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Factor X
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==History== American and British scientists described deficiency of factor X independently in 1953 and 1956, respectively. As with some other coagulation factors, the factor was initially named after these patients, a Mr Rufus Stuart (1921) and a Miss Audrey Prower (1934). At that time, those investigators could not know that the human genetic defect they had identified would be found in the previously characterized enzyme called thrombokinase. Thrombokinase was the name coined by Paul Morawitz in 1904 to describe the substance that converted prothrombin to thrombin and caused blood to clot.<ref>{{cite journal |last1=Morawitz |first1=Paul |title=Beitrage zur Kenntnis der Blutgerinnung |journal=Deutsches Archiv für Klinische Medizin |volume=79 |pages=432–442}}</ref> That name embodied an important new concept in understanding blood coagulation – that an enzyme was critically important in the activation of prothrombin. Morawitz believed that his enzyme came from cells such as platelets yet, in keeping with the state of knowledge about enzymes at that time, he had no clear idea about the chemical nature of his thrombokinase or its mechanism of action. Those uncertainties led to decades during which the terms thrombokinase and thromboplastin were both used to describe the activator of prothrombin and led to controversy about its chemical nature and origin.<ref>{{cite journal | vauthors = Milstone JH | title = On the evolution of blood clotting theory | journal = Medicine | volume = 31 | issue = 4 | pages = 411–447 | date = December 1952 | pmid = 13012730 | doi = 10.1097/00005792-195212000-00004 | doi-access = free }}</ref> In 1947, J Haskell Milstone isolated a proenzyme from bovine plasma which, when activated, converted prothrombin to thrombin. Following Morawitz’s designation, he called it prothrombokinase <ref>{{cite journal | vauthors = Milstone JH | title = Prothrombokinase and the Three Stages of Blood Coagulation | journal = Science | volume = 106 | issue = 2762 | pages = 546–547 | date = December 1947 | pmid = 17741228 | doi = 10.1126/science.106.2762.546-a | s2cid = 35643683 | bibcode = 1947Sci...106..546M }}</ref> and by 1951 had purified the active enzyme, thrombokinase. Over the next several years he showed that thrombokinase was a proteolytic enzyme that, by itself, could activate prothrombin. Its activity was greatly enhanced by addition of calcium, other serum factors, and tissue extracts,<ref>{{cite journal | vauthors = Milstone LM | title = Factor Xa: Thrombokinase from Paul Morawitz to J Haskell Milstone | journal = Journal of Thrombosis and Thrombolysis | volume = 52 | issue = 2 | pages = 364–370 | date = August 2021 | pmid = 33484373 | doi = 10.1007/s11239-021-02387-6 | s2cid = 231682954 }}</ref> which represented the thromboplastins that promoted the conversion of prothrombin to thrombin by their interaction with thrombokinase. In 1964 Milstone summarized his work and that of others: “There are many chemical reactions which are so slow that they would not be of physiological use if they were not accelerated by enzymes. We are now confronted with a reaction, catalyzed by an enzyme, which is still too slow unless aided by accessory factors.” <ref>{{cite journal | vauthors = Milstone JH, Oulianoff N, Milstone VK | title = Thrombokinase as prime activator of prothrombin: historical perspectives and present status | journal = The Journal of General Physiology | volume = 47 | issue = 2 | pages = 315–327 | date = November 1963 | pmid = 14080818 | pmc = 2195336 | doi = 10.1085/jgp.47.2.315 }}</ref>
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