PROTEIN-A-MEDIATED TARGETING OF BACTERIOCHLOROPHYLL-IGG TO STAPHYLOCOCCUS-AUREUS - A MODEL FOR ENHANCED SITE-SPECIFIC PHOTOCYTOTOXICITY

A model for studying the efficiency of photodynamic action with a phot osensitizer placed exclusively on the bacterial cell mall has been use d, Bacteriochlorophyllide molecules, conjugated to rabbit immunoglobul in G (IgG), were synthesized. The conjugated pigment bacteriochlorophy ll (Bchl)-IgG bound with high specificity to protein-A residues natura lly exposed on the cell wall of the bacterium Staphylococcus aureus Co wan I, In bacterial suspensions the phototoxicity of the targeted conj ugates (0.5-2.5 pigment per IgG molecule) was dose dependent (LD50 = 1 .7 mu M in the presence of Light (lambda > 550 nm) and inhibited by na tive IgG but not by ovalbumin, suggesting selective interaction with p rotein-A on the bacterial cell wall, No dark toxicity was noticed even with the highest conjugate concentration tested, In contrast, the pho tocytotoxicity of bacteriochlorophyll-serine (Bchl-Ser, LD50 = 0.07 mu M) used as a nontargeted control was not inhibited by IBG. In spite o f its lower apparent potency, Bchl-IgG was found to be 30 times more e fficacious than Bchl-Ser: At LD50, only 66 000 Bchl-IgG molecules were bound per bacterium compared to 1900 000 molecules of Bchl-Ser, The h igher efficacy of Bchl-IgG is explained by its exclusive position on t he bacterial cell wall, Consequently, photogeneration of oxidative spe cies is confined to the cell wall and its vicinity, a seemingly highly susceptible domain for photodynamic action, In considering the design of cell-specific sensitizers for bacterial and cancer therapies, it w ould be beneficial to identify the more discretely sensitive subcellul ar domains as targets.