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 photosensitizer placed exclusively on the bacterial cell wall has been used. Bacteriochlorophyllide molecules, conjugated to rabbit immunoglobulin G (IgG), were synthesized. The conjugated pigment bacteriochlorophyll (Bchl)-IgG bound with high specificity to protein-A residues naturally exposed on the cell wall of the bacterium Staphylococcus aureus Cowan I. In bacterial suspensions the phototoxicity of the targeted conjugates (0.5-2.5 pigment per IgG molecule) was dose dependent (LD50 = 1.7 microM) in the presence of light (lambda > 550 nm) and inhibited by native IgG but not by ovalbumin, suggesting selective interaction with protein-A on the bacterial cell wall. No dark toxicity was noticed even with the highest conjugate concentration tested. In contrast, the photocytotoxicity of bacteriochlorophyll-serine (Bchl-Ser, LD50 = 0.07 microM) used as a nontargeted control was not inhibited by IgG. In spite of its lower apparent potency, Bchl-IgG was found to be 30 times more efficacious than Bchl-Ser: At LD50, only 66,000 Bchl-IgG molecules were bound per bacterium compared to 1,900,000 molecules of Bchl-Ser. The higher efficacy of Bchl-IgG is explained by its exclusive position on the bacterial cell wall. Consequently, photogeneration of oxidative species 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 would be beneficial to identify the more discretely sensitive subcellular domains as targets.
Gross
S
S
Department of Biological Regulation, Weizmann Institute of Science, Rehovot, Israel.
Brandis
A
A
Chen
L
L
Rosenbach-Belkin
V
V
Roehrs
S
S
Scherz
A
A
Salomon
Y
Y
D001429:Bacteriochlorophylls / Q000378:metabolism*; D001665:Binding Sites; D002473:Cell Wall / Q000378:metabolism; D007074:Immunoglobulin G / Q000378:metabolism*; D008027:Light; D008954:Models, Biological; D017319:Photosensitizing Agents / Q000378:metabolism*; D013205:Staphylococcal Protein A / Q000378:metabolism*; D013211:Staphylococcus aureus / Q000378:metabolism* / Q000528:radiation effects
D001429:Bacteriochlorophylls; D007074:Immunoglobulin G; D017319:Photosensitizing Agents; D013205:Staphylococcal Protein A