Apamin: Difference between revisions

| Verifiedfields = changed

| Watchedfields = changed

| verifiedrevid = 399510913

| Reference = <ref>[https://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=16129677 Apamin - Compound Summary], [[PubChem]].</ref>

|Section1={{Chembox Identifiers

| PubChem = 16129677

| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}

| ChEMBL_Ref = {{ebicite|changed|EBI}}

| ChEMBL = 525408

| IUPHAR_ligand = 2311

| StdInChI_Ref = {{stdinchicite|correct|chemspider}}

| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}

| InChI = 1S/C79H131N31O24S4/c1-35(2)26-49-70(127)107-51-31-136-135-30-41(81)63(120)105-50(28-57(84)114)71(128)108-53(73(130)99-42(12-7-8-22-80)64(121)96-38(5)77(134)110-25-11-15-54(110)75(132)102-47(18-21-58(115)116)69(126)109-59(39(6)111)76(133)95-37(4)62(119)104-49)33-138-137-32-52(106-66(123)44(14-10-24-92-79(88)89)98-65(122)43(13-9-23-91-78(86)87)97-61(118)36(3)94-72(51)129)74(131)101-45(16-19-55(82)112)67(124)100-46(17-20-56(83)113)68(125)103-48(60(85)117)27-40-29-90-34-93-40/h29,34-39,41-54,59,111H,7-28,30-33,80-81H2,1-6H3,(H2,82,112)(H2,83,113)(H2,84,114)(H2,85,117)(H,90,93)(H,94,129)(H,95,133)(H,96,121)(H,97,118)(H,98,122)(H,99,130)(H,100,124)(H,101,131)(H,102,132)(H,103,125)(H,104,119)(H,105,120)(H,106,123)(H,107,127)(H,108,128)(H,109,126)(H,115,116)(H4,86,87,91)(H4,88,89,92)/t36?,37-,38-,39+,41-,42-,43-,44?,45-,46-,47-,48-,49-,50-,51-,52-,53-,54-,59?/m0/s1

| InChIKey1 = YVIIHEKJCKCXOB-WNKOHBQLSA-N

| CASNo_Ref = {{cascite|correct|??}}

| UNII_Ref = {{fdacite|correct|FDA}}

| UNII = X644P85KUR

|Section2={{Chembox Properties

|Section3={{Chembox Hazards

| AutoignitionPt = }}

'''Apamin''' is an 18 amino acid globular [[peptide]] [[neurotoxin]] found in [[apitoxin]] ([[bee]] [[venom]]).<ref name="pmid6095335">{{cite journal | vauthors = Habermann E | title = Apamin | journal = Pharmacology & Therapeutics | volume = 25 | issue = 2 | pages = 255–70 | year = 1984 | pmid = 6095335 | doi = 10.1016/0163-7258(84)90046-9 }}</ref> Dry bee venom consists of 2–3% of apamin.<ref name="pmid17555825">{{cite journal | vauthors = Son DJ, Lee JW, Lee YH, Song HS, Lee CK, Hong JT | title = Therapeutic application of anti-arthritis, pain-releasing, and anti-cancer effects of bee venom and its constituent compounds | journal = Pharmacology & Therapeutics | volume = 115 | issue = 2 | pages = 246–70 | date = Aug 2007 | pmid = 17555825 | doi = 10.1016/j.pharmthera.2007.04.004 }}</ref> Apamin selectively blocks [[SK channel]]s, a type of [[Ca activated K channel|Ca²⁺-activated K⁺ channel]] expressed in the [[central nervous system]]. Toxicity is caused by only a few amino acids, in particular cysteine₁, lysine₄, arginine₁₃, arginine₁₄ and histidine₁₈. These amino acids are involved in the binding of apamin to the Ca²⁺-activated K⁺ channel. Due to its specificity for SK channels, apamin is used as a drug in biomedical research to study the electrical properties of SK channels and their role in the [[afterhyperpolarization]]s occurring immediately following an [[action potential]].<ref name="pmid2469212">{{cite journal | vauthors = Castle NA, Haylett DG, Jenkinson DH | title = Toxins in the characterization of potassium channels | journal = Trends in Neurosciences | volume = 12 | issue = 2 | pages = 59–65 | date = Feb 1989 | pmid = 2469212 | doi = 10.1016/0166-2236(89)90137-9 | s2cid = 22356085 }}</ref>

