Abstract
Bovine thrombin is rapidly inactivated by nitrous acid at pH 4.35 and 0°. The clotting activity falls off more rapidly than the activity towards N-benzoylarginine ethyl ester. The loss of esterase activity parallels the loss of the N-terminal isoleucine residue. N-Benzoylarginine ethyl ester gives almost full protection of the esterase activity and the N-terminal isoleucine, but only partial protection of the clotting activity. These results suggest strongly that this residue is essential for activity and indicate that thrombin resembles the pancreatic serine proteinases in this respect. The initial rate of inactivation of thrombin by nitrous acid is higher than that of trypsin, and considerably higher than that of elastase and α-chymotrypsin under the same conditions, suggesting that the N-terminal residue is more readily accessible to the reagent. The experiments strengthen the hypothesis that all mechanisms of prothrombin activation are proteolytic and suggest that they involve the liberation of the essential N-terminal isoleucine.
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