Human
factor IX is activated to
factor IXa beta when
factor XIa cleaves two
peptide bonds, Arg 145-Ala 146 and Arg 180-Val 181, to release an activation
peptide. In
factor IX Chapel Hill (IXCH), isolated from a
hemophilia B patient with a mild
bleeding disorder, the
arginine 145 residue has been replaced with a
histidine. Thus factor IXCH is activated by
factor XIa by cleaving only at the Arg 180-Val 181 bond, leaving the activation
peptide attached, and resulting in an activated species,
factor IXa alpha CH, that, like normal
factor IXa alpha, is only 20% as active as
factor IXa beta. It is reported that both
factor IX and factor IXCH could be activated by
trypsin to forms of
factor IXa beta and
factor IXa beta CH that had clotting activities identical to
factor XIa-
activated factor IX. Amino-terminal amino acid sequence analysis showed that
trypsin cleaved
factor IX at the same bonds as did
factor XIa; factor IXCH was cleaved at the Arg 180-Val 181 bond, as normal, and was cleaved near the
histidine 145, at the Lys 142-Leu 143 bond, releasing a slightly larger activation
peptide than from normal
factor IXa beta.
Metal ions had no effect on the rate of activation of
factor IX by
trypsin; however,
metal ions had a profound effect on the rate at which further incubation with
trypsin inactivated
factor IXa.
Calcium and
manganese protected
factor IXa from inactivation by
trypsin more effectively than
magnesium, which was more effective than no
metal ion. It is concluded that
trypsin can activate normal
factor IX and factor IXCH to fully active IXa beta forms.