Peptide heterogeneity in a preparation of synthetic fibrinopeptide B

A commercially available preparation of synthetic human fibrinopeptide B (FpB) was shown by hplc to contain two chromatographically distinct peaks, one of which was identical to FpB. Our results suggest that the contaminant peptide (FpB-2), which represented approximately 43% of the total peptide composition, is FpB containing an alpha-aminosuccinimide (Asc) residue. This Asc residue probably arose as a result of the cyclization of 5Asp-6Asn during either the coupling or deprotection reactions. FpB-2 was rapidly hydrolysed by carboxypeptidase B to des-Arg-FpB-2. It was stable under acidic conditions but in dilute alkali was converted to equimolar amounts of FpB and FpB containing beta-Asp at residue 5. Since it has been suggested that 5Asp-6Asn is a major immunorecognition site in FpB, our observations emphasize the need to establish the purity of synthetic FpB preparations destined for use in the immunoassay of either FpB or des-Arg-FpB.