Binding of porphyrins by the tumor-specific lectin, jacalin [Jack fruit (Artocarpus integrifolia) agglutinin].

Jacalin (Artocarpus integrifolia agglutinin) specifically recognizes the tumor-associated T-antigenic disaccharide structure, Gal beta13GalNAc. Porphyrins and their derivatives are currently used as photosensitizers in photodynamic therapy to treat malignant tumors. In this study, the interaction of several free base porphyrins and their metal derivatives with jacalin is investigated by absorption and fluorescence spectroscopy. Each lectin subunit was found to bind one porphyrin molecule and the association constants were estimated to be in the range of 2.4 x 10(3) M(-1) to 1.3 x 10(5) M(-1) at room temperature for the interaction of different porphyrins with jacalin. These values are in the same range as those obtained for the interaction of monosaccharides to jacalin. Both free lectin and lectin saturated with the specific saccharide were found to bind different porphyrins with comparable binding strength indicating that porphyrin binding takes place at a site different from the sugar binding site. Further, both anionic and cationic porphyrins were found to interact with the lectin with comparable affinity, clearly indicating that the charge on the porphyrin does not play any role in the binding process and that most likely the interaction is mediated by hydrophobic forces. These results suggest that jacalin and other lectins may potentially be useful for targeted delivery of porphyrins to tumor tissues in photodynamic therapy.