Fluorescence and absorption spectroscopic studies on the interaction of porphyrins with snake gourd (Trichosanthes anguina) seed lectin

The interaction of several free-base porphyrins and their corresponding cop per(II) and zinc(II) derivatives with the galactose-specific lectin from sn ake gourd (Trichosanthes anguina) seeds has been investigated by absorption and fluorescence spectroscopic techniques. The lectin dimer contains two a pparently equivalent binding sites for the porphyrins. Association constant s obtained for the interaction of various porphyrins with the lectin are in the range 1.7 x 10(4)-6.2 X 10(5) M-1, with the metalloporphyrins being se en to have higher affinity for the lectin compared with the free-base analo gues. Both positively charged and negatively charged porphyrins bind to sna ke gourd seed lectin (SGSL) with comparable affinities, suggesting that bin ding occurs primarily via hydrophobic interactions. Further, binding of por phyrins is found to be largely unaffected by the presence of the sugar liga nd, lactose, indicating that the binding sites for the carbohydrate and por phyrin are different. This study thus suggests that the lectin may serve as a receptor for some endogenous non-carbohydrate, hydrophobic ligand in viv o, in addition to the saccharide ligands. It also opens up the possibility of employing the T. anguina lectin in applications such as photodynamic the rapy, which involve the use of porphyrins. (C) 2000 Elsevier Science S.A. A ll rights reserved.