Immunochemical studies on the combining site of the blood group H-Specific lectin 1 from Ulex europeus seeds

The specificity of purified Ulex lectin I has been studied by quantitative precipitin, quantitative precipitin inhibition, and competitive binding assays using tritium-labeled hog mucin H substance. The lectin is precipitated by human and hog H substances, by human A 2 substances, by a cow substance with A activity, and by B substances of human and horse origin. The lectin did not precipitate with A 1 substances, with Le a substances, with a precursor substance with I activity, or with PI fractions obtained by mild acid hydrolysis of blood group B substance. Ulex lectin I was most specific for the blood group H oligosaccharide HR L 0.75, having the structure lFucαl ↓ 2 dGalβ1 → 4 dGlcNAcβ1 → 6R; this was determined by inhibition of precipitation of the lectin with a human H substance and by inhibition of the binding of tritiated hog H substance to the Ulex lectin I coupled to Sepharose beads using blood group and milk oligosaccharides as inhibitors. The lectin was also reactive with lFucα1 → 2 dGalβ1 → 4[ lFucαl → 3] dGlcNAcβ1 → 6R, 2′-fucosyllactose, lactodifucotetraose, and methyl α lFuc, all of which had essentially parallel slopes by each assay method. lFuc and fucosyl oligosaccharides with type 1 chains gave lines of different slopes but had the same range of activity. The Ulex lectin I site differs from the Lotus tetragonolobus site, which does not react at all with H oligosaccharides with the type 1 chain. Inhibition assays using tritiated hog H substance and various oligosaccharides were about 60 to 80 times more sensitive than assays by inhibition of precipitation.