Enzymatic Melanin Synthesis

This biosynthetic approach was first exploited with remarkable success by Raper (216). Later, using advanced research techniques, biosynthetic studies of eumelanin polymers have revealed many facts regarding 

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It appears from the previous discussion that melanogenesis in vivo or in vitro is regulated by various factors. Hence, to study the dynamics of melanin formation (monooxygenase reaction, i.e. constructive metabolism) and breakdown (dioxygenase reaction, i.e. catabolism of melanin precursors), a non-enzymatic melanin synthesis from tyrosine and tryptophan, respectively, was devised by Roy et al. (227), using a prototype of a monooxygenase reaction, i.e. the Udenfriend reaction (Fe+ /EDTA/ ascorbic acid) (272). 

Synthetic melanins are obtained by biomimetic oxidation reactions using known precursors. So far, four different methods for melanin synthesis have been reported, i.e. in vitro enzymatic, autooxidative, electrochemical, and photochemical methods. Of these, the first two have been generally used, for large scale preparations of the pigment polymers and have been reviewed elsewhere (70, 211). The latter two methods which are discussed here, have been used effectively to understand the mechanism of the melanization process in biological systems. 

Melanoma can be diagnosed throngh the monitoring of tyrosinase, a cytoplasmic melanocyte differentiation protein, which is a key enzyme in melanin synthesis and has been listed as important melanoma biomarker. Mossberg et al. (2014) developed an electrochemical biosensor platform with an amperometric detection mode to detect the enzymatic activity of tyrosinase in fresh biopsy samples withont pretreatment of the samples. The combination of this method with modem portable devices can provide interesting POC sensors in the fnture. 

Enzymes present in melanosomes synthesize two types of melanin, eumelanin and pheomelanin. Figure 2 illustrates the proposed biosynthetic pathways of eumelanin and pheomelanin. The synthesis of eumelanin requires tyrosinase, an enzyme located in melanosomes. Tyrosinase catalyzes the conversion of tyrosine to dopa, which is further oxidized to dopaquinone. Through a series of enzymatic and nonenzymatic reactions, dopaquinone is converted to 5,6-indole quinone and then to eumelanin, a polymer. This polymer is always found attached to proteins in mammalian tissues, although the specific linkage site between proteins and polymers is unknown. Polymers affixed to protein constitute eumelanin, but the exact molecular structure of this complex has not been elucidated. Pheomelanin is also synthesized in melanosomes. The initial steps in pheomelanin synthesis parallel eumelanin synthesis, since tyrosinase and tyrosine are required to produce dopaquinone. Dopaquinone then combines with cysteine to form cysteinyldopa, which is oxidized and polymerized to pheomelanin. The exact molecular structure of pheomelanin also has not been determined. 

The fundamental work of Raper 216) on the in vitro enzymatic synthesis of melanin, using tyrosinase, tyrosine and oxygen, was the first synthetic approach investigating melanogenesis and the structure of... 

The synthesis, autoxidation and enzymatic oxidation of the symmetrical DOPA dimer (25) to melanin [64] has been described. The enzyme employed had little effect on the rate of oxidation of (25) which is unlikely to be an intermediate in DOPA melanin formation [64]. 

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