Comparative studies on the chemical and enzymatic stability of alpha- and beta-arbutin.
OBJECTIVE
The aim of this study was to establish a comparative analysis of the chemical and enzymatic stability of α- and β-arbutins as potential sources of the substance of concern hydroquinone (HQ). The study was performed using an array of techniques including HPLC-PDA, nuclear magnetic resonance (NMR) and optical rotation (OR). Both arbutins are emerging as popular and effective skin whiteners, acting as tyrosinase inhibitors in a fashion similar to the popular whitening agent HQ. Due to their structural similarity to the regulated agent HQ, both arbutins may be regarded as potential sources of the active aglycone after chemical or metabolic conversion.


METHODS
Various cosmetic formulations including creams, sera, gels and lotions were analysed by HPLC-PDA for their arbutin and HQ content in freshly opened and aged samples stored for 16 months. Solutions of pure compounds were also aged and periodically checked for degradation products using 1D and 2D NMR experiments and OR measurements. The metabolic stability was investigated using pear peels as a biological model.


RESULTS
Both arbutins were found to be stable in water and methanol solutions in the absence of buffer or stabilizers. Their stability in cosmetic formulations, however, was found to depend on the type of formulation and pH. Both compounds were unstable under strong hydrolytic conditions, with consequent release of HQ. Enzymatic instability of both arbutins was also observed, although no formation of HQ was observed under the chosen experimental conditions.


CONCLUSIONS
Both arbutins were found to possess similar stability profiles, and to be more prone to in vivo rather than in chemico degradation, although no HQ was found after enzymatic hydrolysis. Also, no epimerization was observed in any of the tested conditions. Diverse experimental approaches can be applied to analyse the chemical and enzymatic stability of arbutins in regard to the potential release of HQ in different types of preparations. These result showed the potential use of NMR and OR as complementary investigative tools for the stability and safety assessment of arbutin along with more established HPLC methods.
National Center for Natural Products Research, School of Pharmacy, The University of Mississippi, University, MS, 38677, USA.
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Avonto
National Center for Natural Products Research, School of Pharmacy, The University of Mississippi, University, MS, 38677, USA.
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Wang
National Center for Natural Products Research, School of Pharmacy, The University of Mississippi, University, MS, 38677, USA.
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Avula
National Center for Natural Products Research, School of Pharmacy, The University of Mississippi, University, MS, 38677, USA.
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Wang
The Center for Food Safety and Applied Nutrition, US Food and Drug Administration, 5100 Paint Branch Parkway, College Park, MD, 20740, USA.
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Rua
National Center for Natural Products Research, School of Pharmacy, The University of Mississippi, University, MS, 38677, USA.|Division of Pharmacognosy, Department of BioMolecular Sciences, School of Pharmacy, The University of Mississippi, University, MS, 38677, USA.|Department of Pharmacognosy, College of Pharmacy, King Saud University, Riyadh, Saudi Arabia.
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Khan
D001104:Arbutin / Q000737:chemistry / Q000502:physiology*; D003358:Cosmetics; D004798:Enzymes / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy
D003358:Cosmetics; D004798:Enzymes; D001104:Arbutin
chemical analysis; formulation/stability; hydroquinone; whitening cosmetics; α-arbutin; β-arbutin