Histochemical specificity of beta-galactose binding lectins from Arachis hypogaea (peanut) and Ricinus communis (castor bean)

Previous histochemical studies have demonstrated disparities in the binding of two lectins with a nominal specificity for terminal beta-D-galactose. Biochemical studies have shown that the most complementary structure for binding peanut agglutinin (PNA) is the terminal disaccharide Gal-(beta 1----3)-GalNAc, whereas the most complementary structure for binding Ricinus communis agglutinin I (RCA I) is the terminal disaccharide Gal-(beta 1----4)-GlcNAc. However, it is not known if only these differences in affinity account for the different histochemical staining reactions observed on tissue sections. In the present study we compared the staining patterns of PNA and RCA I by inhibiting in situ the binding of each lectin conjugated to horseradish peroxidase (HRP) with increasing concentrations of unlabeled PNA or RCA I. The PNA-HRP conjugate did not stain most tissue sites suspected of containing an abundance of glycoconjugates with terminal Gal-(beta 1----4)-GlcNAc. Moreover, unlabeled PNA failed to significantly inhibit strong RCA I-HRP staining in these sites. In loci thought to contain variable amounts of glycoconjugates with terminal Gal-(beta 1----3)-GalNAc, unlabeled RCA I decreased PNA-HRP reactivity only slightly or not at all, whereas weak to strong RCA I-HRP staining was diminished or abolished by unlabeled PNA. The results suggest that PNA staining is restricted to glycoconjugates with terminal Gal-(beta 1----3)-GalNAc. RCA I apparently reacts most strongly with glycoconjugates having the terminal disaccharide Gal-(beta 1----4)-GlcNAc, but also stains sites containing a moderate to abundant amount of glycoconjugates with the terminal Gal-(beta 1----3)-GalNAc sequence.