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Vanya Bogoeva1*, Lidia Petrova1, Constance John2, George Russev1, Antonio Varriale3, and Sabato D`Auria3
1Department of Molecular Biology of Cell Cycle, Institute of Molecular Biology, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria; 2MandalMed, Inc, San Francisco, CA 94107USA; 3Laboratory for Molecular Sensing, Institute of Protein Biochemistry, CNR, 80131 Naples, Italy Lectins are carbohydrate-binding proteins, known for their ability to interact with specific oligosaccharides. Recently, in addition to their carbohydrate binding capacity, it has been discovered that some plant lectins have hydrophobic sites for adenine and physiological compounds. The affinity towards these physiological non-carbohydate ligands was higher than their affinity for carbohydates indicating that the interaction with hydrophobic ligands may be biologically relevant. Similarly, some animal lectins have non-carbohydrate binding domains, and it is shown that they bind non-carbohydrate ligands through hydrophobic interactions. Therefore, we postulated that human lectins, in particular galectin-1, could possibly interact with non-carbohydrate ligands. The present work shows a novel binding activity of the tumor specific lectin - human galectin-1 (hGal-1) to three porphyrin compounds: Zn-porphyrin, Mn-porphyrin and Au-porphyrin, widely applied in photodynamic therapy of cancer (PDT). Our data indicate that hGal-1 similar to some plant lectins, a bacterial lectin from Pseudomonas aeruginosa and an animal lectin from Helix pomatia possesses dual functions binding to both carbohydrate and non-carbohydrate ligands. The interaction of ZnTPPS with hGal-1 was studied by the specific fluorescence emission of the porphyrin. The protein binding properties to Mn/Au porphyrins were measured by intrinsic protein fluorescence quenching. The values determined for the apparent dissociation constants (KD) of 0.6-1.5 µM are similar to the KD for complexes of Concanavalin A and porphyrin, and are indicative of the high affinity of hGal-1 for these porphyrins and adenine. In addition, the analysis of the hyperbolic binding curves suggests the presence of one hGal-1 binding site for porphyrins. Additionally, we found that hGal-1 interacts with the fluorescent probe 2-(p-toluidinyl) naphthalene sulfonic acid (TNS), that was used to identify the hydrophobic regions within hGal-1. In summary, similar to some plant lectins hGal-1 shows a novel porphyrin binding activity, in addition to its carbohydrate one. In the last decade most investigations on human tumors revealed a direct relationship between hGal-1 expression and tumor stage, for which it becomes a valuable tumor marker, as well as a potential target for therapy. Regarding this, and its binding to cancer antigens, as well as its porphyrin binding capacity, hGal-1 can be considered as a possible candidate for PDT and could be viewed as a potentially interesting delivery molecule for the treatment of cancer cells. |
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