DEFINING CARBOHYDRATE SPECIFICITY OF RICINUS-COMMUNIS AGGLUTININ AS GA1-BETA-1-]4GLCNAC (II)GREATER-THAN-GAL-BETA-1-]3GLCNAC (I)GREATER-THAN-GAL-ALPHA-1-]3GAL (B)GREATER-THAN-GAL-BETA-1-]3GALNAC (T)

To define carbohydrate specificity of Ricinus communis agglutinin (RCA1), the combining site of RCA, was further characterized by quantitative precipitin (QPA) and precipitin-inhibition assays (QPIA). Among the oligosaccharides tested for QPIA, Galbeta1-->4GlcNAc (II, human blood group type II precursor sequence) was found to be 7.1 times more active than Galbeta1-->3GalNAc (T, Thomsen-Friedenreich sequence) and about 1.7 times more active than the other three disaccharides tested-Galbeta1-->4Man, Galbeta1-->3DAra and Galbeta1-->6GalNAc. Galalpha1-->4Gal, the receptor of the uropathogenic E. coli ligand was 3.6 times less active than the II sequence. These results indicate that the beta1-->4 linkage of the terminal Gal to subterminal GlcNAc is important as this beta1-->4GlcNAc sequence is at least 1.6 times more active than other types of disaccharides. Among the glycoproteins examined for QPA, native and desialized bovine submandibular glycoproteins, native and desialized human plasma alpha1-acid glycoproteins, as well as crude hog stomach mucin and its three mild acid hydrolyzed products reacted well with the lectin. These glycoproteins precipitated over 75% of the lectin nitrogen added indicating that RCA1 has the ability to recognize Galbeta1-->4/3GlcNAc and/or the related residues at the non-reducing ends and at positions in the interior of the chains. However, Tn (GalNAcalpha1-->Ser/Thr sequence) rich glycoproteins such as desialized ovine submandibular glycoprotein and desialized armadillo salivary glycoprotein, in which over 90% of the carbohydrate side chains are Tn determinants with none or only a trace of I/II or T determinants, precipitated poorly with RCA1. From the present and previous results obtained, the carbohydrate specificity of RCA, can be constructed and summarized in decreasing order by lectin determinants as follows: II (Galbeta1-->4GlcNAc) > I (Galbeta1-->3GlcNAc) > E (Galalpha1-->4Gal) and B (Galalpha-->3Gal) > T (Galbeta1-->3GalNAc), while Tn (GalNAcalpha1-->Ser/Thr) is a poor inhibitor.