Everything about Apolipoprotein H totally explained
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| Name = Apolipoprotein H (beta-2-glycoprotein I)
| HGNCid = 616
| Symbol = APOH
| AltSymbols =; B2G1; BG
| OMIM = 138700
| ECnumber =
| Homologene = 26
| MGIid = 88058
| GeneAtlas_image1 = PBB_GE_APOH_205216_s_at_tn.png
| Function =
| Component =
| Process =
| Orthologs =
}}
Apolipoprotein H (Apo-H), previously known as (β
2-glycoprotein I, beta-2 glycoprotein I), is a multifunctional
apolipoprotein. One of its functions is to bind
cardiolipin. When bound the structure of cardiolipin and Apo-H both undergo large changes in structure. 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).
Apo-H has a complex involvement in agglutination, it appears to alter ADP mediated agglutenation of platlets. 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 activities
Apo-H appears to completely inhibit
serotonin release by the platelets 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. 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.
Apo-H also inhibits the generation of
factor Xa in the presence of platelets. Apo-H also inhibits that activation of
factor XIIa.
In addition, Apo-H inhibits the activation of protein C blocking its activity on phosphatidylserine:phosphatidylcholine vesicles 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. This inhibitory activity was diminished by adding phospholipids suggesting the Apo-H inhibition of protein C is phospholipid competitive. This indicates that under certain conditions Apo-H takes on a procoagulation properties.
Pathology
Anti-cardiolipin antibodies are found in both infectious (
syphilis) and autoimmune disease(
sclerosis,
lupus). The activity of
anti-cardiolipin antibodies in autoimmune
antiphospholipid syndrome requires apolipoprotein H. 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 β
2 glycoprotein I antibodies) strongly associate with thrombitic forms of
lupus and
sclerosis.
Further Information
Get more info on 'Apolipoprotein H'.
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