Calcium modulates plasmin cleavage of the fibrinogen D fragment γ chain N-terminus: mapping of monoclonal antibody J88B to a plasmin sensitive domain of the γ chain

Abstract Plasmin sensitive sites are found on the Aα, Bβ and γ chains of fibrinogen at regions joining the two C-terminal D fragments with the central E fragment. We have developed a monoclonal antibody (MoAb) reactive with this plasmin sensitive region on the human fibrinogen y chain and mapped its epitope. MoAb J88B reacts with γ chains of both native as well as with reduced and denatured fibrinogen and fibrin, the CNBr fragment of the fibrinogen central domain, plasmin cleaved fragments D, γ-γ dimers, but not with plasmic fragments E. These data indicate that J88B maps to the plasmin sensitive domain localized to γ 63–78 . MoAb J88B failed to react with synthetic peptide γ 70–78 , which suggests that the epitope includes the newly exposed N-terminal residues γ 63–70 of the early plasmic fragment DIA. As calcium has a marked influence on plasmin cleavage of C-terminal sites on the γ chain, the effects of calcium on modulating plasmin cleavage of DIA to D1 were assessed in the absence or presence of J88B. The results indicated that calcium delays and J88B (±calcium) protects the γ chain from plasmin cleavage at the N-terminus of DIA, suggesting that this enzymatically labile site is calcium-sensitive. Thus, MoAb J88B should prove useful in studies examining the structure of plasmin cleaved fibrinogen and fibrin.