The Acidic Region of Factor VIII Light Chain Contains the Thrombin-Binding Site(s) Responsible for the Cleavage at Arg1689

Thrombin-catalyzed activation of factor (F)VIII by cleavages at Arg³⁷², Arg⁷⁴⁰, and Arg¹⁶⁸⁹ is essential for the propagation phase of blood coagulation cascade. Activated FVIII (FVIIIa) forms the tenase complex and markedly amplifies the activation of FX as a cofactor of FIX. We had already demonstrated that thrombin interacts with FVIII through the residues 392-394 and 484-509 in the A2 domain and the C2 domain, and each association regulates cleavage at Arg⁷⁴⁰, Arg³⁷², and Arg¹⁶⁸⁹, respectively (Nogami K, JBC 2000, 2005; BJH 2008), and recently reported that the A1 acidic clustered region 340-350 involving the sulfated tyrosine regulate the cleavage of Arg³⁷² (Minami et al. 55th ASH).  On the other hand, Fay and colleague suggested that recombinant FVIII lacking the C2 domain retains greater than 50% cofactor activity (JBC 2010), supporting the presence of other thrombin-binding region responsible for cleavage at Arg¹⁶⁸⁹ of the light chain. In this study, we attempted to identify this thrombin-binding site(s). We focused on the acidic residues 1659-1669 and 1675-1685 within the light chain, which had similar sequence to the A1 residues 340-350 in terms of involving the clustered acidic residues and sulfated tyrosine as well as hirugen residues 54-65. We prepared four of synthetic peptides corresponding to the residues 1659-1669 and 1675-1685 with sulfated tyrosine, P(1659-69s) and P(1675-85s), and with non-sulfated tyrosine, P(1659-69) and P(1675-85). The inhibitory effect on the thrombin-catalyzed FVIII activation by each peptide was evaluated in a one-stage clotting assay. Each peptide showed a dose-dependent inhibition on thrombin-catalyzed activation. These inhibitory effects were greater in order of P1675-85s, P1659-69s, P1675-85, P1659-69, and the IC₅₀ were 25, 67, 71 and 225 µM, respectively. The peptides with sulfated tyrosine had approximately 3-fold greater inhibition of the FVIII activation by thrombin than with non-sulfated tyrosine. The IC₅₀ in the presence of mixture of P1675-85s and P1659-69s was 30.4 µM, suggesting that these peptides had no an additive effect. The impacts of P1659-69s and P1675-85s on the thrombin-catalyzed cleavage at Arg¹⁶⁸⁹ were examined by SDS-PAGE/western blotting. These peptides blocked the cleavage at Arg¹⁶⁸⁹ in dose-dependent fashions. In timed-course assay, the presence of P1659-69s and P1675-85s decreased the cleavage rate of Arg¹⁶⁸⁹ by 61.3 % and 81.8 %, respectively compared to its absence. The direct binding of P1659-69s and P1675-85s to thrombin was examined by surface resonance plasmon (SPR)-based assay and by the zero-length cross-linking reagent EDC. In SPR-based assay using a Biacore T200^TM, thrombin bound to immobilized P1659-69s and P1675-85s directly with high affinity. The K_d values adjusted to 1:1 binding model of global fitting were 203 nM and 94 nM, respectively. EDC cross-linking in fluid-phase assay revealed that formation of EDC cross-linking products between biotinylated P1659-69s or P1675-85s and thrombin were observed in dose-dependent fashions. The products between the biotinylated peptides (800 nM) and thrombin were competitively reduced by the addition of non-biotinylated peptides. Moreover, N-terminal sequence analysis of cross-linking products between both peptides-thrombin indicated that thrombin bound to the residues 1664-1669 and 1683-1684.  Taken together, we demonstrated that the A3 residues 1659-1669 (QEEIDYDDTIS) and residues 1675-1685 (EDFDIYDEDEN) contained the thrombin binding-sites responsible for proteolytic cleavage at Arg¹⁶⁸⁹ of the A3 domain.