Tyrosinase and

Kojic acid is a fungal metabolite (5-hydroxy-4 pyran 4-1-2 methyl) known to inhibit tyrosinase and used to treat melasma at concentration of 2-4% twice a day. The stability is one of its advantages if compared with hydroquinone. Unfortunately, it is considered to have a high sensitizing potential. 

V. Melanin Synthesis—The Role of Tyrosinase and Related Proteins.158... 

V. MELANIN SYNTHESIS—THE ROLE OF TYROSINASE AND RELATED PROTEINS... 

Furthermore, a melanocyte line that has reverted in culture from albino to wild type has restored the cysteine codon at position 85 (Jackson and Bennett, 1990). In vivo studies (see below) proved that the c-locus encodes tyrosinase and showed that expression of a tyrosinase minigene rescues the albino phenotype (Yokoyama et al., 1990 Tanaka et al., 1990 Beermann et al., 1990). 

Adults require 1-2 mg of copper per day, and eliminate excess copper in bile and feces. Most plasma copper is present in ceruloplasmin. In Wilson s disease, the diminished availability of ceruloplasmin interferes with the function of enzymes that rely on ceruloplasmin as a copper donor (e.g. cytochrome oxidase, tyrosinase and superoxide dismutase). In addition, loss of copper-binding capacity in the serum leads to copper deposition in liver, brain and other organs, resulting in tissue damage. The mechanisms of toxicity are not fully understood, but may involve the formation of hydroxyl radicals via the Fenton reaction, which, in turn initiates a cascade of cellular cytotoxic events, including mitochondrial dysfunction, lipid peroxidation, disruption of calcium ion homeostasis, and cell death. 

Oxygenation rates were first examined for the system [Cu (R-XYL-H)]2+. In the model described here, two bis [2-(2-pyridyl)ethyl] amine (PY2) units are linked by a xylyl spacer group (R = H). Although initially proposed as a crude hemocyanin model, this system now is studied as a model for tyrosinase and is an example of hydrocarbon oxygenation taking place under mild conditions—that is, < 1 atm 02... 

There has been enormous activity in the field of copper(I)-dioxygen chemistry in the last 25 years, with our information coming from both biochemical-biophysical studies and to a very important extent from coordination chemistry. This has resulted in the structural and spectroscopic characterization of a large number of copper dioxygen complexes, some of which are represented in Figure 14.2. The complex F, first characterized in a synthetic system was subsequently established to be present in oxy-haemocyanin, and is found in derivatives of tyrosinase and catechol oxidase, implying its involvement in aromatic hydroxylations in both enzymes and chemical systems. 

The values with tyrosinase are also given in the rate per Cu equivalent, considering that the enzyme contains 4 Cu ions (Mol. wt. 12,800) ( ). Much difference between the values with tyrosinase and those with other Cu complexes indicates that the rate of... 

Enzymatic browning. Phenol-oxidizing enzymes (such as tyrosinase and peroxidase) oxidize tyrosine residues into reactive quinone derivatives, which will condense into colored polymers (melanins). Melanins are rich in carboxyl groups and therefore have high affinity for divalent metal ions such as calcium. 

Tyrosinase is a monooxygenase which catalyzes the incorporation of one oxygen atom from dioxygen into phenols and further oxidizes the catechols formed to o-quinones (oxidase action). A comparison of spectral (EPR, electronic absorption, CD, and resonance Raman) properties of oxy-tyrosinase and its derivatives with those of oxy-Hc establishes a close similarity of the active site structures in these proteins (26-29). Thus, it seems likely that there is a close relationship between the binding of dioxygen and the ability to "activate" it for reaction and incoiporation into organic substrates. Other important copper monooxygenases which are however of lesser relevance to the model studies discussed below include dopamine p-hydroxylase (16,30) and a recently described copper-dependent phenylalanine hydroxylase (31). 

