Pathways

Ximelagatran is an anticoagulant drug used to prevent and treat blood clots, and was the first drug in the anticoagulant drug class to be able to be ingested orally. It was discontinued from distribution by its parent company AstraZeneca in 2006 as it was found to raise liver enzyme levels in patients and cause liver damage as a result. Ximelagatran inhibits prothrombin. Then zooming in even further to the endoplasmic reticulum within the liver, vitamin K1 2,3-epoxide uses vitamin K epoxide reductase complex subunit 1 to become reduced vitamin K (phylloquinone), and then back to vitamin K1 2,3-epoxide continually through vitamin K-dependent gamma-carboxylase. This enzyme also catalyzes precursors of prothrombin and coagulation factors VII, IX and X to prothrombin, and coagulation factors VII, IX and X. From there, these precursors and factors leave the liver cell and enter into the blood capillary bed. Once there, prothrombin is inhibited by ximelagatran, and is catalyzed into the protein complex prothrombinase complex which is made up of coagulation factor Xa/coagulation factor Va (platelet factor 3). These factors are joined by coagulation factor V and ximelagatran inhibits prothrombin. Through the two factors coagulation factor Xa and coagulation factor Va, thrombin is produced and inhibited by ximelagatran, which then uses fibrinogen alphabet, and gamma chains to create fibrin (loose). This is then turned into coagulation factor XIIIa, which is activated through coagulation factor XIII A and B chains. From here, fibrin (mesh) is produced which interacts with endothelial cells to cause coagulation. Plasmin is then created from fibrin (mesh), then joined by tissue-type plasminogen activator through plasminogen and creates fibrin degradation products. These are enzymes that stay in your blood after your body has dissolved a blood clot. Coming back to the factors transported from the liver, coagulation factor X is catalyzed into a group of enzymes called the tenase complex: coagulation factor IX and coagulation factor VIIIa (platelet factor 3). This protein complex is also contributed to by coagulation factor VIII, which through prothrombin is catalyzed into coagulation factor VIIIa. Prothrombin is inhibited by ximelagatran here as well. From there, this protein complex is catalyzed into prothrombinase complex, the group of proteins mentioned above, contributing to the above process ending in fibrin degradation products. Another enzyme transported from the liver is coagulation factor IX which becomes coagulation factor IXa, part of the tense complex, through coagulation factor XIa. Coagulation factor XIa is produced through coagulation factor XIIa which converts coagulation XI to become coagulation factor XIa. Coagulation factor XIIa is introduced through chain of activation starting in the endothelial cell with collagen alpha-1 (I) chain, which paired with coagulation factor XII activates coagulation factor XIIa. It is also activated through plasma prekallikrein and coagulation factor XIIa which activate plasma kallikrein, which then pairs with coagulation factor XII simultaneously with the previous collagen chain pairing to activate coagulation XIIa. Lastly, the previously transported coagulation factor VII and tissue factor coming from a vascular injury work together to activate tissue factor: coagulation factor VIIa. This enzyme helps coagulation factor X catalyze into coagulation factor Xa, to contribute to the prothrombinase complex and complete the pathway.

Ximelagatran is a prodrug that is converted to melagatran. This drug is a direct thrombin inhibitor that binds directly to the active site of thrombin, preventing coagulation and formation of blood clots due to the inactivation of the enzyme that catalyzes activation of coagulation factors V, XIII and fibrinogen. Ximelagatran is the first direct thrombin inhibitor that could be taken orally, and was thought to be a replacement for warfarin, due to not having as many dietary restrictions involved. However, it was withdrawn from testing and distribution following evidence of liver damage. After oral administration, ximelagatran is absorbed and converted to melagatran through an unknown series of reactions. From there, melagatran directly binds to thrombin, preventing its use as an enzyme. This prevents catalyzation of factor V to factor Va, which would form the prothrombinase complex and create more thrombin. It also prevents the catalysis fibrinogen or factor I to fibrin, which then polymerizes to form the blood clot. The lack of useable thrombin also prevents the catalysis of factor XIII to factor XIIIa, which is necessary to crosslink the polymerized fibrin to form a water insoluble clot.

Xanthophyll cycle is a pathway that transforms zeaxanthin to violaxanthin and antheraxanthin through enzymes. Xanthophyll cycle mainly takes place in diatoms and dinoflagellates of plants in high-light condition. Zeaxanthin is obatined from zeaxanthin biosynthesis that transforms lycopene to zeaxanthin (indirectly). Zeaxanthin is catalyzed into antheraxanthin and antheraxanthin catalyzed into violaxanthin both by the enzyme, zeaxanthin epoxidase with cofactor FAD. Violaxanthin deepoxidase/antheraxanthin deepoxidase can reverse the above reactions (i.e. violaxanthin to antheraxanthin and antheraxanthin to zeaxanthin).

Xanthinuria Type II is a rare inborn error of metabolism (IEM) and autosomal recessive disorder and caused by a defective xanthine dehydrogenase. Xanthine dehydrogenase catalyzes the conversion of hypoxanthine into xanthine and conversion of xanthine into uric acid. This disorder is characterized by a large accumulation of xanthine and hypoxanthine; as well as dissipation of uric acid. Symptoms of the disorder include blood in the urine, recurrent urinary tract infections and abdominal pain. It is estimated that xanthinuria types I and II affects 1 in 69,000 individuals.

Xanthinuria Type II is a rare inborn error of metabolism (IEM) and autosomal recessive disorder and caused by a defective xanthine dehydrogenase. Xanthine dehydrogenase catalyzes the conversion of hypoxanthine into xanthine and conversion of xanthine into uric acid. This disorder is characterized by a large accumulation of xanthine and hypoxanthine; as well as dissipation of uric acid. Symptoms of the disorder include blood in the urine, recurrent urinary tract infections and abdominal pain. It is estimated that xanthinuria types I and II affects 1 in 69,000 individuals.

Xanthinuria Type II is a rare inborn error of metabolism (IEM) and autosomal recessive disorder and caused by a defective xanthine dehydrogenase. Xanthine dehydrogenase catalyzes the conversion of hypoxanthine into xanthine and conversion of xanthine into uric acid. This disorder is characterized by a large accumulation of xanthine and hypoxanthine; as well as dissipation of uric acid. Symptoms of the disorder include blood in the urine, recurrent urinary tract infections and abdominal pain. It is estimated that xanthinuria types I and II affects 1 in 69,000 individuals.

Xanthinuria Type II (Xanthine Dehydrogenase Deficiency) is a rare inborn error of metabolism (IEM) and autosomal recessive disorder and caused by a defective xanthine dehydrogenase. Xanthine dehydrogenase catalyzes the conversion of hypoxanthine into xanthine and conversion of xanthine into uric acid. This disorder is characterized by a large accumulation of xanthine and hypoxanthine; as well as dissipation of uric acid. Symptoms of the disorder include blood in the urine, recurrent urinary tract infections and abdominal pain. It is estimated that xanthinuria types I and II affects 1 in 69,000 individuals.