INTERACTION OF PORPHYRINS WITH CONCANAVALIN-A AND PEA LECTIN

Currently porphyrins are used as photosensitizers in photodynamic ther apy for the treatment of cancer. However, this approach suffers due to the inability of many porphyrin-based drugs to accummulate preferenti ally in tumours. In view of this, we considered if the carbohydrate-bi nding proteins, lectins, which preferentially recognize malignant cell s, could be used for the targeting of porphyrins to tumour cells. In t he present study, we have investigated the interaction of a free base porphyrin, meso-tetrasulphonatophenylporyphyrin and the corresponding metal derivative, meso-zinc-tetrasulphonatophenylporphyrin with two le gume lectins, concanavalin A and pea (Pisum sativum) lectin. Each lect in subunit was found. to bind one porphyrin molecule and the associati on constant, K-a, estimated from absorption and fluorescence titration s at room temperature (28 +/- 1 degrees C) was in the range of 1.2 x 1 0(4) M-1 to 6.3 x 10(4)4 M-1. Both free lectin and lectin saturated wi th the specific saccharide were found to bind the porphyrin with compa rable binding strength, indicating that porphyrin binding takes place at a site different from the sugar-binding site, These results indicat e that lectins may potentially serve as drug-delivery agents for porph yrin sensitizers in photodynamic therapy.