The first symptoms of apitoxin (bee venom), that are now thought to be caused by apamin, were described back in 1936 by Hahn and Leditschke. Apamin was first isolated by Habermann in 1965 from ''Apis mellifera'', the [[Western honey bee]]. Apamin was named after this bee. Bee [[venom]] contains many other compounds, like histamine, [[phospholipase A2]], hyaluronidase, [[MCD peptide]], and the main active component [[melittin]]. Apamin was separated from the other compounds by gel filtration and ion exchange chromatography.<ref name="pmid6095335"/>

==Structure and active site==

Apamin is a [[polypeptide]] possessing an [[amino acid]] sequence of H-Cys-Asn-Cys-Lys-Ala-Pro-Glu-Thr-Ala-Leu-Cys-Ala-Arg-Arg-Cys-Gln-Gln-His-NH₂ (one-letter sequence CNCKAPETALCARRCQQH-NH₂, with [[disulfide bonds]] between Cys₁-Cys₁₁ and Cys₃-Cys₁₅). Apamin is very rigid because of the two disulfide bridges and seven hydrogen bonds. The three-dimensional structure of apamin has been studied with several spectroscopical techniques: [[HNMR]], [[Circular Dichroism]], [[Raman spectroscopy]], [[FT-IR]]. The structure is presumed to consist of an alpha-helix and beta-turns, but the exact structure is still unknown.<ref>{{cite journal | url = https://books.google.com/books?isbn=978-0-12-385096-6 | title = Apamin | journal = Handbook of Biologically Active Peptides | vauthors = Kastin AJ | pages = 417–418 | edition = 2013}}</ref>

By local alterations it is possible to find the [[amino acids]] that are involved in toxicity of apamin. It was found by Vincent et al. that guanidination of the ε-amino group of lysine₄ does not decrease toxicity. When the ε-amino group of lysine₄ and the α-amino group of cysteine₁ are acetylated or treated with fluorescamine, toxicity decreases with a factor of respectively 2.5 and 2.8. This is only a small decrease, which indicates that neither the ε-amino group of lysine₄ nor the α-amino group of cysteine₁ is essential for the toxicity of apamin. Glutamine₇ was altered by formation of an amide bond with glycine ethyl ester, this resulted in a decrease in toxicity of a factor 2.0. Glutamine₇ also doesn't appear to be essential for toxicity. When histidine₁₈ is altered by carbethoxylation, toxicity decreases only by a factor 2.6. But when histidine₁₈, the ε-amino group of lysine₄ and the α-amino group of cysteine₁ all are carbethoxylated and acetylated toxicity decreases drastically. This means that these three [[amino acids]] are not essential for toxicity on their own, but the three of them combined are. Chemical alteration of arginine₁₃ and arginine₁₄ by treatment of [[1,2-Cyclohexanedione|1,2-cyclohexanedione]] and cleavage by [[trypsin]] decreases toxicity by a factor greater than 10. The amino acids that cause toxicity of apamin are cysteine₁, lysine₄, arginine₁₃, arginine₁₄ and histidine₁₈.<ref name="Vincent, 1975">{{cite journal | vauthors = Vincent JP, Schweitz H, Lazdunski M | title = Structure-function relationships and site of action of apamin, a neurotoxic polypeptide of bee venom with an action on the central nervous system | journal = Biochemistry | volume = 14 | issue = 11 | pages = 2521–5 | date = Jun 1975 | pmid = 1138869 | doi = 10.1021/bi00682a035 }}</ref>

== Toxicodynamics ==

Apamin is the smallest neurotoxin polypeptide known, and the only one that passes the blood-brain barrier.<ref name="Vincent, 1975"/> Apamin thus reaches its target organ, the central nervous system. Here it inhibits small-conductance [[Ca activated K channel|Ca²⁺-activated K⁺ channels]] (SK channels) in neurons. These channels are responsible for the afterhyperpolarizations that follow action potentials, and therefore regulate the repetitive firing frequency.<ref name="Stocker 1999">{{cite journal|title= An apamin-sentisitive Ca²⁺-activated K⁺ current in hippocampal pyramidal neurons |author1=M. Stocker |author2=M. Krause |author3=P. Pedarzani | journal = PNAS | volume = 96 | issue = 8 | year = 1999 | doi= 10.1073/pnas.96.8.4662 |pmc=16389 | pmid=10200319 | pages=4662–4667|bibcode=1999PNAS...96.4662S |doi-access=free }}</ref>

Three different types of SK channels show different characteristics. Only SK2 and SK3 are blocked by apamin, whereas SK1 is apamin insensitive. SK channels function as a tetramer of subunits. Heteromers have intermediate sensitivity.<ref name="Stocker 1999" /> SK channels are activated by the binding of intracellular Ca²⁺ to the protein [[calmodulin]], which is constitutively associated to the channel.<ref name="Stocker 2004">{{cite journal | vauthors = Stocker M | title = Ca(2+)-activated K+ channels: molecular determinants and function of the SK family | journal = Nature Reviews. Neuroscience | volume = 5 | issue = 10 | pages = 758–70 | date = Oct 2004 | pmid = 15378036 | doi = 10.1038/nrn1516 | s2cid = 22211829 }}</ref> Transport of potassium ions out of the cell along their concentration gradient causes the membrane potential to become more negative. The SK channels are present in a wide range of excitable and non-excitable cells, including cells in the central nervous system, intestinal myocytes, endothelial cells, and hepatocytes.

