Synthetic immunochemistry of glycohexapeptide analogues characteristic of oncofetal fibronectin. Solid-phase synthesis and antigenic activity.

Monoclonal antibody FDC-6, and its second-generation antibodies FDB-1 and FDB-4, are able to distinguish between fibronectin (FN) from fetal or cancer tissue (onco-FN) vs. FN from normal adult tissue and plasma (nor-FN). The epitope structure recognized by the above antibodies is the glycohexapeptide H-Val-(GalNAc-alpha)Thr-His-Pro-Gly-Tyr-OH (P2). In order to define further the specificity of the reactive site, we synthesized various glycopeptides based on the unglycosylated hexapeptide sequence (P1) and compared their reactivities with these antibodies. In continuation of our structure-activity relationship studies the (Asn3,Ala5)-glycohexapeptide analogue (P3) was synthesized by a solid-phase procedure. The [Ala(CN)3,Ala5]-glycopeptide (P4), owing to dehydration of the asparagine side chain amide during carboxyl activation of Fmoc-Asn-OH, was also isolated. Fmoc-[GalNAc(Ac)3-alpha]Thr-OH was used for incorporating the glycosylated amino acid residue. For the sake of comparison the epitope P2 and the hexapeptide sequence P1 were also synthesized. The final products were characterized by elemental and amino acid analyses, optical rotation, analytical HPLC, proton NMR and fast-atom bombardment mass spectroscopy. Synthetic analogues were applied to inhibit onco-FN specific MAbs FDB-1, FDB-4 and FDC-6 binding to immobilized onco-FN, and their activities were compared with onco-FN and nor-FN. P2 exhibited an activity similar to that of an intact molecule of onco-FN. Deglycosylation (P1) or replacement of amino acid (P3, P4) greatly reduced activity. Data clearly showed that P2 was the minimal essential structure of the epitope in onco-FN defined by MAbs FDB-1, FDB-4 and FDC-6.(ABSTRACT TRUNCATED AT 250 WORDS)