Binding of negatively charged macromolecules to factor XII induces a conformational change such that it becomes a substrate for trace amounts of activated factor present in plasma (less than 0.01%). As activated factor XII (factor XIIa or factor XIIf) forms, it converts prekallikrein (PK) to kallikrein and kallikrein cleaves high molecular weight kininogen (HK) to release bradykinin. A far more rapid activation of the remaining unactivated factor XII occurs by enzymatic cleavage by kallikrein (kallikrein-feedback) and sequential cleavage yields two forms of activated factor XII; namely, factor XIIa followed by factor XII fragment (factor XIIf). PK circulates bound to HK and binding induces a conformational change in PK so that it acquires enzymatic activity and can stoichiometrically cleave HK to produce bradykinin. This reaction is prevented from occurring in plasma by the presence of C1 inhibitor (C1 INH). The same active site leads to autoactivation of the PK-HK complex to generate kallikrein if a phosphate containing buffer is used. Theoretically, formation of kallikrein by this factor XII-independent route can activate surface-bound factor XII to generate factor XIIa resulting in a marked increase in the rate of bradykinin...Continue Reading
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