Reactivity of fibrinogen and fibrinopeptide A containing fibrinogen fragments with antisera to fibrinopeptide A

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Abstract
Two antisera [rabbit] used in the radioimmunoassay for human fibrinopeptide A (FPA) which appear to have different immunochemical specificities were tested for cross-reactivity with fibrinogen and with 3 fragments of fibrinogen which contain the FPA sequence. These fragments were the 3-chain, NH2-terminal disulfide knot (N-DSK) produced by CNBr cleavage of fibrinogen, the reduced, carboxymethyl A.alpha. chain portion of the N-DSK, and fragment E produced by plasmin digestion of fibrinogen. One antiserum (R-2) showed high specificity for free FPA and less than 2% cross-reactivity with fibrinogen or the FPA-containing fragments. The other antiserum (r-33) possessed a much higher degree of cross-reactivity with the FPA-containing fragments. Synthetic and native fibrinopeptides were indistinguishable in the assay system with either antiserum. An hypothesis was developed concerning the antigenic determinants on FPA which favor measurement of free FPA and limit cross-reactivity with larger, FPA-containing peptides.

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