Enzymes tyrosinases

An interesting combination of enzymatic with non-enzymatic transformation in a one-pot three-step multiple sequence was reported by Waldmann and coworkers [82]. Phenols 125 in the presence of oxygen and enzyme tyrosinase are hydroxylated to catechols 126 which are then oxidized in situ to ortho quinones 127. These intermediates subsequently undergo a Diels-Alder reaction with inverse electron demand by reaction with different dienophiles (Table 4.19) to give endo bicyclic 1,2-diketones 128 and 129 in good yields. 

Fig. 4 Energy-minimized 3D structures of compounds 3-7, which showed potential inhibitory activities against the enzyme tyrosinase [43]...

Khan MTH (2007) Recent Advances on the Sugar-Derived Heterocycles and Their Precursors as Inhibitors Against Glycogen Phosphorylases (GP). 9 33-52 Khan MTH (2007) Heterocyclic Compounds Against the Enzyme Tyrosinase Essential for Melanin Production Biochemical Features of Inhibition. 9 119-138 Khan MTH (2007) Molecular Modeling of the Biologically Active Alkaloids. 10 75-98 Khan MTH, Ather A (2007) Microbial Transformation of Nitrogenous Compoimds. 10 99-122... 

Heterocyclic Compounds against the Enzyme Tyrosinase Essential for Melanin Production Biochemical Features of Inhibition... 

The pathway of melanin synthesis starts from the amino acid tyrosine (Fig. 1). The first two reactions are catalyzed by the copper-containing enzyme tyrosinase (EC 1.14.18.1). Tyrosine is hydroxylated to 3,4-dihy-... 

Encapsulated Cu—chlorophthalocyanines oxidize hexane at C-l using 02 and at C-2 using H202 as oxidants. The dimeric structure of copper acetate is intact when it is incorporated into the zeolite. This is a regioselective aromatic hydroxylation catalyst, which mimics the specificity of the monooxygenase enzyme tyrosinase.82,89 Zeolite NaY catalysts made with a tetranuclear Cu(II) complex were synthesized and characterized.90... 

Since the oxidative polymerization of phenols is the industrial process used to produce poly(phenyleneoxide)s (Scheme 4), the application of polymer catalysts may well be of interest. Furthermore, enzymic, oxidative polymerization of phenols is an important pathway in biosynthesis. For example, black pigment of animal kingdom "melanin" is the polymeric product of 2,6-dihydroxyindole which is the oxidative product of tyrosine, catalyzed by copper enzyme "tyrosinase". In plants "lignin" is the natural polymer of phenols, such as coniferyl alcohol 2 and sinapyl alcohol 3. Tyrosinase contains four Cu ions in cataly-tically active site which are considered to act cooperatively. These Cu ions are presumed to be surrounded by the non-polar apoprotein, and their reactivities in substitution and redox reactions are controlled by the environmental protein. 

Oxidative polymerization of phenol derivatives is also important pathway in vivo, and one example is the formation of melanin from tyrosine catalyzed by the Cu enzyme, tyrosinase. The pathway from tyrosine to melanin is described by Raper (7) and Mason (8) as Scheme 8 the oxygenation of tyrosine to 4-(3,4-dihydro-xyphenyl)-L-alanin (dopa), its subsequent oxidation to dopaqui-none, its oxidative cyclization to dopachrome and succeeding decarboxylation to 5,6-dihydroxyindole, and the oxidative coupling of the products leads to the melanin polymer. The oxidation of dopa to melanin was attempted here by using Cu as the catalyst. 

Dianisyltellurium oxide (DAT) is a mild and selective oxidant for quinone for-mation. Treatment of the A,A-di-n-propyldopamine (2) with DAT leads to the betaine (3), which is identical with the product of oxidation by the enzyme tyrosinase both of (2) and of the monohydric phenol A,A-dimethyltyramine. The implications and relevance to the mode of action of tyrosinases have been discussed. 

Hguie 1. Exan les of hypothetical iixrdes of reaction for the enzyme tyrosinase... 

As the melanin structure grows, it becomes more colored giving various shades of brown color to our skin. This brown coloration acts to help protect deeper skin elements from being damaged by the UV radiation. The absence of the enzyme tyrosinase that converts tyrosine to melanin can lead to albinism. 

The mechanism of action of these compounds appears to involve inhibition of the enzyme tyrosinase, thus interfering with the biosynthesis of melanin. In addition, monobenzone may be toxic to melanocytes, resulting in permanent loss of these cells. Some percutaneous absorption of these compounds takes place, because monobenzone may cause hypopigmentation at sites distant from the area of application. Both hydroquinone and monobenzone may cause local irritation. Allergic sensitization to these compounds can occur. Prescription combinations of hydroquinone, fluocinolone... 

