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Apolipoprotein H
Apolipoprotein H (Apo-H), previously known as (β2-glycoprotein I, beta-2 glycoprotein I), is a multifunctional apolipoprotein. One of its function is to bind cardiolipin. When bound the structure of cardiolipin and Apo-H both undergo large changes in structure.[1] Within the structure of Apo-H is a stretch of positively charged amino acids, (protein sequence positions 282-287) Lys-Asn-Lys-Glu-Lys-Lys, are involved in phospholipid binding (See image on right).[2] Apo-H has a complex involvement in agglutination, it appears to alter ADP mediated agglutenation of platlets.[3] Normally Apo-H assumes an anti-coagulation activity in serum (by inhibiting coagulation factors), however changes in blood factors can result of a reversal of that activity. Inhibitory activitiesApo-H appears to completely inhibit serotonin release by the platelets[4] and prevents subsequent waves of the ADP-induced aggregation. The activity of Apo-H appears to involve the binding of agglutenating, negatively charged compounds, and inhibits agglutenation by the contact activation of the intrinsic blood coagulation pathway. [5] Apo-H causes a reduction of the prothrombinase binding sites on platelets and reduces the activation caused by collagen when thrombin is present at physiological serum concentrations of Apo-H suggesting a regulatory role of Apo-H in coagulation.[6] Apo-H also inhibits the generation of factor Xa in the presence of platelets.[7] Apo-H also inhibits that activation of factor XIIa.[8] In addition, Apo-H inhibits the activation of protein C blocking its activity on phosphatidylserine:phosphatidylcholine vesicles[9] however once protein C is activated, Apo-H fails to inhibit activity. Since protein C is involved in factor Va degradation Apo-H indirectly inhibits the degradation of factor Va. [10] This inhibitory activity was diminished by adding phospholipids suggesting the Apo-H inhibition of protein C is phospholipid competitive.[11] This indicates that under certain conditions Apo-H takes on a procoagulation properties. PathologyAnti-cardiolipin antibodies are found in both infectious (syphilis) and autoimmune disease(sclerosis, lupus).[12] The activity of anti-cardiolipin antibodies in autoimmune antiphospholipid syndrome requires apolipoprotein H.[13][14] The subset of antibodies that bind Apo-H and alter its activity are condsidered different from antibodies that bind thrombin, serum phospholipids and are called anti-apolipoprotein antibodies. In autoimmune disease, anti-apolipoprotein antibodies (Anti β2 glycoprotein I antibodies) strongly associate with thrombitic forms of lupus and sclerosis. References
Categories: Genes on chromosome 17 | Human proteins | Autoimmune diseases | Apolipoproteins | Autoantigens |
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Apolipoprotein_H". A list of authors is available in Wikipedia. |
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