Carbohydrate structural units in glycosphingolipids as receptors for Gal and GalNAc reactive lectins.

Glycosphingolipids (GSLs) contain many carbohydrate epitopes or crypto-glycotopes for Gal and GalNAc reactive lectins. Many of them are in the nervous system and function as important receptors in various life processes. During the past two decades, 11 mammalian structural units have been used to express the binding domain of applied lectins. They are: F, GalNAc alpha1 --> 3GalNAc; A, GalNAc alpha1 --> 3Gal; T, Gal beta1 --> 3GalNAc; I, Gal beta1 --> 3GlcNAc; II, Gal beta1 --> 4GlcNAc; B, Gal alpha1 --> 3Gal; E, Gal alpha1 --> 4Gal; L, Gal beta1 --> 4Glc; P, GalNAc beta1 --> 3Gal; S, GalNAc beta1 --> 4Gal, and Tn, GalNAc alpha1 --> Ser(Thr). Although 10 of them occur in GSLs, only 3 (Lbeta, Sbeta, and Tbeta) are found in human brain, and 2 (Lbeta and IIbeta) are present in the inner structures of human blood group active GSLs. In the families of gangliosides, Lbeta and IIbeta represent 55% of the total structural units, while the other three units (Tbeta, Palpha, and Sbeta) constitute the rest. To facilitate the selection of lectins that could serve as structural probes, the carbohydrate binding specificities of Gal/GalNAc reactive lectins have been classified according to their highest affinity for the structural units and their binding properties expressed by decreasing order of reactivity. Hence, the binding relation between GSLs and Gal/GalNAc specific lectins can be established.