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Techniques for
studying the photochemistry and phototoxicity of topically applied herbal
components
Chignell, Colin1
Laboratory of Pharmacology and Chemistry, NIEHS, NIH1
Abstract-
Many medical applications of herbal preparations involve their use in
eyewashes, skin lotions and hair dyes. The topical application of these
preparations to the skin or eyes raises the possibility that adverse
phototoxic and/or photoallergic reactions may result from an interaction
between their constituents and sunlight. We have studied the photochemistry
and phototoxicity of several components of herbal preparations, including
berberine (an alkaloid found in Goldenseal) and curcumin (from
Curcuma longa, present in the spice turmeric). We have employed
direct electron spin resonance (ESR) and the ESR spin trapping technique,
using 5,5-dimethyl-1-pyrroline N-oxide (DMPO), to identify the
free radical intermediates formed by Type I reactions. In solvents such
as acetonitrile or benzene oxygen-centered free radicals, including
superoxide, are photogenerated by UVA irradiation of berberine or curcumin.
Singlet oxygen generated during a Type II reaction was measured by direct
detection of its phosphorescence at 1280nm. In aqueous solution neither
berberine nor curcumin generate singlet oxygen, but singlet oxygen is
produced by both alkaloids with moderate quantum yields in non-polar
solvents. These findings suggest that, in the cell, the photochemistry
of berberine and curcumin will be strongly dependent on their subcellular
localization. We have used HaCaT keratinocytes to examine the phototoxicity
of herbal components. Fluorescence spectroscopy indicated that in keratinocytes
berberine is present in a hydrophobic environment, while confocal microscopy
revealed that berberine binds to mitochondria. UVA irradiation of HaCaT
cells in the presence of berberine (50  M)
resulted in an 80% decrease in cell viability and a threefold increase
in DNA damage as measured by the Comet assay. These findings suggest
that exposure to sunlight or artificial light sources emitting UVA should
be avoided when topical preparations derived from Goldenseal
are used. In contrast, the antiseptic properties of topically applied
ointments derived from Curcuma longa may be due in part to the
interaction of curcumin with sunlight.
Keywords: berberine,
curcumin, singlet oxygen, free radicals
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