Copper-catalyzed oxidations of phenols by dioxygen have attracted considerable interest owing to their relevance to enzymic tyrosinases (which transform phenols into o-quinones equation 24) and laccases (which dimerize or polymerize diphenols),67 and owing to their importance for the synthesis of specialty polymers [poly(phenylene oxides)]599 and fine chemicals (p-benzoquinones, muconic acid). A wide variety of oxidative transformations of phenols can be accomplished in the presence of copper complexes, depending on the reaction conditions, the phenol substituents and the copper catalyst.56... 

Ortho hydroxylation of phenols can be accomplished by molecular oxygen in the presence of copper(I) chloride and metallic copper in acetonitrile. This particular ortho selectivity, which is similar to that of enzymic tyrosinases, has been attributed to the formation of a stable copper(II) catecholate resulting from the reaction of copper(I) phenates with dioxygen (equation 274).605... 

The phenol-oxidizing enzyme tyrosinase has two types of activity (/) phenol o-hydroxylase (cresolase) activity, whereby a monophenol is converted into an o-diphenol via the incorporation of oxygen, and (2) cathecholase activity, whereby the diphenol is oxidized. The two reactions are illustrated in Figure 2-6, in the conversion of tyrosine (2.40) to L-DOPA (3,4-dihydroxyphenylalanine (2.41), dopaquinone (2.42), and indole-5,6-quinone carboxylate (2.43), which is further converted to the brown pigment... 

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,... 

Another fascinating topic which attracted the attention of many researchers is the difference in the enzymatic activities between catechol oxidase and the structurally related enzyme tyrosinase. Although both enzymes contain type 3 active... 

Tyrosinase inhibitors prevent browning in foodbecause they inhibit the oxidation caused by the enzyme tyrosinase. Cuminaldehyde is identified as a potent mushroom tyrosinase monophenol monooxygenase inhibitor from cumin seeds, ft inhibits the oxidation of L-3,4-dihydroxyphenylalanine (l-DOPA) by mushroom tyrosinase with an ID50 of 7.7g/ml (0.05 mM). Its oxidized analogue, cumic acid (p-isopropylbenzoic acid), also inhibits this oxidation with an 1D50 of 43g/ml (0.26mM). These two inhibitors affect mushroom tyrosinase activity in different ways (Kubo and Kinst-Hori, 1998). 

Tyrosine, itself a degradation product of phenylalanine (Sec. 15.1), is initially converted to 3.4-dihydroxyphenylalanine (dopa), and the corresponding do pa quinone, by the copper-containing enzyme tyrosinase. Tyrosinase is found in melanocytes and is a mixed-function oxidase. It catalyzes the following reaction ... 

Albinism is also caused by lack of the enzyme tyrosinase, resulting from a genetic defect. 

Hemocyanin, the rather distantly related oxygen-carrying protein from arthropods and moluscs, is included since recent data show that the copper pairs in this protein are probably closely related to those of the fungal enzyme, tyrosinase. This suggests that the two molecules are evolutionarily related 54). 

Both enzymes are prominent in the early history of oxidative biochemistry. The early work of Bertrand and Keilin and Mann on laccase and of Bourquelot, Bertrand, and Kubowitz on tyrosinase has been adequately covered in excellent reviews of both enzymes [tyrosinase (106) laccase (J07)]. Much of the early work was on the nature of the reactions catalyzed by these enzymes, and only brief mention is made here as to the nature of the reactions. 

Because of its bioisosteric similarity to the normal physiological substrate L-dopa (4), L-mimosine (5) inhibits catechol oxidation by the enzyme tyrosinase (7). These compounds exemplify a situation in which bioisosteresdis-play opposite pharmacologic effects at the same receptor. 

The present review will focus on the analysis of l-DOPA and L-tyrosine by HPLC in plasma. Both are involved in the first step of melanogenesis controlled by a melanocyte-specific enzyme, tyrosinase. The plasma L-DOPA/L-tyrosine ratio has been evaluated as a tumor marker in melanoma during a 10-year collaboration between the Biochemistry Laboratory, the Dermatology Department of Saint-Louis Hospital (AP-HP) in Paris (France) and the Dermatology Department of the National Center of Oncology in Sofia (Bulgaria). 

Figure 4.1 Early phases of the melanogenesis pathway. The first steps are critically regulated by the melanocyte-specific enzyme tyrosinase. l-DOPA is directly formed from L-tyrosine (1) and/or indirectly via L-DOPAquinone (2). Adapted from Melanoma Research, 9, Letellier S, Gamier JP, Spy J, Stoitchkov K, Le Bricon T, Baccard M, Revol M, Kernels Y, Bousquet B. Development of metastases in mahgnant melanoma is associated with an increase of plasma L-DOPA/L-tyrosine ratio, pages 389-394, 1999, with permission from Lippincott Williams Wilkins...

---