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Lens α‐Crystallin and Hypericin: A Photophysical Mechanism Explains Observed Lens Damage ¶†
Author(s) -
TrevithickSutton Colleen C.,
Chin Khin K.,
Contos Sotiria D.,
Foote Christopher S.
Publication year - 2004
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.2004.tb00112.x
Subject(s) - hypericin , singlet oxygen , photosensitizer , photochemistry , chemistry , photodynamic therapy , crystallin , biophysics , oxygen , organic chemistry , biochemistry , biology , pharmacology
Determining whether α‐crystallin (the major lens protein) affects the photophysics of hypericin, a photosensitizing agent found in various plants, such as St. John's Wort, is important. Hypericin shows promise in cancer and human immunodeficiency virus therapy but may harm individuals taking St. John's Wort extracts (for mild to moderate depression). Hypericin causes hypericism, which is characterized by cellular damage in light‐exposed areas. Ocular tissues are at risk for photosensitized damage; thus, we investigated the effects on hypericin photophysics by α‐crystallin. We measured the transient absorption spectra and the 1270 nm luminescence of singlet ( 1 Δ g ) oxygen produced from hypericin in the presence of α‐crystallin. α‐Crystallin complexes hypericin, extending the lifetime of its triplet excited state; the Stern‐Volmer slope is negative, but not linear, after a saturation curve. Damage to the lens protein by hypericin is known to occur via singlet oxygen, which oxidizes methionine, tryptophan and histidine residues. Binding to α‐crystallin does not inhibit singlet oxygen formation by hypericin. α‐Crystallin reacts with singlet oxygen with a rate constant of 1.3 × 10 8 M −1 s −1 . Thus, we anticipate that hypericin will be an effective photosensitizer in the lens.