Role in Nature

Mammalian tyrosinases are located in specialized melanocytes and are responsible for the photoprotective pigmentation of hair, skin, and retina (García-Borrón and Solano 2002). Disorders in tyrosinase-catalyzed melanin synthesis are not only an aesthetic problem; they are linked with serious skin diseases, such as the well-known malignant melanoma. Vitiligo is another such disease, characterized by hypopigmentation and total melanocyte depletion in the basal layer of the epidermis. Immunological studies of vitiligo show the generation and presence of autoantibodies directed against tyrosinase antigens in patient sera. This indicates that tyrosinase acts as an autoantigen and can serve as a marker for vitiligo (Parvez et al. 2007). Albinism, the total loss of pigmentation, is caused by different gene defects that do not primarily affect tyrosinase activity but rather transport of the enzyme into the mel-anosomes (Kushimoto et al. 2003).

Plant tyrosinases may be involved in biosynthetic processes and in defense against herbivores. During browning reactions, the injured tissues build up a melanin layer as protection against microbial pathogens (Mayer and Harel 1979; Mayer 2006).

In sponges and many invertebrates, tyrosinases are important components of wound healing and the primary immune response (Cerenius and Soderhall 2004). In arthropods they are involved in sclerotization of the cuticle after molting or injury (Anderson et al. 1996). After their activation from inactive proenzymes by a cascade of serine proteases, insect phenoloxidases generate cytotoxic quinones and other reactive intermediates to immobilize and kill invading pathogens and parasites. Bacterial cell wall components are effective activators of these systems (Jiang et al 1998; Soderhall and Cerenius 1998; Sugumaran 2002).

Fungal tyrosinases are generally associated with spore pigmentation, formation, and stability, as well as with defense and virulence mechanisms, or wound healing by melanin production (Seo et al. 2003; Halaouli et al. 2006; Mayer 2006).

The biological roles of bacterial tyrosinases are rather diverse. In soil environments, extracellular Streptomyces tyrosinases are probably involved in the polymerization and detoxification of plant phenolic compounds and the formation of humic matter (Kutzner 1968; Sjoblad and Bollag 1981).

Bacteria of the genus Rhizobium living in the root nodules of Papillionaceae plants carry tyrosinase genes in plasmids required for symbiosis (Mercado-Blanco et al. 1993). It was recently shown that the tyrosinase from Rhizobium etli plays a role in nodulation efficiency and symbiosis-associated stress resistance. Tyrosinase probably protects symbiotic microorganisms against toxic phenolic compounds in the soil environment and phytoalexins produced by plants (Pinero et al. 2007). The same mechanism is expected to be present in other plant-associated bacteria, like Ralstonia solanacearum.

The best-documented function of the enzyme is restricted to the formation of eumelanins. The dark pigments protect cells and spores against UV radiation, heat, enzymatic hydrolysis, antimicrobial compounds, heavy metals, or phagocytosis (Butler and Day 1998; Ruan et al. 2004; Wan et al. 2007), and contribute to microbial pathogenesis (Nosanchuk and Casadevall 2003; Plonka and Grabacka 2006).

An attractive theory suggests that bacteria may use melanin as a redox polymer for adaptating to different oxygen concentrations:

• The aerobic soil bacterium Azotobacter chroococcum contains an active polyphenol oxidase (tyrosinase?) and forms melanin from catechol (Shivprasad and

Page 1989). This microorganism produces particularly large amounts of melanin when cultured under aerobic conditions. Although the intensity of melanogenesis does not seem to be directly correlated with the activity of nitrogenase (the key enzyme of atmospheric nitrogen fixation), it is possible that Azotobacter employs melanogenesis to enhance the utilization of oxygen and to maintain the reducing conditions necessary to bind atmospheric nitrogen.

• Soil bacteria can use humic acids as an electron acceptor for anaerobic respiration (Coates et al. 2002). A similar function can be assumed for the melanins.

• In Proteus mirabilis, an important cause of infections of the urinary tract, tyro-sinase was identified as the enzyme responsible for pigmentation. The melanin decreases the level of reactive oxygen species, which probably makes the pathogen more resistant to the oxygen burst connected with the immunological response of the host (Agodi et al. 1996).

Body Detox Made Easy

Body Detox Made Easy

What exactly is a detox routine? Basically a detox routine is an all-natural method of cleansing yourbr body by giving it the time and conditions it needs to rebuild and heal from the damages of daily life and the foods you eat and other substances you intake. There are many different types of known detox routines.

Get My Free Ebook


Post a comment