos,c-jun,andc-myc.
StructuralandFunctionalDiversityofSnakeSarafotoxins 9
Effect of Binding to ET-B
ET-B subtype receptors are present on the surface of endothelial cells and on the
surface of smooth muscle cells. ET-B binding on smooth muscle cells has similar
effecttoET-AbindingwhileET-Bbindingonendothelialcellshasdifferenteffects.
Onendothelialcells,ET-Bbindingactivatesthephosphoinositide3kinase(PI3/Akt)
that stimulates phosphorylation of eNOS with subsequent NO production (Liu
etal.2005).ET-Bbindingalsoactivatescyclooxygenase2thatinducesthesynthesis
of prostacyclin, resulting in the activation of adenylate cyclase that leads to a
decrease in calcium concentration. This phenomenon is responsible for smooth
muscle cells relaxation. Moreover, after binding on ET-B, ET-1 is rapidly internal-
ized and degraded by endosomes. This is the main way of endothelin clearance
especiallyinlungsandkidneys(MazzzucaandKhalil2012).
Biological Activity: Differences Between Short and Long
Sarafotoxins
In mammals, endothelins are the most potent endogenous vasoconstrictors to date
(Yanagisawaetal.1988),whenSRTXsdisplayvariousvasoconstriction potencies.
Thus,SRTX-bhasthesamepotentvasoconstrictionactivitythanETs,whileSRTX-c
is a very weak constrictor and showsat high doses a vasodilatory effect(Wollberg
etal.1989).Hanetal.haveshownonisolatedperfusedheartthatSRTX-bandET-1
had potent positive inotropic and negative chronotropic effects (Han et al. 1990).
Animal studies have shown that intravenous injection of ET-1 is responsible of an
increaseofbloodpressureduetoaperipheralvasoconstrictionwhile intracoronary
injection provokes a coronary vasoconstriction (Konrad et al. 2005). Recently, an
invivostudyonanesthetizedratsundermechanicalventilation,combininginvasive
catheterizationoftheleftventricleandDopplerechocardiography,hasinvestigated
the precise cardiovascular effect of short and long SRTXs (Mahjoub et al. 2015).
TheseauthorsshowedthatintravenousinfusionofSRTX-bdecreasescardiacoutput
andimpairsleftventricularsystolicanddiastolicfunction(negativelusitropiceffect)
whileincreasingtotalperipheral resistance.Hence,SRTX-b(themostpotentvaso-
constrictoramongSRTXs)impairsleftventricularfunctioninvivobyincreasingleft
ventricularafterload.ConcerninglongSRTXs,theauthorshaveshownthatSRTX-m
hasnoeffectonleftventricularafterloadorcontractilefunction.Interestingly,these
authors have shown that SRTX-m induces an acute right ventricular dysfunction
while SRTX-b has no effect on the right ventricle. SRTX-m seems to have also a
significant effect on bronchial smooth muscle cell as intravenous injection of
SRTX-m significantly increases airway pressure. Moreover, a truncated form of
SRTX-m (SRTX-m without its longer C-terminus extension) has exactly the same
effectsasSRTX-b(Mahjoubetal.2015).
Together, these results pave the way for several hypotheses. First, an in vivo
maturation of thelong SRTXs by an endogenous enzyme may liberate a short-like
SRTX.ThepreviousresultsdonotsupportthishypothesisasSRTX-mandSRTX-b
10 Y.Mahjoubetal.
havedifferentinvivoeffects.Second,onemayconsidertheinvolvementofanother
subtype of receptors (non-A, non-B endothelin receptors). Some authors have
postulated this hypothesis: Zeng et al. have cloned and characterized a human
genethatencodesaputative“humanET-Blikeprotein”fromahumanhippocampus
tissue cDNA library. The deduced amino acid sequence of this protein is 52 %
similar and 26.7 % identical to human ET-B, but ligands and physiological signif-
icance of this protein remains unknown (Zeng et al. 1997). Moreover, Bdolah
et al. have found that to thecontrary to ET-1, SRTX-b failed to induce contraction
ofEgyptianmongoose(Herpestesichneumon)aortalpreparation.Theseauthorsalso
foundET-1andSRTXspecificbindingsitesinthemongoosebrain.Thus,theymade
thehypothesisthatsomestructuralfeaturesofET/SRTXreceptorsinthemongoose
enablethemtodifferentiatebetweenthetwopeptides(Bdolahetal.1997).Further
studiesremainnecessarytoconfirmthishypothesisespeciallyforothermammalian
species.Finally,a hetero-oligomerization process may modulate thepharmacology
ofinteractingligands.Fukurodaetal.providedevidencethatthecombinedtreatment
with ET-A and ET-B antagonists antagonized the contraction of human isolated
bronchiinducedbyET-1butnosignificantinhibitionwasobservedwiththeuseof
the respective antagonist alone suggesting collaboration in the recognition of ET-1
betweenET-AandET-Breceptors(Fukurodaetal.1996).Haradaetal.havestudied
bindingpropertiesofET-AandET-Breceptorsofratpituitaryglands.Theyconclude
thatalthoughthereareET-AandET-Breceptorswithafunctionalbindingcapability
for ET-receptor–ligands, the ET-B receptor does not independently recognize ET-1
withouttheaidoftheET-Areceptor.TheymadethehypothesisthatET-1canbridge
between the two receptors to form an ET-A-ET-B receptor heterodimer (Harada
etal.2002).Thephenomenonofhetero-oligomerizationofG-protein-couplerecep-
tors has been described for functional opioid receptors (k and δ) (Jordan and Devi
1999).Furtherstudiesremainmandatorytoinvestigatethesehypotheses.
Nevertheless,theseresultsemphasizethemajorandelaboratedrole“played”by
theC-terminusextensionindefiningtheinvivoeffectsofSRTXs.
Conclusion and Future Directions
SRTXsextractedfromthevenomofatractaspididaesnakesbelongtotheendothelin-
like peptide family. They are potent vasoconstrictors. Short SRTXs have high
affinity for endothelin receptors especially for ET-B and high toxicity. Long
SRTXs have an extended C-terminal domain with to three or four additional
amino acids. This extended C-terminal domain is responsible for a dramatic decay
in agonist affinity but did not impair significantly the toxicity of these peptides. In
vivo experiments have shown that the cardiovascular effects of short and long
SRTXs are completely different. Long SRTXs impair right ventricular function
andprovokebronchoconstrictionwhileshortSRTXsimpairleftventricularfunction
withnoeffectontherightventricle.Someauthorshavemadethehypothesisfornew
subtypereceptorsorhetero-olgomerizationprocess.Furtherexperimentsareneces-
sarytocharacterizetheroleoftheC-terminusextensionindefiningtheinvivotoxic