About the Lectin

The fava bean lectin is a weak, non-specific hemagglutinin that shares some of the carbohydrate binding properties of the lentil (LcH) and pea (PSA) lectins. VFA can be inhibited by mannose, glucose, and GlcNAc, as well as several other saccharides. A fucose residue attached α(1,6) to an asparagine linked GlcNAc enhances binding ¹ . This specificity is comparable to that of both LcH and PSA. VFA is inhibited by a lower concentration of mannose than LcH, PSA, or Con A. While LcH and PSA react equally well with glucose and mannose, as determined by agglutination inhibition assays, glucose is only 25% as potent as mannose for inhibition of VFA. This is similar to the reactivity of Con A, which also has a lower affinity for glucose than mannose. Comparative binding studies have been performed on VFA, LcH, PSA, and Con A. Agglutination inhibition assays indicate that VFA is similar to LcH and PSA. However, VFA does not exhibit the same increase in reactivity with several carbohydrate derivatives, relative to mannose, as LcH and PSA ² . All four of the mannose specific lectins bind to the cell surface of 6C3HED mouse ascites tumor cells ³ . This binding can be inhibited by methyl α–D-mannose. A comparison has been made to determine how effectively each of the lectins can compete for receptor sites on the tumor cell. Results of this comparison indicate that VFA is not quite as effective as either LcH or PSA, but is at least twice as effective as Con A. Crude extracts of the fava bean, and the purified lectin, are mitogenic for human and murine lymphocytes ^4,5 . The incorporation of [³H] thymidine is somewhat lower for VFA than for the other lectins, but the response to VFA does not appear to be as sensitive to the amount of lectin used ⁵ . VFA is a tetramer composed of two dissimilar subunits. The size of the subunits is comparable to the α and β subunits found in both LcH and PSA. In addition, VFA is about the same aggregate molecular weight as LcH and PSA, but less than half that of Con A. The structural similarities of VFA, LcH, and PSA coincide with their carbohydrate binding properties. Con A is structurally distinct from these three lectins and reacts differently with mannose containing glycoconjugates. The stability of VFA is affected by pH and ionic conditions, unlike LcH and PSA. The lyophilized preparation forms insoluble aggregates between pH 5-10. Under neutral pH conditions the lyophilized lectin is more soluble in 1.0 M NaCl than in lower or higher salt concentrations. When stored at pH 5 the lectin does not form aggregates ⁵ . VFA is a glycoprotein containing mannose and GlcNAc. The reactivity of the lectin with these carbohydrates may help explain the formation of aggregates in the purified preparations. Aggregates can be solubilized in 10% sucrose. VFA has been shown by circular dichroism measurements to contain up to 23% α-helix structure⁶, unlike most other lectins. Interestingly, binding of GlcNAc or methyl-α-mannoside changes its structure to almost exclusively β–sheets, as does certain arabinogalactans or galactomannans such as guar gum. The gums also enhance the erythroagglutinating activity of VFA 16- to 32- fold ⁷ . The X-ray crystallographic structure of VFA with bound glucose has been solved to 2.8 Å; it shows extensive β-structure, and is very similar to the concanavalin A structure ⁸ . All preparations of VFA contain various fragments and polymers of the two subunits ⁹ . This explains the heterogeneous nature of VFA when analyzed by SDS-PAGE.

REFERENCES

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