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.