Chemical modification studies on the glucose mannose specific lectins fromfield and lablab beans

Seeds of Dolichos lablab var. lignosus (field beans) and variety typicus (l ablab beans) contain glucose/mannose specific lectins that have been affini ty purified and well characterised (Siva Kumar N., and Rajagopal Rao, D., J .Biosci., 1986, 10, 95-109, (1) Rajasekhar etal., (Biochem.Archives. 1997, 13, 233-240) (2). Purified lectins are glycoproteins with a native molecula r mass of 60 kDa and are made of two types of subunits (Gowda etal., 1994, J.Biol.Chem. 269, 18789-18793) (3). Chemical modifications of various group s in purified lectins (using group specific reagents) such as lysine (citra conic anhydride), carboxyl groups (water soluble carbodi-imide) tyrosine (N -acetyl imidazole) and tryptophan (2-hydroxy 5-nitro benzylbromide) reveale d that 14 out of 21 residues of lysines 7 out of 92 residues of carboxyl gr oups, 16 out of 24 tyrosine residues and 2 out of 10 tryptophan residues we re modified. Lysine and carboxyl group modification led to 95% loss in haem aglutinating activity compared to control while tyrosine and tryptophan mod ifications led to complete loss of lectin activity. Arginine and histidine modifications led to only 50% loss in activity. The extent of modification and loss in activity was same when the lysine and carboxyl groups were modi fied in the presence and absence of the inhibitory sugar methyl alpha-D-glu copyranoside at 0.1M concentration. However protection of modification and lectin activity was observed when the tyrosine and tryptophan residues were modified in the presence of the inhibitory sugar. Earlier CD studies carri ed out (1) and extensive chemical modification studies reported here substa ntiate the involvement of tyrosine and tryptophan residues in the sugar bin ding site of these lectins.