The enzyme requires two copper ions per subunit for full expression of activity (18), but, unlike tyrosinase and hemocyanin, there is an absence of magnetic coupling between the two Cu(II) sites and both appear to be separate, isolated mononuclear copper sites (17). The process of dioxygen binding and activation appears to involve interaction of the doxygen molecule with only one copper ion, and it is also found that a proton is requir for the hydroxylation of substrate (19). 

Figure 3.2 Coupled reaction of tyrosinase and laccase in conversion of tyrosol to dimeric products [48].

Figu re 3.3 Conceptual process model for application of a coupled tyrosinase-laccase reaction converting tyrosol. Immobilized enzymes are first characterized with respect to substrate conversion rates, using tyrosol and hydroxytyrosol as substrates for tyrosinase and laccase, respectively. One hundred percent conversion can be achieved in Reactor 1 by use of sufficient tyrosinase... 

Hemocyanin [30,31], tyrosinase [32] and catechol oxidase (2) [33] comprise this class of proteins. Their active sites are very similar and contain a dicopper core in which both Cu ions are ligated by three N-bound histidine residues. All three proteins are capable of binding dioxygen reversibly at ambient conditions. However, whereas hemocyanin is responsible for O2 transport in certain mollusks and arthropods, catechol oxidase and tyrosinase are enzymes that have vital catalytic functions in a variety of natural systems, namely the oxidation of phenolic substrates to catechols (Scheme 1) (tyrosinase) and the oxidation of catechols to o-quinones (tyrosinase and catechol oxidase). Antiferromagnetic coupling of the two Cu ions in the oxy state of these metalloproteins leads to ESR-silent behavior. Structural insight from X-ray crystallography is now available for all three enzymes, but details... 

While only tyrosinase catalyzes the ortho-hydroxylation of phenol moieties, both tyrosinase and catechol oxidase mediate the subsequent oxidation of the resulting catechols to the corresponding quinones. Various mono- and dinu-clear copper coordination compounds have been investigated as biomimetic catalysts for catechol oxidation [21,194], in most cases using 3,5-di-tert-butylcatechol (DTBC) as the substrate (Eq. 16). The low redox potential of DTBC makes it easy to oxidize, and its bulky tert-butyl groups prevent un-... 

Azelaic acid is a newer treatment for hyperpigmentation, primarily for post-inflammatory hyperpigmentation. It works by blocking the activity of tyrosinase and does not cause photosensitivity of the skin or residual changes in the skin. There is decreased incidence of allergic reactions associated with azelaic acid. Corticosteroids also block the activity of tyrosinase. Corticosteroids are used in combination with other drugs to minimize the side effects. The combination of azelaic acid and hydrocortisone acetate (10%) may also be useful in the treatment of post-inflammatory hyperpigmentation of skin... 

The oxidation of catecholamines in the presence of tyrosinases and polyphenolases has been widely studied, and the subject has been adequately discussed in the literature. References 12 and 31-41 will serve as a guide to further reading on the subject. 

To set up the inhibition assay, prepare a table similar to Table E5. 1. Inhibitor should appear in the list of reagents before tyrosinase. Use the same level of tyrosinase and the same dopa stereoisomer as in part C. Vary the amount of dopa as in part C. A constant amount of inhibitor (cinnamic acid or thiourea) should be added to each cuvette. You will have to determine this level of inhibitor by trial and error. The desired inhibition rate with saturating substrate is about 50% of the uninhibited rate. Add all reagents except tyrosinase, mix well, and determine the blank rate, if any. Add tyrosinase, mix, and immediately record AA75 for 2 minutes. From recorder traces or graphs of A475 vs. time, calculate AA/min for each assay. 

Kinetic analysis of tyrosinase and calculation of constants will be described using graphical analysis by the Michaelis-Menten equation, Lineweaver-Burk equation, or the direct linear curve. Procedures for preparing these graphs are described below. Alternatively, students may use available computer software to graph data and calculate kinetic constants. Recommended enzyme kinetic computer software packages include Enzyme... 

