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

Aplysia gonad lectin (AGL), which has been shown to stimulate mitogenesis i n human peripheral lymphocytes, to suppress tumor cells, and to induce neur ite outgrowth and improve cell viability in cultured Aplysia neurons, exhib its a peculiar galacturonic acid/galactose specificity. The carbohydrate bi nding site of this lectin was characterized by enzyme-linked lectino-sorben t assay and by inhibition of AGL-glycan interactions. Examination of the le ctin binding with 34 glycans revealed that it reacted strongly with the fol lowing glycoforms: most human blood group precursor (equivalent) glycoprote ins (gps), two Gal alpha 1-->4Gal-containing gps, and two D-galacturonic ac id (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-gl ycan binding, GalUA mono- to trisaccharides were the most potent ones. They were 8.5 x 10(4) times more active than Gal and about 1.5 x 10(3) more act ive than the human blood group pk active disaccharide (E, Gal alpha 1-->4Ga l). This disaccharide was 6, 28, and 120 times more efficient than Gal beta 1-->3GlcNAc(I), Gal beta 1-->3GalNAc(T), and Gal beta 1--> 4GlcNAc (II), r espectively, and 35 and 80 times more active than melibiose (Gal alpha 1--> 6Glc) and human blood group B active disaccharide (Gal alpha 1-->3Gal), res pectively, showing that the decreasing order of the lectin affinity toward alpha-anomers of Gal is alpha 1-->4 > alpha 1-->6 > alpha 1-->3. From the d ata provided, the carbohydrate specificity of AGL can be defined as GalUA a lpha 1-->4 trisaccharides to mono GalUA > branched or cluster forms of E, I , and II >> monomeric E, I, and II, whereas GalNAc is inactive.