Detection, isolation and characterization of multiple lectins from the haemolymph of the cockroach Blaberus discoidalis

Abstract

Three agglutinins (lectins), designated BDL1, BDL2 and BDL3, were identified in the haemolymph of the cockroach Blaberus discoidalis by erythrocyte cross-adsorption and sugar inhibition tests. With the use of (NH4)2SO4 fractionation, anion-exchange and affinity chromatography, BDL1 and BDL2 have been purified to homogeneity, and BDL3 has been partially purified to three bands on SDS/PAGE. BDL1 has a molecular-mass estimate of 390 kDa by gel filtration and approx. 158 kDa by SDS/PAGE under non-reducing conditions, further reduced to subunits of 36 kDa under reducing conditions. BDL2 has a molecular mass of approx. 140 kDa and is composed of subunits of 67 kDa which can be further reduced to identical subunits of 23 kDa. Isoelectric focusing in agarose gels revealed that BDL1 and BDL2 both focused as single bands at pH 6.0 and pH 5.2 respectively. The purified forms of BDL1 and BDL2 were stained by the periodic acid/Schiff's reagent showing that both lectins are glycoproteins. In addition, BDL1 was deglycosylated by endo-beta-N-acetylglucosaminidase H. Immunological tests showed that these three lectins are not structurally related. All three lectins bind galactose but have different specificities for binding other sugars and for a range of vertebrate erythrocytes. BDL1 is specifically inhibited by D-(+)-glucose, D-(+)-mannose and N-acetyl-D-mannosamine, but not by N-acetyl-D-glucosamine, and BDL2 is inhibited by N-acetyl-D-glucosamine, but not by D-(+)-glucose, D-(+)-mannose or N-acetyl-D-mannosamine. BDL3 is strongly inhibited by N-acetyl-D-galactosamine, but not by any of the other above-mentioned sugars. Erythrocyte specificities showed that BDL1 is more specific for rabbit than mouse erythrocytes, whereas BDL2 and BDL3 are more specific for mouse than rabbit erythrocytes. The haemagglutinating activities of both the serum and isolated lectins are Ca(2+)-dependent. Localization of BDL1 and BDL2 with fluorescein isothiocyanate-labelled antibodies showed that both lectins are associated with the granules and other areas of the cytoplasm of all blood cell types.