FLUORESCENCE QUENCHING AND TIME-RESOLVED FLUORESCENCE STUDIES ON MOMORDICA-CHARANTIA (BITTER GOURD) SEED LECTIN

Chemical modification studies implicated tryptophan (Trp) residues in the sugar binding activity of Momordica charantia lectin (MCL) [Mazumd ar, T., Gaur, N. & Surolia, A. (1981) Eur. J. Biochem. 113, 463-470]. In the present study, the accessibility and environment of Trp residue s in MCL were investigated by intrinsic fluorescence quenching and tim e-resolved fluorescence. The emission lambda(max) of native MCL in the absence as well as in the presence of 0.1 M lactose was around 335nm, which shifted to 365nm in the presence of 8 M urea, suggesting that t he Trp residues which are predominantly buried in the hydrophobic core of the native lectin get exposed to the aqueous environment upon dena turation. At a quencher concentration of 0.5 M, the extent of quenchin g observed for the native MCL with acrylamide, I- and Cs+ was 46%, 17% and 12%, respectively. In the presence of 0.1 M lactose this quenchin g was smaller, suggesting that the sugar ligand provides a partial pro tection to the Trp residues. In time-resolved fluorescence measurement s, the decay curves could be fitted well to a biexponential function w ith the estimated life times 0.92 ns and 4.64 ns for the native protei n and 1.15 ns and 5.1 ns in the presence of 0.1 M lactose. All these r esults are consistent with the involvement of Trp residues in the suga r-binding activity of MCL.