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

Jacalin (Artocarpus integrifolia agglutinin) specifically recognizes the tu mor-associated T-antigenic disaccharide structure, Gal beta 13GalNAc. Porph yrins and their derivatives are currently used as photosensitizers in photo dynamic therapy to treat malignant tumors. In this study, the interaction o f several free base porphyrins and their metal derivatives with jacalin is investigated by absorption and fluorescence spectroscopy. Each lectin subun it was found to bind one porphyrin molecule and the association constants w ere estimated to be in the range of 2.4 x 10(3) M-1 to 1.3 x 10(5) M-1 at r oom temperature for the interaction of different porphyrins with jacalin. T hese 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 d ifferent from the sugar binding site. Further, both anionic and cationic po rphyrins were found to interact with the lectin with comparable affinity, c learly indicating that the charge on the porphyrin does not play any role i n the binding process and that most likely the interaction is mediated by h ydrophobic forces. These results suggest that jacalin and other lectins may potentially be useful for targeted delivery of porphyrins to tumor tissues in photodynamic therapy.