Analysis of pectin structure part 2 - Analysis of pectic epitopes recognised by hybridoma and phage display monoclonal antibodies using defined oligosaccharides, polysaccharides, and enzymatic degradation

The structure of epitopes recognised by anti-pectin monoclonal, antibodies (mAbs) has been investigated using a series of model lime-pectin samples wi th defined degrees and patterns of methyl esterification, a range of define d oligogalacturonides and enzymatic degradation of pectic polysaccharides. In immune-dot-assays, the anti-homogalacturonan (HG) mAbs JIM5 and JIM7 bot h bound to samples with a wide range of degrees of methyl esterification in preference to fully de-esterified samples. In contrast, the anti-MG phage display mAb PAM1 bound most effectively to fully de-esterified pectin. In c ompetitive inhibition ELISAs using fully methyl-esterified or fully de-este rified oligogalacturonides with 3-9 galacturonic acid residues, JIM5 bound weakly to a fully de-esterified nonagalacturonide but JIM7 did not bind to any of the oligogalacturonides tested. Therefore, optimal JIM5 and JIM7 bin ding occurs where specific but undefined methyl-esterification patterns are present on HG domains, although fully de-esterified HG samples contain sub -optimal JIM5 epitopes. The persistence of mAb binding to epitopes in pecti c antigens, with 41% blockwise esterification (P41) and 43% random esterifi cation (F43) subject to fragmentation by endo-polygalacturonase II (PG II) and endo-pectin lyase (PL), was also studied. Time course analysis of PG II digestion of P41 revealed that JIM5 epitopes were rapidly degraded, but a low level of PAM1 and JIM7 epitopes existed even after extensive digestion, indicating that some HG domains were more resistant to cleavage by PG II. The chromatographic separation of fragments produced by the complete digest ion of P41 by pectin lyase indicated that a very restricted population of f ragments contained the PAM1 epitope while a (1-->4)-beta-D-galactan epitope occurring on the side chains of pectic polysaccharides was recovered in a broad range of fractions. (C) 2000 Elsevier Science Ltd. All rights reserve d.