Interactions of a plant peroxidase with oligogalacturonides in the presence of calcium ions

The interaction between homogalacturonans and an anionic isoperoxidase puri fied from zucchini hypocotyls was characterized. The binding of the enzyme to the pectic molecules was studied by gel filtration through Sephacryl S20 0 and by centrifugation. It took place only in the presence of calcium ions , at pi-Is ranging from 4.5 to 7.0. Ca2+ was necessary because it induced t he cross-linking of polygalacturonan chains into a structure which could be recognized by the isoperoxidase. A comparison between mixtures of large (d egree of polymerization: DP = 9-22) and small (DP = 2-8) oligogalacturonide s showed that only the former ones were able to form the Ca2+-induced struc ture that could be pelleted upon centrifugation and to bind the isoperoxida se. Large oligogalacturonides (OGAs) and polygalacturonic acid (PGA) had al most the same capacity to retain the isoperoxidase. Gel filtration experime nts showed that the binding of the isoperoxidase to large OGAs occurred eve n at low Ca2+ concentrations (0.05 mM). Competition experiments showed that polyanions like dextran sulfate or heparin even at a hundred times higher concentration did not completely prevent the binding of peroxidase to Ca2+- pectate. Alginic acid was also unable to suppress this binding and, althoug h its structure is similar to that of OGAs and although it was also cross-l inked by Ca2+ ions and formed a pelletable gel, it did not offer a structur e suitable for peroxidase binding. (C) 1999 Elsevier Science Ltd. All right s reserved.