Purpose: To determine the uptake, location and fluorescence of hypericin, the active ingredient in St. John's Wort, in situ in the isolated intact calf lens.
Methods: The absorption and fluorescence spectra of hypericin 10(-5 ) M were measured in DMSO/phosphate buffer, pH 7.4) [PBS] (1/10 in volume) in the presence of alpha-crystallin (0.5 and 1.1 mg/ml). Bovine lenses were incubated in the dark for 24 hours in 10(-4) M hypericin in a DMSO/PBS (1/10 in volume) mixture. Diffused hypericin fluorescence emission was detected with a fluorescence stereomicroscope from the PBS washed lens surface. A lens-holder specially built for front-surface excitation-detection was used to measure fluorescence emission and excitation spectra of intact lenses incubated with hypericin solutions.
Results: As increasing concentrations of alpha-crystallin were added, the absorption and fluorescence spectra of hypericin in DMSO/PBS (1/10 in volume) changed, indicating a binding between the chromophore and the lens protein. Fluorescence emission spectra detected from the lens surface (lambda( em) = 601 and 651 nm; lambda(exc) = 550 nm) confirmed that hypericin does bind to the ocular tissues.
Conclusions: The results we obtained in simplified model systems can provide clues to investigate the effects of hypericin on lens properties in physiological conditions. Hypericin could in fact bind to lens protein thus increasing the retention time of hypericin in the eye and possibly altering a-crystallin properties as a chaperone. Should therefore hypericin be taken up by the lens, this can be detected, non-invasively by its fluorescence. Therefore, ophthalmologists may use a slit-lamp or scanning fluorometry to monitor the uptake of hypericin in the eyes of patients using St. John's Wort or receiving high doses of hypericin while undergoing photodynamic therapy.