Determination of a factor VIII-interactive region within plasmin responsible for plasmin-catalysed activation and...

Plasmin, an active form of plasminogen, activates and inactivates factor VIII (FVIII) by limited proteolysis. We have previously identified lysine-binding site-independent plasmin-interactive sites on the FVIII A2 domain responsible for cleavages at Arg336 and Arg372, together with lysine-binding site-dependent plasmin sites on the light chain responsible for cleavage at Lys36. We have now characterised FVIII-interactive regions on plasmin. SDS-PAGE analysis demonstrated that a monoclonal antibody (mAb) against kringle (K)5-catalytic domain (K5-CD) of plasmin significantly blocked plasmin-catalysed cleavages at Arg336 and Arg372. K5-CD fragment and this mAb blocked plasmin-catalysed activation and inactivation of FVIII(a). Anti-K1-2-3 and anti-K4 mAbs blocked plasmin-catalysed cleavages at Lys36, and K1-2-3 and K4 fragments inhibited plasmin-catalysed inactivation of A11-336FVIIIa. The K5-CD preferentially bound to the A2 domain (Kdapp; 52 nM), whilst the K1-2-3 and K4 bound to the light chain (Kdapp; 75 and 106 nM, respectively) in ELISA. Binding was attributed to the A2 484-509 region and A3 1690-1705/1804-1818 region, respectively. 6-aminohexanoic acid, a lysine analogue, significantly inhibited the light chain/K1-2-3 (and K4) binding by approximately 90%, whilst A2/K5-CD binding was moderated by only approximately 35%. Furthermore, an anti-CD antibody blocked plasmin-catalysed cleavage by inhibiting the A2/K5-CD interaction. These data demonstrate that the K5-CD of plasmin (and plasminogen) interacts with the A2 domain independent of lysine-binding site, whilst interactions of K1-2-3 and K4 with the light chain are lysine-binding site-dependent. Interactions between the K5-CD and A2 likely constitute the major regulatory mechanism for activation and inactivation of FVIII(a) mediated by cleavage at Arg372 and Arg336.