The structure-complement activation relationship of Plantago-mucilage A, a partially O-acetylated glucuronoarabinoxylan isolated from the seed of Plantago asiatica, has been studied. The anti-complementary activity was markedly enhanced when the polysaccharide was deacetylated, but carboxyl-reduction and partial acid hydrolysis had little effect. Periodate oxidation decreased, but subsequent reduction restored, the activity; the activity of this polyalcohol was not changed in the absence of Ca2+. When serum was incubated with the deacetylated polymer, more marked C4 consumption was observed in the presence of Ca2+ than with Plantago-mucilage A, but the polyalcohol did not show any C4 consumption. After incubation of the serum with the polyalcohol in the absence of Ca2+, a cleavage of C3 in the serum was detected by immunoelectrophoresis and by the consumption of complement when rabbit erythrocytes were used in the assay system, and these effects were much higher than for the original polysaccharide. Although the product of carboxyl-reduction did not contain O-acetyl groups, its C4 consumption was almost the same as that of Plantago-mucilage A. Thus, the O-acetyl groups in Plantago-mucilage A prevent the activation of complement via the classical pathway but the carboxyl groups are essential for the activation. The polyalcohol increases the activation of complement via the alternative pathway, but decreases the activation via the classical pathway.
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