Binding of apamin to SK channels is mediated by amino acids in the pore region as well as extracellular amino acids of the SK channel.<ref name="Nolting 2007">{{cite journal | vauthors = Nolting A, Ferraro T, D'hoedt D, Stocker M | title = An amino acid outside the pore region influences apamin sensitivity in small conductance Ca2+-activated K+ channels | journal = The Journal of Biological Chemistry | volume = 282 | issue = 6 | pages = 3478–86 | date = Feb 2007 | pmid = 17142458 | pmc = 1849974 | doi = 10.1074/jbc.M607213200 | doi-access = free }}</ref> It is likely that the inhibition of SK channels is caused by blocking of the pore region, which hinders the transport of potassium ions. This will increase the neuronal excitability and lower the threshold for generating an [[action potential]]. Other toxins that block SK channels are [[tamapin]] and [[scyllatoxin]].

== Toxicokinetics ==

The kinetics of labeled derivatives of apamin were studied in vitro and in vivo in mice by Cheng-Raude et al. This shed some light on the kinetics of apamin itself. The key organ for excretion is likely to be the [[kidney]], since enrichment of the labeled derivatives was found there. The peptide apamin is small enough to pass the [[glomerular]] barrier, facilitating renal excretion. The central nervous system, contrarily, was found to contain only very small amounts of apamin. This is unexpected, as this is the target organ for neurotoxicity caused by apamin. This low concentration thus appeared to be sufficient to cause the toxic effects.<ref>{{cite journal | vauthors = Cheng-Raude D, Treloar M, Habermann E | title = Preparation and pharmacokinetics of labeled derivatives of apamin | journal = Toxicon | volume = 14 | issue = 6 | pages = 467–76 | year = 1976 | pmid = 1014036 | doi = 10.1016/0041-0101(76)90064-7 }}</ref>

However, these results disagree with a study of Vincent et al. After injection of a supralethal dose of radioactive acetylated apamin in mice, enrichment was found in the [[spinal cord]], which is part of the target organ. Some other organs, including kidney and brain, contained only small amounts of the apamin derivative.<ref name="Vincent, 1975"/>

==Symptoms==

Symptoms following bee sting may include:

*Local effects: burning or stinging [[pain]], [[Swelling (medical)|swelling]], redness.

*Severe systemic reactions: swelling of the [[tongue]] and [[throat]], difficulty [[breathing]], and [[Shock (circulatory)|shock]].

*Development of [[optic neuritis]] and [[atrophy]].

*[[Atrial fibrillation]], [[cerebral infarction]], acute [[myocardial infarction]], [[Fisher's syndrome]], acute inflammatory [[polyradiculopathy]] ([[Guillain–Barré syndrome]]), [[Ulnar nerve entrapment|claw hand]] (through a central action of apamin on the [[spinal cord]] and a peripheral action in the form of median and [[ulna]]r [[neuritis]], causing spasms of the long [[flexor]]s in the [[forearm]]).<ref name="Saravanan 2004">{{cite journal | vauthors = Saravanan R, King R, White J | title = Transient claw hand owing to a bee sting. A report of two cases | journal = The Journal of Bone and Joint Surgery. British Volume | volume = 86 | issue = 3 | pages = 404–5 | date = Apr 2004 | pmid = 15125129 | doi = 10.1302/0301-620x.86b3.14311 | doi-access = free }}</ref>

Apamin is an element in [[bee]] [[venom]]. A person can come into contact with apamin through bee venom, so the symptoms that are known are not caused by apamin directly, but by the venom as a whole. Apamin is the only neurotoxin acting purely on the central nervous system. The symptoms of apamin toxicity are not well known, because people are not easily exposed to the toxin alone.<ref name="Habermann, 1977">{{cite journal | vauthors = Habermann E | title = Neurotoxicity of apamin and MCD peptide upon central application | journal = Naunyn-Schmiedeberg's Archives of Pharmacology | volume = 300 | issue = 2 | pages = 189–91 | date = Nov 1977 | pmid = 593441 | doi=10.1007/bf00505050| s2cid = 11709787 }}</ref>

