Purification and characterization of three galactose specific lectins from mulberry seeds (Morus sp.)

Three lectins were extracted and purified from mulberry seeds by gel filtration of 100% ammonium sulfate saturated crude protein extract followed by ion-exchange chromatography on DEAE and CM-cellulose. The lectins were found to be homogeneous as judged by polyacrylamide disc gel electrophoresis. The molecular masses of the lectins as determined by gel filtration were 175 000 for MSL-1, 120 000 for MSL-2 and 89 500 for MSL-3. MSL-1 is dimer in nature, with the two monomers held together by disulfide bond(s), while MSL-2 and MSL-3 contain four nonidentical subunits that are held together by nonionic hydrophobic interactions. The lectins agglutinated rat red blood cells and this agglutination was inhibited specifically by galactose, methyl-alpha-d-galactopyranoside, methyl-beta-d-galactopyranoside, lactose and raffinose. The lectins MSL-1, MSL-2 and MSL-3 contained 5.7, 5.4 and 4.5% neutral sugars, respectively, and the sugar composition of the lectins was glucose and mannose for MSL-1 and galactose for both MSL-2 and MSL-3. The lectins exhibited strong cytotoxic effect in brine shrimp lethality bioassay.