Defining the carbohydrate specificities of Aplysia gonad lectin exhibiting a peculiar D-galacturonic acid affinity

Aplysia gonad lectin (AGL), which has been shown to stimulate mitogenesis in human peripheral lymphocytes, to suppress tumor cells, and to induce neurite outgrowth and improve cell viability in cultured Aplysia neurons, exhibits a peculiar galacturonic acid/galactose specificity. The carbohydrate binding site of this lectin was characterized by enzyme-linked lectino-sorbent assay and by inhibition of AGL-glycan interactions. Examination of the lectin binding with 34 glycans revealed that it reacted strongly with the following glycoforms: most human blood group precursor (equivalent) glycoproteins (gps), two Galα1→4Gal-containing gps, and two D- galacturonic acid (GalUA)-containing polysaccharides (pectins from apple and citrus fruits), but poorly with most human blood group A and H active and sialylated gps. Among the GalUA and mammalian saccharides tested for inhibition of AGL-glycan binding, GalUA mono- to trisaccharides were the most potent ones. They were 8.5 x 10⁴ times more active than Gal and about 1.5 x 10³ more active than the human blood group P(k) active disaccharide (E, Galα1←4Gal). This disaccharide was 6, 28, and 120 times more efficient than Galβ1→3GlcNAc(I), Galβ1→3GalNAc(T), and Gal 1/4 →4GlcNAc (II), respectively, and 35 and 80 times more active than melibiose (Galα1→6Glc) and human blood group B active disaccharide (Galα1→3Gal), respectively, showing that the decreasing order of the lectin affinity toward α-anomers of Gal is α1→4 > α1→6 > α1→3. From the data provided, the carbohydrate specificity of AGL can be defined as GalUAα1→4 trisaccharides to mono GalUA > branched or cluster forms of E, I, and II >> monomeric E, I, and II, whereas GalNAc is inactive.