Through research about the neurotoxicity of apamin some symptoms were discovered. In mice, the injection of apamin produces convulsions and long-lasting spinal spasticity. Also, it is known that the polysynaptic spinal reflexes are disinhibited in cats.<ref name="Habermann, 1977"/> Polysynaptic reflex is a reflex action that transfers an impulse from a sensory neuron to a motor neuron via an interneuron in the spinal cord.<ref>{{cite web|url=http://www.encyclopedia.com/doc/1O6-polysynapticreflex.html | title = polysynaptic reflex}}</ref> In rats, apamin was found to cause tremor and ataxia, as well as dramatic haemorrhagic effects in the [[lung]]s.<ref name="pmid8560501">{{cite journal | vauthors = Lallement G, Fosbraey P, Baille-Le-Crom V, Tattersall JE, Blanchet G, Wetherell JR, Rice P, Passingham SL, Sentenac-Roumanou H | title = Compared toxicity of the potassium channel blockers, apamin and dendrotoxin | journal = Toxicology | volume = 104 | issue = 1–3 | pages = 47–52 | date = Dec 1995 | pmid = 8560501 | doi = 10.1016/0300-483X(95)03120-5 }}</ref>

Furthermore, apamin has been found to be 1000 times more efficient when applied into the ventricular system instead of the peripheral nervous system. The ventricular system is a set of structures in the brain containing cerebrospinal fluid. The peripheral nervous system contains the nerves and ganglia outside of the brain and spinal cord.<ref name="Habermann, 1977"/> This difference in efficiency can easily be explained. Apamin binds to the SK channels, which differ slightly in different tissues. So apamin binding is probably stronger in SK channels in the ventricular system than in other tissues.

==Toxicity rates==

In earlier years it was thought that apamin was a rather nontoxic compound (LD₅₀ = 15&nbsp;mg/kg in mice) compared to the other compounds in bee venom.<ref>{{cite journal|url=http://www.dtic.mil/cgi-bin/GetTRDoc?AD=AD746242 |archive-url=https://web.archive.org/web/20130409222326/http://www.dtic.mil/cgi-bin/GetTRDoc?AD=AD746242 |url-status=dead |archive-date=April 9, 2013 | title = Beta adrenergic and antiarrhythmic effect of apamin, a component of bee venom | author = department of the army Edgewood Arsenal biodemical laboratory | year = 1972}}</ref> The current lethal dose values of apamin measured in mice are given below.<ref>{{cite web | url = http://datasheets.scbt.com/sc-200994.pdf | work = Material Safety Data Sheet | title = Apamin }}</ref> There are no data known specific for humans.

Intraperitoneal (mouse) LD₅₀: 3.8&nbsp;mg/kg

Subcutaneous (mouse) LD₅₀: 2.9&nbsp;mg/kg

Intravenous (mouse) LD₅₀: 4&nbsp;mg/kg

Intracerebral (mouse) LD₅₀: 1800&nbsp;ng/kg

Parenteral (mouse) LD₅₀: 600&nbsp;mg/kg

==Therapeutic use==

Recent studies have shown that SK channels do not only regulate afterhyperpolarization, they also have an effect on [[synaptic plasticity]]. This is the activity-dependent adaptation of the strength of synaptic transmission. Synaptic plasticity is an important mechanism underlying learning and memory processes. Apamin is expected to influence these processes by inhibiting SK channels. It has been shown that apamin enhances learning and memory in rats and mice.<ref name="Stocker 1999" /><ref name="Faber, 2007">{{cite journal | vauthors = Faber ES, Sah P | title = Functions of SK channels in central neurons | journal = Clinical and Experimental Pharmacology & Physiology | volume = 34 | issue = 10 | pages = 1077–83 | date = Oct 2007 | pmid = 17714097 | doi = 10.1111/j.1440-1681.2007.04725.x | s2cid = 5553791 }}</ref> This may provide a basis for the use of apamin as a treatment for memory disorders and cognitive dysfunction. However, due to the risk of toxic effects, the therapeutic window is very narrow.<ref name="Faber, 2007"/>

SK channel blockers may have a therapeutic effect on [[Parkinson's disease]]. Dopamine, which is depleted in this disease, will be released from midbrain dopaminergic neurons when these SK channels are inhibited. SK channels have also been proposed as targets for the treatment of [[epilepsy]], [[emotional disorder]]s and [[schizophrenia]].<ref name="Faber, 2007"/>

== References ==

{{Reflist|33em}}

== External links ==

[[Category:Peptides]]