Research communicationPhotostability and photoprotection factor of boldine and glaucine
Introduction
Boldine, (S)-2,9-dihydroxy-1,10-dimethoxy-aporphine, occurs abundantly in the leaves and bark of boldo (Peumus boldus Mol.) a widely distributed native tree of Chile. Infusions of boldo leaves traditionally have been employed in folk medicine for their reportedly chloretic, diuretic, sedative and digestive properties [1], [2]. At low micromolar concentrations, boldine prevents both enzymatic and non-enzymatic mediated damage to biological systems. In vitro, boldine inhibits the free-radical-mediated initiation and propagation of the peroxidative damage induced to various membrane types (such as liver homogenates, hepatic microsomes and ghost erythrocytes) and it blocks the free-radical-dependent lysis of red blood cells and intact hepatocytes [1], [3], [4]. In other words, boldine is a very potent antioxidant substance for biological systems. Boldine undergoes peroxidative free-radical-mediated damage, the boldine molecule acts as an efficient hydroxyl radical scavenger [4].
Glaucine is the dimethoxy analogue of boldine (Scheme 1). The principal difference between both molecules is their solubility, while boldine is hydrosoluble, glaucine is liposoluble. Both molecules are relatively non-toxic [5], [6], [7]. Ca2+ channel antagonism by glaucine appears to be responsible for the relaxant effect of glaucine in human isolated bronchus while PDE4 inhibition contributes to the inhibitory effects of glaucine in human granulocytes [8].
We investigated the photoprotector capacity of boldine hydrochloride against UVB using in vitro and in vivo methods. The calculated membrane protection factor, in vitro was higher than octylmethoxycinnamate [9], while the SPF in vivo determined on volunteers was lower than Nivea sun filter [9].
Boldine was found to be photounstable at wavelengths up to λ = 300 nm. Here, we report the photostability and photoprotection capacity of boldine, boldine hydrochloride and glaucine hydrochloride. Three main photoproducts F1, F2 and F3 (Scheme 1) were isolated and characterized from boldine.
Section snippets
Material and methods
Boldine was purchased from Sigma. Boldine hydrochloride and Glaucine hydrochloride were a kind gift from Dr. Bruce Cassels (Facultad de Ciencias, Universidad de Chile). Methanolic solutions of the three compounds (6.7 × 10−6 M) (See Fig. 1) were irradiated at λ = 300 nm under nitrogen, oxygen and air, using a Srinivaran–Griffin Rayonet Photochemical Reactor during 16 min. Solutions containing 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) (25 mM) were irradiated under the same conditions.
Fluorescence
Photodegradation kinetics
Kinetics of photodegradation were determined through fluorescence spectra with the following excitation wavelengths: boldine (λ = 304 nm); boldine hydrochloride (λ = 344 nm); glaucine hydrochloride (λ = 304 nm) and emissions: boldine (λ = 344 nm); boldine hydrochloride (λ = 353 nm); glaucine hydrochloride (λ = 345 nm).
Comparison between kinetics of photodegradation of boldine and boldine hydrochloride solutions irradiated at λ = 300 nm demonstrate that, under air conditions, boldine hydrochloride (k = 6.2 ± 1.4
Discussion
All tested alkaloids were photounstable (mean irradiance = 14.6 W/m2).
Oxygen was involved in the mechanism of photodegradation, in accordance with the results obtained by Zanocco et al. [11], demonstrated that boldine and glaucine are very effective quenchers for excited oxygen particularly in polar solvents, as was in this case. Furthermore, the addition of TEMPO (a high efficient radical scavenger) demonstrated free radicals participation. The kinetics of photodegradation of boldine and boldine
Conclusion
Boldine and glaucine are photounstable at irradiation conditions, the photodegradation mechanism, oxygen dependant, involves free radicals. The aporphine structure remains unchanged, causing that the antioxidant capacity related with phenolic structure remains unchanged. Glaucine’s photoproducts exhibited higher SPF than the compound without irradiation, for these reasons both compounds boldine and glaucine represent good natural sunscreen to be used in the future.
Abbreviations
- ABTS
2,2′-azino-di-(3-ethylbenzotiazoline-6-sulfonic acid)
- PDE4
phosphodiesterase 4
- SPF
Sun protection factor
- TEMPO
2,2,6,6-tetramethyl-1-piperidinyloxy
Acknowledgment
This work was supported by a Project DIPUV 20/93.
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