The diphenol oxidases (DPOs) includes two major groups of enzymes the ortho-DPOs (also known as catecholases, polyphenol oxidases, and tyrosinases) and the para-DPOs (more usually known as laccases). The names catecholase and laccase are used in this unit. 

PV Ramsohoye, IA Kozlov. Isoprotein composition and cross-linking of thaumatins using mushroom tyrosinase and dimethyl suberimidate. Int J Food Sci Technol 26(3) 271-282, 1991. 

Copper has an essential role in a number of enzymes, notably those involved in the catalysis of electron transfer and in the transport of dioxygen and the catalysis of its reactions. The latter topic is discussed in Section 62.1.12. Hemocyanin, the copper-containing dioxygen carrier, is considered in Section 62.1.12.3.8, while the important role of copper in oxidases is exemplified in cytochrome oxidase, the terminal member of the mitochondrial electron-transfer chain (62.1.12.4), the multicopper blue oxidases such as laccase, ascorbate oxidase and ceruloplasmin (62.1.12.6) and the non-blue oxidases (62.12.7). Copper is also involved in the Cu/Zn-superoxide dismutases (62.1.12.8.1) and a number of hydroxylases, such as tyrosinase (62.1.12.11.2) and dopamine-jS-hydroxylase (62.1.12.11.3). Tyrosinase and hemocyanin have similar binuclear copper centres. 

This enzyme exhibits no hydroxylase activity and is involved in the final synthesis of many naturally occurring /t-quinoncs. e.g. the naphthaquinone juglone in walnut (1.58) and the benzoquinone arbutin (hydroquinone-(3-D-glucopyranoside 2.46). Arbutin is a plant cryo-protectant that stabilizes membranes (Hincha et al., 1999). This compound has medicinal properties and has, for example, been used to treat urinary tract infections in humans. It is also used to lighten skin color, because it inhibits tyrosinase and hence the formation of melanin. The derivative deoxyarbutin (2.47 note the difference in the sugar molecule) was recently reported to be considerably more effective as a skin-lightening compound (Boissy et al., 2005). 

Tyrosine can also be acted on by the enzyme tyrosinase and converted into a dark pigment. This enzyme is quite interesting to study because it is vulnerable to a genetic mutation that makes it heat labile (i.e., it only works correctly in the cooler areas of the body). The consequence of this mutation is a lack of pigmentation in humans (albinism) and, conversely, the characteristic pattern of dark pigmentation at the ends of the nose,... 

Tyrosinase and amine oxidase appear to be true antihypertensive substances they are useless at the present time for clinical application. Many cardiotoxic and depressor agents are known which will lower blood pressure at the expense of kidneys, heart, or blood volume. A few newer compounds, however, are now being studied which on preliminary trial appear to fit the definition of antihypertensive substances. It is believed that, in the not too distant future, a practical method for the control of this prevalent condition will be found. 

Tumor tissue has also been demonstrated to take up naked pDNA following direct intratumoral injection, but this ability may be dependent on tumor type and the pDNA construct. In an important study by Vile and Hart (1993), mice bearing subcutaneous (s.c.) B16F1 melanoma or Colo 26 colon carcinoma were injected intratumorally with naked P -gal pDNA or P -gal pDNA/calcium phosphate precipitates. The tumors were collected on days 2, 4, 6 and 10 after the pDNA intratumoral injection. A gradual increase in blue-staining cells was found in the transfected melanomas with 10-15% of the cells expressing /3-gal by day ten. In contrast, none of the colon carcinoma tumors was positive for /3-gal. One explanation for the lack of in vivo transfection of the colon carcinoma is that the /3-gal pDNA constmcts contained melanoma-specific promoters (tyrosinase and TRP promoters). This study demonstrated that using an appropriate promoter established tumors could take up and express naked pDNA. 

Comparison of reaction mechanisms and products of transformation of catechol by biotic (tyrosinase) and abiotic (birnessite) catalysts... 

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