13.08.24 김준섭

▶Objective : Melanogenesis 에 대한 과정을 화학반응과 반응속도에 관한 논점에서 바라본 논문을 알아보기

INHA University - Bioengineer-ingABSTRACT

Melanogenesis proceeds in three distinctive steps•The initial step is the production of cysteinyldopas by the rapid ad-dition of cysteine to dopaquinone, which continues as long as cys-teine is present (1 uM).

•The second step is the oxidation of cysteinyldopas to give pheome-lanin, 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.

INHA University - Bioengineer-ingINTRODUCTION

•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.

INHA University - Bioengineer-ingINTRODUCTION

Fig. 1. Pathways for production of eumelanin and pheomelanin in melanocytes.

The pathway of melano-genesis

INHA University - Bioengineer-ingDISCUSSION

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 cate-chols through redox exchange proceeds as fast as the thiol addition.

addition of sulphydryl com-pounds

•The addition of amines does not proceed so fast

INHA University - Bioengineer-ingDISCUSSION

The Branching Point in Melano-genesis

Fig. 3. Schematic outline of the branching point of production of eumelanin and pheomelanin

1. cysteinyldopa is pre-ferred as longas the cysteine concentrationis higher than 1 uM.

2. Cysteinyldopas thus accumulate in the early phase of pheomelano-genesis

3. pheomelanogenesis is preferred over eume-lanogenesisas long as the cys-teinyldopa concentra-tion is higherthan 10 uM.

INHA University - Bioengineer-ingDISCUSSION

The Chemistry of Pheomelano-genesis

The Chemistry of Eumelanogen-esis

•Ratio 5-S-cysteinyldopa : 2-S-cysteinyldopa = 5 : 1•The formation of 5-Scysteinyldopaquinone become predominant.

Fig. 4. Proposed pathway for mixed melanogen-esis.

•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.

INHA University - Bioengineer-ingDISCUSSION

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 theSoluene-350 solubilization

•Slt;Slaty mouse is known to de-crease the activity of Dct.

DHI : DHICA ≒ 3 : 1•Dct : Dct appears to in-crease the ratio of DHICA in eumelanin and to in-crease the production of eumelanin.

INHA University - Bioengineer-ingDISCUSSION

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.

INHA University - Bioengineer-ingDISCUSSION

The Cytotoxicity of o-Quinone Melanin Pre-cursors•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 cate-chols.

INHA University - Bioengineer-ingDISCUSSION

Development of Anti-melanoma Agents Based on Melano-genesis•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

INHA University - Bioengineer-ingCONCLUSION

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 pheome-lanin to eumelanin.

(4) The cytotoxicity of o-quinones is correlated to their binding to pro-teins through the cysteine residues.