Targeted cross-linking of a molten globule form of acetylcholinesterase by the virucidal agent hypericin.
The natural product hypericin is a photosensitive polycyclic aromatic dione compound, which has been widely investigated because of its virucidal and antitumor properties. Although it has been suggested that singlet oxygen or a radical species might be responsible for its biological action, its mechanism of action remains unknown. Due to its amphiphilic characteristics, we considered the possibility that it might interact preferentially with partially unfolded proteins which exhibit exposed hydrophobic surfaces. We here demonstrate that hypericin binds to a molten globule species generated from Torpedo acetylcholinesterase, but not to the corresponding native enzyme. Irradiation with visible light, under aerobic conditions, causes chemical cross-linking of the catalytic subunits, to dimers and heavier species, under conditions where no cross-linking is observed for the native enzyme. Both anaerobiosis and sodium azide greatly reduce the extent of cross-linking, suggesting that singlet oxygen is responsible for the phenomenon. This agrees with our observation, using spin traps, that mainly singlet oxygen is produced by the complex of hypericin with the molten globule of acetylcholinesterase. Cross-linking is enhanced in the presence of liposomes to which the molten globule of acetylcholinesterase is quantitatively adsorbed. This may be due to high local concentrations of both hypericin and the protein resulting in close proximity, and hence in a high yield of cross-linking. Molten globule species are believed to be intermediates in both protein folding and translocation through biological membranes. Thus, hypericin may serve as a valuable tool for trapping such intermediates. This might also explain its therapeutic effectiveness toward virus-infected or tumor cells.
Weiner
L
L
Departments of Chemical Services, Neurobiology, and Organic Chemistry, The Weizmann Institute of Science, Israel. cilev@weizmann.weizmann.ac.il
Roth
E
E
Mazur
Y
Y
Silman
I
I
D000110:Acetylcholinesterase / Q000737:chemistry* / Q000378:metabolism; D000818:Animals; D000873:Anthracenes; D000998:Antiviral Agents / Q000737:chemistry* / Q000378:metabolism; D002499:Centrifugation, Density Gradient; D003432:Cross-Linking Reagents / Q000737:chemistry* / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D004591:Electrophoresis, Polyacrylamide Gel; D019791:Guanidine / Q000737:chemistry; D010569:Perylene / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D011489:Protein Denaturation; D017510:Protein Folding; D012995:Solubility; D013050:Spectrometry, Fluorescence; D014101:Torpedo
D000873:Anthracenes; D000998:Antiviral Agents; D003432:Cross-Linking Reagents; D010569:Perylene; C004965:hypericin; D000110:Acetylcholinesterase; D019791:Guanidine