Due to recent observations which suggest that the calcium binding property of prothrombin results from the action of vitamin K, the binding of calcium to prothrombin has been examined more carefully. The binding curve is sigmoidal, and at 1 mM calcium, 4 mol of calcium is bound per mol of prothrombin. There was a strong dependence of calcium binding on pH with maximum binding occurring between pH's 7.5 and 9.5. The loss of calcium binding on either side of this range is very marked, suggesting an ionization event at these pH's. A biologically inactive prothrombin produced by dicumarol-treated cows has a lower calcium binding affinity, and at 1 mM less than 1 mol of calcium is bound per mol. In contrast to normal prothrombin, this abnormal prothrombin does not adsorb onto barium citrate, and this property appears to be related to the same sites that cause calcium binding. Modification of prothrombin with 8 M urea or 8 M urea plus reduction of disulfide bonds revealed that tertiary structure and disulfide bonds are necessary for maximum calcium binding. However, if the proteins were allowed to refold and reoxidize, calcium binding was fully restored. Similar treatment of the abnormal prothrombin did not result in any detectable calcium binding. These observations indicate that the abnormal prothrombin differs from the normal protein in some way other than its disulfide-bond arrangement or tertiary structure.
|Original language||English (US)|
|Number of pages||4|
|State||Published - 1972|