A Class of Lectins Present in the Tissues of Seed Plants

Abstract

The seeds of both angiosperms and gymnosperms (91 of 104 families tested) contain a specific glycoprotein lectin, which gel-diffusion experiments suggest to be widely present in plant tissues. This lectin will form an insoluble complex with tris-(4-glycosyloxyphenylazo)phloroglucinol where glycosyl = ß-D-gluco-, D-galacto-, D-xylo-, malto-, lacto- or cellobiopyranosyl. In legume seeds the major part of the lectin is concentrated in the intercellular spaces as distinct globular bodies. The purification and analysis of the glycoprotein from a selection of species is described. Both protein and carbohydrate composition show little variation over the range. Hydroxyproline and glucosamine are present and the major sugars are galactose and arabinose. From ultracentrifugation studies the weight-average molecular weight of the Brassica napus lectin is 126 000. Since the carbohydrate:protein ratio is c. 8: 1, the protein moiety has a molecular weight of approx. 15 000. The sole N-terminal amino acid detected in the very similar Allium porrum lectin is tyrosine, suggesting that a single type of peptide chain is present. Neither the function of the lectin nor the reasons for its extraordinary evolutionary stability are known.