Heparin

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Heparin

Heparin is a highly sulfated glycosaminoglycan widely used as an injectable anticoagulant. It is also used to form an inner anticoagulant surface on various experimental and medical devices such as test tubes and renal dialysis machines. Pharmaceutical grade heparin is commonly derived from the tissue of slaughter house animals e.g. porcine intestine or bovine lung..

History

Heparin is one of the oldest drugs currently still in widespread clinical use. Its introduction predates the establishment of the United States Food and Drug Administration.. It was originally isolated from liver cells, hence its name (hepar or "ηπαρ" is Greek for "liver"). Scientists were looking for an anticoagulant that could work safely in humans, and Jay McLean, a second-year medical student from Johns Hopkins University working under the guidance of William Henry Howell, found a compound extracted from liver that acted as an anticoagulant.

Medical uses

Heparin acts as an anticoagulant, preventing the formation of clots and extension of existing clots within the blood. While heparin does not break down clots that have already formed, it allows the body's natural clot lysis mechanisms to work normally to break down clots that have already formed. Heparin is used for anticoagulation for the following conditions:

Administration

Heparin is given parenterally; it is digested when taken by mouth. It can be injected intravenously or subcutaneously (under the skin). Intramuscular injections (into muscle) are avoided because of the potential for forming hematomas. Because of its short biologic half-life of approximately one hour, heparin must be given frequently or as a continuous infusion.

If long-term anticoagulation is required, heparin is often only used to commence anticoagulation therapy until the oral anticoagulant warfarin takes effect.

Adverse reactions

A serious side-effect of heparin is heparin-induced thrombocytopenia (HIT syndrome). HITS is caused by an immunological reaction that makes platelets aggregate within the blood vessels, thereby using up coagulation factors. Formation of platelet clots can lead to thrombosis, while the loss of coagulation factors and platelets may result in bleeding. HITS can (rarely) occur shortly after heparin is given, but also when a person has been on heparin for a long while. Immunologic tests are available for the diagnosis of HITS. There is also a benign form of thrombocytopenia associated with early heparin use which resolves without stopping heparin.

Other side effects include alopecia and osteoporosis.

Treatment of overdose

In case of overdose, protamine sulfate can be given to counteract the action of heparin.

Heparin structure

Native heparin is a polymer with a molecular weight ranging from 6 kDa to 40 kDa although the average molecular weight of most commercial heparin preparations is in the range of 12 kDa to 15 kDa. Heparin is a member of the glycosaminoglycan family of carbohydrates (which includes the closely related molecule heparan sulfate) and consists of a variably sulfated repeating disaccharide unit. GlcA-GlcNS.png IdoA-GlcNS.png | IdoA-GlcNS(6S).png IdoA(2S)-GlcNS(6S).png

Abbreviations

GlcA=β-L-glucuronic acid, IdoA=α-L-iduronic acid, IdoA(2S)=2-O-sulfo-α-L-iduronic acid, GlcNAc=2-deoxy-2-acetamido-α-D-glucopyranosyl, GlcNS=2-deoxy-2-sulfamido-α-D-glucopyranosyl, GlcNS(6S)=2-deoxy-2-sulfamido-α-D-glucopyranosyl-6-O-sulfate.

3-dimensional structure

The three dimensional structure of heparin is complicated by the fact that iduronic acid may be present in either of two low energy conformations when internally positioned within an oligosaccharide. The conformational equilibrium being influenced by sulfation state of adjacent glucosamine sugars. Nevertheless the solution structure of a heparin dodecasacchride composed solely of six GlcNS(6S)-IdoA(2S) repeat units has been determined using a combination of NMR spectroscopy and molecular modelling techniques. Two models were constructed one in which all IdoA(2S) were in the 2S0 conformation (A and B below) and one in which they are in the 1C4 conformation (C and D below). These models correspond to the protein data bank code 1HPN

]1HPNmod1_VDW.jpeg 1HPNmod2_lines.jpeg

In these models heparin adopts a helical conformation, the rotation of which places clusters of sulfate groups at regular intervals of about 17 Angstroms on either side of the helical axis.

Mechanism of action

Heparin binds to the enzyme inhibitor antithrombin III (AT-III) causing a conformational change which results in its active site being exposed. The activated AT-III then inactivates thrombin and other proteases involved in blood clotting, most notably factor Xa. The rate of inactivation of these proteases by AT-III increases 1000-fold due to the binding of heparin..

AT-III binds to a specific pentasaccharide sulfation sequence contained within the heparin polymer

GlcNAc/NS(6S)-GlcA-GlcNS(3S,6S)-IdoA(2S)-GlcNS(6S)

The conformational change in AT-III on heparin binding mediates its inhibition of factor Xa. For thrombin inhibition however, thrombin must also bind to the heparin polymer at a site proximal to the pentasacchride. The formation of a ternary complex between AT-III, thrombin and heparin results in the inactivation of thrombin. For this reason heparin's activity against thrombin is size dependant, the ternary complex requiring at least 18 sacchride units for efficient formation.In contrast anti factor Xa activity only requires the pentasacchride binding site.

This size difference has led to the development of low molecular weight heparins and more recently to fondaparinux (a synthetic pentasacchride identical in structure to the AT-III binding pentasacchride) as pharmaceutical anticoagulants. These products target anti factor Xa activity rather than anti thrombin (IIa) activity with the aim of facilitating a more subtle regulation of coagulation and an improved therapeutic index.

With these unfractioned heparin alternatives, there is a reduced risk of osteoporosis and heparin-induced thrombocytopenia (HIT). Monitoring of the APTT is also not required and indeed does not reflect the anticoagulant effect, as APTT is insensitive to alterations in factor Xa.

The effects of heparin are measured in the lab by the partial thromboplastin time (aPTT), (the time it takes the blood plasma to clot).

Heparins exact physiological role is still unclear, because blood anti-coagulation is mostly achieved by endothelial cell-derived heparan sulfate proteoglycans.

Other uses/information

Test tubes, Vacutainers, and capillary tubes that use the lithium salt of heparin (lithium heparin) as an anticoagulant are usually marked with green stickers and green tops. Heparin has the advantage over EDTA as an anticoagulant, as it does not affect levels of ions (such as calcium). Heparin can interfere with some immunoassays, however. As lithium heparin is usually used, a person's lithium levels cannot be obtained from these tubes; for this purpose, royal-blue topped Vacutainers containing sodium heparin are used.

Heparin gel

Heparin gel (topical) may sometimes be used to treat sports injuries. It is known that the diprotonated form of histamine binds site specifically to heparin.The release of histamine from mast cells at a site of tissue injury contributes to an inflammatory response. The rationale behind the use of such topical gels may be to block the activity of released histamine and so help to reduce inflammation.

Heparin and PCR

Heparin inteferes with the PCR reaction and care should be taken when heparinized blood is used as a PCR material. Specimens with large amounts of DNA exhibit less interference. The addition of 0.1 to 0.0016 U of heparin per reaction mixture (50 mL) suppresses DNA amplification in a dose-dependent fashion. However considerable interference differences were found among Taq DNA polymerases. The most dramatic inhibitory effect of heparin was observed when QIATaq was used, followed by Ph Taq, Takara Taq, Wako Taq, PE Taq, and ExTaq in this order.

References

External links

History of heparin

Anticoagulants | Glycosaminoglycans

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