2. INHA University - Bioengineering
ABSTRACT
Melanogenesis proceeds in three distinctive steps
•The initial step is the production of cysteinyldopas by the rapid addition of
cysteine to dopaquinone, which continues as long as cysteine is present (1
uM).
•The second step is the oxidation of cysteinyldopas to give pheomelanin,
which continues as long as cysteinyldopas are present (10 uM).
•The last step is the production of eumelanin, which begins only after most
cysteinyldopas are depleted.
Dopachrome tautomerase (Dct)
•Dct catalyses the tautomerization of dopachrome to give mostly 5,6-
dihydroxyindole-2-carboxylic acid (DHICA).
•The role of Dct is to increase the ratio of DHICA in eumelanin and to increase
the production of eumelanin.
3. INHA University - Bioengineering
INTRODUCTION
•Melanin pigments are composed of many different units, and these units are
connected through strong carbon– carbon bond
•Eumelanin consists of DHI and DHICA units in a reduced or oxidized state,.
•Pheomelanin consists mostly of benzothiazine units, but those units are
degraded to benzothiazole units to some extent.
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INTRODUCTION
Fig. 1. Pathways for production of eumelanin and pheomelanin in melanocytes.
The pathway of melanogenesis
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DISCUSSION
The Intrinsic Reactivity of o-Quinones
Fig. 2. Intrinsic chemical reactivity of o-quinones.
•Rate constants of thiol addition
: the range 4 x 105 to 3 x 107⁄M⁄s
(in the case of cysteine at pH 7)
•The reduction to parent catechols
through redox exchange proceeds
as fast as the thiol addition.
addition of sulphydryl compounds
•The addition of amines
does not proceed so fast
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DISCUSSION
The Branching Point in Melanogenesis
Fig. 3. Schematic outline of the branching point of
production of eumelanin and pheomelanin
1. cysteinyldopa is
preferred as longas the
cysteine concentration
is higher than 1 uM.
2. Cysteinyldopas thus
accumulate in the early
phase of
pheomelanogenesis
3. pheomelanogenesis is
preferred over
eumelanogenesis
as long as the
cysteinyldopa
concentration is higher
than 10 uM.
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DISCUSSION
The Chemistry of Pheomelanogenesis
The Chemistry of Eumelanogenesis
•Ratio 5-S-cysteinyldopa : 2-S-cysteinyldopa = 5 : 1
•The formation of 5-Scysteinyldopaquinone become predominant.
Fig. 4. Proposed pathway for mixed melanogenesis.
•The ratio of DHI to DHICA is 70:1
•In the presence of Dct, dopachrome undergoes tautomerization, that is,
isomerization with a shift of a hydrogen atom, to produce mostly DHICA.
•The ratio of DHI to DHICA is thus determined by the activity of Dct.
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DISCUSSION
The Roles of Dct
Fig. 5. Role of dopachrome tautomerase
(Dct). Effect of the slaty mutation is shown.
TM stands for total melanin analysed by the
Soluene-350 solubilization
•Slt;Slaty
mouse is known to
decrease the activity of Dct.
DHI : DHICA ≒ 3 : 1
•Dct
: Dct appears to increase
the ratio of DHICA in
eumelanin and to increase
the production of eumelanin.
9. INHA University - Bioengineering
DISCUSSION
The Roles of Tryp1
Fig. 6. Proposed role of Tyrp1. Copolymerization of DHI and
DHICA is suggested.
•Tyrp1 catalyses oxidation of DHICA
•Copolymerization of these four intermediates should yield a copolymer of DHI
and DHICA.
10. INHA University - Bioengineering
DISCUSSION
The Cytotoxicity of o-Quinone Melanin Precursors
•Some phenols and catechols are cytotoxic to melanocytes.
•o-quinones are the ultimate toxic metabolites.
•o-Quinones undergo addition reaction with GSH and SH enzymes.
Fig. 7. Mechanism of melanocytotoxicity of phenols and catechols.
11. INHA University - Bioengineering
DISCUSSION
Development of Anti-melanoma Agents Based on Melanogenesis
•4-S-cysteaminylphenol (4-S-CAP)
•4-S-CAP is a good substrate for tyrosinase and is oxidized to an o-quinone,
dihydro-1,4-benzothiazine-6,7-dione (BQ).
•BQ, the ultimate toxic metabolite, exerts cytotoxicity by bindingto GSH and SH
enzymes
12. INHA University - Bioengineering
CONCLUSION
In conclusion,
(1) o-quinones are highly reactive chemical species. The addition of SH
compounds to o-quinones proceeds very fast.
(2) In the melanosome, the high reactivity of dopaquinone chemically controls the
early process of melanogenesis.
(3) The availability of cysteine determines the proportion of pheomelanin to
eumelanin.
(4) The cytotoxicity of o-quinones is correlated to their binding to proteins through
the cysteine residues.
