Platelet-activating factor: Definition from Answers.com

Platelet-activating factor

Identifiers
CAS number	74389-68-7
PubChem	108156
MeSH	Platelet+Activating+Factor
Properties
Molecular formula	C₂₆H₅₄NO₇P
Molar mass	523.68 g mol⁻¹
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Platelet-activating factor, also known as a PAF, PAF-acether or AGEPC (acetyl-glyceryl-ether-phosphorylcholine) is a potent phospholipid activator and mediator of many leukocyte functions, including platelet aggregation, inflammation, and anaphylaxis.

It is produced in response to specific stimuli by a variety of cell types, including neutrophils, basophils, platelets, and endothelial cells.

1 Chemistry
2 Function
3 History
4 Biosynthesis and degradation
5 Antagonists
6 See also
7 References
8 External links

Chemistry

Several molecular species of platelet-activating factor have been identified which vary in the length of the O-alkyl side chain.

Its alkyl group is connected by an ether linkage at the C1 carbon to a sixteen carbon chain.
The acyl group at the C2 carbon is an acetate unit (as opposed to a fatty acid) whose short length increases the solubility of PAF, allowing it to function as a soluble signal messenger.
The C3 has a phosphocholine head group, just like standard phosphatidylcholine.

Function

It is an important mediator of bronchoconstriction.

It causes platelets to aggregate and blood vessels to dilate. Thus it is important to the process of hemostasis. At a concentration of 10^-12 M, PAF causes life threatening inflammation of the airways to induce asthma like symptoms.

Toxins such as fragments of destroyed bacteria induce the synthesis of PAF, which causes a drop in blood pressure and reduced volume of blood pumped by the heart, which leads to shock and maybe death.

History

It was discovered by French immunologist Jacques Benveniste in the early 1970s.^[1]^[2] Its structure was elucidated by Constantinos A. Demopoulos in 1979.^[3]

Biosynthesis and degradation

PAF is biosynthesized from lyso phosphatidylcholine (LPC) and acetyl CoA by the enzyme LPC acetyltransferase (LPCAT).

It is degraded (thereby terminating its capacity to act as a signaling molecule) by a group of enzymes called PAF acetylhydrolases (PAFAHs) which are related to phospholipase A2.

Antagonists

SM-12502 is a PAF antagonist, which is metabolized in the liver by the enzyme CYP2A6. ^[4]
Rupatadine is an antihistamine and PAF antagonist used to treat allergies.

References

^ Benveniste J, Henson PM, Cochrane CG (1972). "Leukocyte-dependent histamine release from rabbit platelets. The role of IgE, basophils, and a platelet-activating factor". J. Exp. Med. 136 (6): 1356–77. doi:10.1084/jem.136.6.1356. PMID 4118412.
^ Benveniste J (1974). "Platelet-activating factor, a new mediator of anaphylaxis and immune complex deposition from rabbit and human basophils". Nature 249 (457): 581–2. doi:10.1038/249581a0. PMID 4275800.
^ Demopoulos CA, Pinckard RN, Hanahan DJ (1979). "Platelet-activating factor. Evidence for 1-O-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine as the active component (a new class of lipid chemical mediators)" (abstract). J. Biol. Chem. 254 (19): 9355–8. PMID 489536. http://www.jbc.org/cgi/content/abstract/254/19/9355.
^ Platelet-Activating Factor Antagonist, SM-12502, Attenuates ...

External links

v • d • e Lipids: phospholipids

Glycerol backbone (Glycerophospholipids/ Phosphoglycerides)	Phosphatidyl-: -ethanolamine/cephalin - -choline/lechithin (dipalmitoyl-) - -serine - -glycerol - -inositol (glyco-) Phosphoinositides: PIP (PI(3)P, PI(4)P, PI(5)P) - PIP₂ (PI(3,4)P₂, PI(3,5)P₂, PI(4,5)P₂) - PIP₃ Cardiolipin Ether lipids: Plasmalogen (Platelet-activating factor)

Sphingosine backbone	Sphingomyelin

v • d • e Autacoids

Kinins	Kininogen (MHWK, LHWK) • Bradykinin • Kallidin • Tachykinins • Urotensin-II

Others	Angiotensin • Eicosanoid • Histamine • Platelet-activating factor • Serotonin

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Platelet-activating factor

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