Tyrosinase is widely distributed in microorganisms, animals, and plants, and is a key enzyme in melanin biosynthesis and pigmentation of mammalian skin and hair. Its oxidative activities have a positive impact on the organoleptic properties of some fermentation products (raisins, cocoa, tea, coffee), but are also responsible for the undesirable enzymatic browning of fruits and vegetables, thereby causing a decrease in their nutritional quality and an inability to sell foods that have turned brown (Mayer and Harel 1979; Martinez and Whitaker 1995). Current conventional techniques of avoiding browning include heat inactivation of tyrosinase, but these processes cause undesirable losses to the quality of the product. Various chemicals such as halide salts and aromatic carboxylic acids as well as reducing compounds such as sulfite, citric acid, ascorbic acid, and cysteine are known to inhibit tyrosi-nase. The benefit of ascorbic acid is the focus of some discussion, and the use of sulfites is being restricted due to potential health hazards (Taylor and Bush 1986).

Widely used tyrosinase inhibitors for in vitro studies include L-mimosine, kojic acid, tropolone, phenylthiourea, and azide. However, as safety is paramount in the food industry, there is a constant search for better inhibitors from natural sources that are largely free of any harmful side effects. A number of tyrosinase inhibitors from natural sources (plants, fungi) that inhibit monophenolase and/or diphenolase have been already identified (e.g., arbutin, oxyresveratrol). Presently, 4-hexylresor-cinol is considered to be safe for use in the food industry for browning control (Mayer 2006; Parvez et al. 2007).

A search for new tyrosinase inhibitors has also been launched by the cosmetic and pharmaceutical industries. Although melanin plays a crucial protective role against UV radiation and as an antioxidant, abnormal melanin pigmentation is a serious aesthetic problem in humans. Thus, tyrosinase inhibitors are important in the cosmetic industry due to their skin whitening and preventive effects (Parvez et al. 2007).

A number of tyrosinase inhibitors from natural sources have been reported, but only a few of them are used as skin-whitening agents, primarily due to various safety concerns. For example, linoleic acid, hinokitiol, kojic acid, arbutin, naturally occurring hydroquinones, and catechols were reported to inhibit enzyme activity but have also exhibited side effects (Maeda and Fukuda 1991). Currently, arbutin (a hydroquinone glycoside) and aloesin (a glycosylated chromone) are used in the cosmetic industry as whitening agents because they are strong inhibitors of tyrosinase (Kahn 1995; Parvez et al. 2007).

Malignant melanoma is an increasingly serious clinical problem, with a high mortality rate among humans due to the failure of melanoma cells to respond to cytotoxic treatment in the form of radiation and chemotherapy. A selective strategy toward the treatment of malignant melanoma is called melanocyte-directed enzyme prodrug therapy (Jordan et al. 2001). Instead of tyrosine itself, a derivate coupled with an inactive prodrug serves as substrate in the biosynthetic pathway that converts tyrosine into melanin (Prota et al. 1994). This would allow selective conversion of inactive prodrugs into cytotoxic drugs in melanoma cells.

The substrate stereospecificity of the monophenolhydroxylase and diphenoloxi-dase activities of tyrosinase are the basis for many industrial applications (Halaouli et al. 2006): as biosensors for the monitoring of phenols; in the pharmaceutical industry for the production of o-diphenols (e.g., L-dopa, dopamine for the treatment of Parkinson's disease), and for the synthesis of biopolymers. Synthetic melanins find application as protective agents against radiation (UV, X-rays, g-rays), cation exchangers, drug carriers, antioxidants, antiviral agents, and immunogens (Nosanchuk and Casadevall 2003; Wang et al. 2000). Their ability to crosslink proteins has opened up new application markets for tyrosinases in food industries (Thalman and Lotzbeyer 2002; Halaouli et al. 2005).

Tyrosinase has been suggested as an environmental tool for the detoxification of phenol-contaminated sites (Duran and Esposito 2000; Gianfreda and Rao 2004). However, due to their broad substrate spectrum and higher stabilities and activities under environmental conditions (such as variations in pH and temperature; presence of soil constituents), laccases appear to be much more suitable for bioremediation purposes (Claus and Filip 1988a,b, 1990a,b, 1991, Filip and Claus 1995).

How To Prevent Skin Cancer

How To Prevent Skin Cancer

Complete Guide to Preventing Skin Cancer. We all know enough to fear the name, just as we do the words tumor and malignant. But apart from that, most of us know very little at all about cancer, especially skin cancer in itself. If I were to ask you to tell me about skin cancer right now, what would you say? Apart from the fact that its a cancer on the skin, that is.

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