A highly pure hexylresorcinol (>99% purity), designed for topical application, is shown to be a safe and effective skin lightener as demonstrated by in vitro and human clinical studies. Melanin inhibitory activity of hexylresorcinol is demonstrated to be due to the inhibition of various points in the melanogenesis pathway. Skin protective property of hexylresorcinol is shown to be due to its stimulatory effects on glutathione, glutathione peroxidase and glutathione reductase.
Alkylresorcinols (ARs) are amphiphilic phenolic lipids present in significant amounts in the bran fractions of rye and other cereals. ARs were reported to have antitumour, antibacterial, antifungal and antiparasitic activities.1 These effects of ARs were attributed to membranemodulating effects due to interactions of their alkyl tails with phospholipids and/or proteins, and to antioxidant effects of the phenolic hydrogen.1 ARs are present in wheat and rye grains at levels of approximately 0.015% to 0.3% of whole kernel dry weight, thus representing a significant proportion of the phenolic compounds present in these cereals. ARs have been found to be bioactive in many in vitro models, and may be important in food and human nutrition.2 4-hexylresorcinol (HR, Fig. 1) is the most studied and well-known AR, which has an 80-year history of use and is reported to have anesthetic, antiseptic and anthelminitic properties.3 It can be used topically on small skin infections, or as an ingredient in throat lozenges.4 HR is an antiseptic that also has local anesthetic effects. Its antiseptic action kills the bacteria that may be associated with sore throats, while its anesthetic action helps relieve the pain. The action of sucking the lozenge allows the active ingredient to work in the area of the discomfort, and also helps to coat, lubricate and soothe a sore throat. HR is reported to be an excellent inhibitor of polyphenol oxidase (PPO) which is responsible for enzymatic browning of fruits and vegetables. PPO (a mixture of monophenol oxidase and catechol oxidase enzymes) is present in nearly all plant tissues and can also be found in bacteria, animals and fungi.5 In fact, browning by PPO is not always an undesirable reaction; the familiar brown colour of tea, coffee and cocoa is developed by PPO enzymatic browning during product processing. When used in combination with ascorbic acid, HR has been shown to be a very effective inhibitor of surface browning on many fresh-cut fruits, including apples and pears.6,7 HR has a synergistic effect with ascorbic acid in the prevention of browning.8,9 Ascorbic acid reduces quinones generated by polyphenoloxidase while HR specifically interacts with polyphenol oxidase, and renders it incapable of catalysing the enzymatic reaction. A post-cutting dip of HR, ascorbic acid and calcium lactate extended the shelf-life of pear slices from 15 to 30 days.10 Red delicious apple slices treated with a combined anti-browning dip (HR, isoascorbic acid, N-acetyl cysteine and calcium propionate) and held at 5°C maintained visual quality for five weeks.11 HR is also reported to inhibit melanosis (black spots) in shrimp.12 HR has a GRAS (Generally Recognized as Safe) status and is considered to be safe and effective in use as an anti-browning agent.13
Recently, the protective effects of resveratrol and HR against oxidative DNA damage in human lymphocytes induced by hydrogen peroxide were investigated.14 The inhibition of oxidative damage in human lymphocytes by resveratrol and HR is shown to be due to the increase in the glutathione levels and the antioxidant enzymes – glutathione reductase and glutathione peroxidase.14 Davydova et al15 have shown long-term preservation of DNA in aqueous solution in the presence of methyl- and hexyl-resorcinols. The initial property of DNA is preserved to the greatest extent in the presence of HR. In another publication, Davydova et al16 have shown that irradiating DNA with UV light in the presence of methyl- and hexyl-resorcinols results in comparatively insignificant DNA destruction as evidenced by the electrophoretic mobility pattern in agarose gel. Protective effect on DNA is more predominant with HR than with methylresorcinol.
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