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Hypericin Can
Bind to and Affect the Lens and its Constituents: an in vitro Spectroscopic
Study
Sgarbossa, Antonella 1
and Lenci, Francesco 1
CNR, Istituto Biofisica, Pisa1
Abstract-
The medicinal plant Hypericum perforatum (St. John's Wort), containing
the perylene quinone pigment hypericin, is easily available as a herbal
treatment of some behaviour disorders. In addition to its antidepressant
effects, hypericin is also a strong photosensitizer and is intensively
investigated as a photodynamic drug with antitumor, antiviral and antibacterial
properties. To assess if hypericin can be safely used without phototoxic
side effects in light-exposed organs and tissues, it is crucial to determine
whether hypericin reaches the human eye and whether it can be phototoxic
to ocular tissues and in particular to lens proteins. For this purpose,
we have performed an in vitro spectroscopic study on intact bovine lens
incubated with hypericin. Our results show that hypericin is taken up
by the intact lens and that it not only sticks onto the outer layers,
but partly penetrates inside the lens. To clarify how the pigment binds
to the lens proteins and to investigate the effects of visible light
on these molecular systems, we have studied by means of absorption and
fluorescence spectroscopy the molecular interactions between hypericin
and alpha-, beta- and gamma-crystallin proteins. For alpha-crystallin,
the major lens protein which plays a key role with its chaperone-like
function in preventing the formation of lens cataract, the binding constant
of hypericin has been evaluated to be of the order of 3.0, corresponding
to a dissociation constant of about 0.3. Following irradiation with
visible light, alpha-crystallin photopolymerization sensitized by hypericin
itself causes a spatial rearrangement of the protein framework. In the
case of beta- and gamma-crystallins similar, even if less marked, effects
have been observed. These photosensitized alterations can induce loss
of solubility and stability of lens proteins and engender randomly distributed
aggregations causing light scattering. Moreover, the sensitized photodamage
to alpha-crystallin can spoil its "one-way
sink" and chaperone-like function.
Keywords: hypericin,
lens proteins, photosensitization, spectroscopy
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