1.3 Biosynthesis of Eumelanin—Formation of Dopaquinone and Dopachrome
11
Körner and Pawelek analyzed dopachrome consumption rate in the presence/absence of DCF by UV-visible spectroscopy, in which the product DHI was
also detected with HPLC. The amount of decarboxylation (reaction to remove the
carboxy group as carbon dioxide) was also quantified by
14 C labeling. As a result, it
was revealed that DCF drastically accelerates dopachrome consumption and slightly
accelerates decarboxylation, respectively. Production of DHI was detected regardless
of the presence or absence of DCF (but faster in the presence of DCF). Considering
that the acceleration of dopachrome consumption is more significant than that of
decarboxylation, at that time DCF was thought to promote conversion of dopachrome
to DHICA, and then DHICA spontaneously decarboxylates to form DHI.
In 1985, Körner and Pawelek identified the product of the DCF-catalyzed reaction as DHICA by spectrophotometric and
13 C NMR analysis [40]. At this time, it
was recognized that DHICA was a transient intermediate and not a monomer. Thus,
DHICA was thought to further convert to DHI via decarboxylation before it polymerizes to eumelanin. However, detailed analyses of eumelanins given by Ito et al.
[41] revealed that DHICA-derived units were present in a content of nearly 50%
in natural eumelanin [41]. It can be noted that the conducted analyses include the
oxidative chemical degradation method, which is now a widely accepted method, as
introduced in Sect. 1.2.
Integrating these results, the building monomers of eumelanin are regarded as DHI
and DHICA. Briefly, DHI is produced in the absence of DCF catalytic action, while
DHICA is formed if DCF acts on dopachrome. In this regard, it can be noted that
there are also other (non-enzymatic) factors that catalyze dopachrome conversion
into DHICA (see Chap. 3.).
Although some of previous studies have indicated that DHICA easily decarboxylates, this might be due to the use of mushroom tyrosinase; extracts of mushrooms
may have effects of promoting decarboxylation from DHICA [42].
The product formed by the action of DCF is now identified as DHICA, and
the conversion of dopachrome to DHICA is a protolytic tautomerization. Based
on the understood action of this enzyme, a more specific new name “dopachrome
tautomerase: DCT” was proposed, instead of DCF [42]. The name DCT is currently
used for mammals, although DCF is exclusively used for insects. This is because, in
the cases of insects melanogenesis, there is an enzyme that promotes dopachrome
conversion to DHI but not to DHICA unlike mammals [43].
DCT has an amino acid sequence, which is very similar to tyrosinase, and belongs
to a family called tyrosinase-related proteins (TRPs). (Therefore, DCT is also called
TRP2.) I.J. Jackson et al. analyzed the amino acid sequence of mouse melanoma
DCT. As a result, the sequence of DCT was found to be similar with tyrosinase and
an enzyme called TRP1 expressed in melanocytes [44]. Analogous with tyrosinase,
where copper ions at the active site are bound with three histidine residues, DCT was
also expected to contain metal ions such as copper because histidine residues were
located in similar places.
Although the possibility of copper or iron ions as DCT-containing metal ions
has been predicted, experiments by F. Solano et al. revealed that the metal ions
11
Körner and Pawelek analyzed dopachrome consumption rate in the presence/absence of DCF by UV-visible spectroscopy, in which the product DHI was
also detected with HPLC. The amount of decarboxylation (reaction to remove the
carboxy group as carbon dioxide) was also quantified by
14 C labeling. As a result, it
was revealed that DCF drastically accelerates dopachrome consumption and slightly
accelerates decarboxylation, respectively. Production of DHI was detected regardless
of the presence or absence of DCF (but faster in the presence of DCF). Considering
that the acceleration of dopachrome consumption is more significant than that of
decarboxylation, at that time DCF was thought to promote conversion of dopachrome
to DHICA, and then DHICA spontaneously decarboxylates to form DHI.
In 1985, Körner and Pawelek identified the product of the DCF-catalyzed reaction as DHICA by spectrophotometric and
13 C NMR analysis [40]. At this time, it
was recognized that DHICA was a transient intermediate and not a monomer. Thus,
DHICA was thought to further convert to DHI via decarboxylation before it polymerizes to eumelanin. However, detailed analyses of eumelanins given by Ito et al.
[41] revealed that DHICA-derived units were present in a content of nearly 50%
in natural eumelanin [41]. It can be noted that the conducted analyses include the
oxidative chemical degradation method, which is now a widely accepted method, as
introduced in Sect. 1.2.
Integrating these results, the building monomers of eumelanin are regarded as DHI
and DHICA. Briefly, DHI is produced in the absence of DCF catalytic action, while
DHICA is formed if DCF acts on dopachrome. In this regard, it can be noted that
there are also other (non-enzymatic) factors that catalyze dopachrome conversion
into DHICA (see Chap. 3.).
Although some of previous studies have indicated that DHICA easily decarboxylates, this might be due to the use of mushroom tyrosinase; extracts of mushrooms
may have effects of promoting decarboxylation from DHICA [42].
The product formed by the action of DCF is now identified as DHICA, and
the conversion of dopachrome to DHICA is a protolytic tautomerization. Based
on the understood action of this enzyme, a more specific new name “dopachrome
tautomerase: DCT” was proposed, instead of DCF [42]. The name DCT is currently
used for mammals, although DCF is exclusively used for insects. This is because, in
the cases of insects melanogenesis, there is an enzyme that promotes dopachrome
conversion to DHI but not to DHICA unlike mammals [43].
DCT has an amino acid sequence, which is very similar to tyrosinase, and belongs
to a family called tyrosinase-related proteins (TRPs). (Therefore, DCT is also called
TRP2.) I.J. Jackson et al. analyzed the amino acid sequence of mouse melanoma
DCT. As a result, the sequence of DCT was found to be similar with tyrosinase and
an enzyme called TRP1 expressed in melanocytes [44]. Analogous with tyrosinase,
where copper ions at the active site are bound with three histidine residues, DCT was
also expected to contain metal ions such as copper because histidine residues were
located in similar places.
Although the possibility of copper or iron ions as DCT-containing metal ions
has been predicted, experiments by F. Solano et al. revealed that the metal ions