Purification and characterization of potato lectin.

Potato lectin (Solanum tuberosum agglutinin, STA), purified by affinity chromatography on tri-N-acetylchitotriose-Sepharose 6B, has Mr approximately 100,000, as estimated by gel filtration on Sephadex G-150 and is an aggregating system with a monomer Mr = 54,000, as estimated by sedimentation equilibrium analysis. Equilibrium dialysis showed that STA (dimer) has two binding sites for a specific sugar per molecule. STA has a high content of sugar, most of which is L-arabinose, and is rich in Hyp and Cys. On interaction with specific sugars, STA induced a UV difference spectrum having positive peaks at 292 and 285 nm characteristic of tryptophyl residues. The association constants with chitin oligosaccharides, determined from the intensities of the difference spectra at various concentrations of sugars, increased with increasing chain length of the sugar. Association constants obtained by frontal affinity chromatography of chitin oligosaccharides with STA-Sepharose were in good agreement with those obtained by difference spectra, whereas the association constants obtained by frontal affinity chromatography of STA with di- and tri-N-acetylchitotriose-Sepharose were much higher, presumably owing to the effect of multivalency of ligands. The CD spectra of STA in the far UV region indicate the presence of 40% of beta- and 60% of unordered form, and no alpha-helix conformation, which supports the structure suggested by the amino acid composition and the high content of sugar.