2.1 Introduction
35
The catalytic activity of Zn(II) on dopachrome conversion was relatively very
weak. This is a little surprising if one should consider the fact that DCT contained
Zn(II) at the active sites. A reexamination given by Ito et al. reproduced the catalytic
activity of Cu(II) on dopachrome conversion, and also showed that Cu(II) promoted
the oxidative polymerization to form eumelanin with higher ratios of DHICA [7].
Since Cu(II) is relatively abundant in melanosomes [11, 12], the DHI/DHICA ratio
of eumelanin may be significantly affected by not only DCT but also Cu(II). Palumbo
conducted a comparative experiment for the two catalytic factors, DCT and Cu(II),
on dopachrome conversion [13]. Their results show that DCT had higher catalytic
activity than Cu(II).
From the above experimental results, the main factors that control dopachrome
conversion are pH (of melanosomes) and the DCT activity, while Cu(II) may also act
as secondary factor. It was also reported that correlation between the DCT activity
and the DHI/DHICA ratio could be not straightforward [14, 15]. Commo et al.
showed that human follicular melanocytes from elderly individuals (aged older than
45 years old) have scarcely detectable DCT proteins. Nevertheless, melanin from
these samples showed relatively high DHICA content (33–45%), clearly demonstrating the existence of an alternative mechanism to convert dopachrome to DHICA
that does not rely on DCT [15, 16]. In other words, Cu(II) ions in melanosomes are
likely to be a complementary factor, which promote DHICA production even in the
absence of DCT activity.
In the case of DCT-catalyzed reaction, the selective DHICA production may be
explained by a relatively straightforward mechanism. From the reported high stereospecificity, DCT presumably has a site capable of recognizing carboxyl group of
dopachrome [17]. (Note that this carboxyl group is located at a chiral carbon.) Therefore, the formation of DHI would be suppressed by the inhibition of decarboxylation due to the interaction with the carboxyl-recognizing site of DCT. In contrast,
it is not clear how (non-protein bound) metal ions such as Cu(II) alone catalyzes
selective formation of DHICA. Although the metal ion-catalyzed DHICA formation was thought to occur through chelation of metal ions at the quinonoid site of
dopachrome, mechanistic significance of such metal-dopachrome complexes on the
DHICA formation is unclear.
pH is another factor affecting dopachrome conversion, although its mechanistic
roles have not yet been clarified. The reported slower conversion at acidic pH would
be due to the suppression of the rate-limiting deprotonation of dopachrome. Vavricka
et al. showed that the conversion rate was also correlated with the concentration of
buffer solution even at the same pH [18]. Interestingly, some types of buffer solution
promoted DHICA production. From these results, there would be various factors
besides pH affecting proton exchange processes between dopachrome and solvent
molecules. Furthermore, the reported preference of DHICA formation under strongly
acidic and basic conditions are also not clear.
Dopachrome conversion is a reaction that proceeds through proton rearrangements
and forms a transient unstable species. There have been experimental difficulties in
investigating the behavior of protons at the level of elementary reactions. To elucidate the conversion mechanism, Sugumaran et al. prepared esterified dopachrome to
35
The catalytic activity of Zn(II) on dopachrome conversion was relatively very
weak. This is a little surprising if one should consider the fact that DCT contained
Zn(II) at the active sites. A reexamination given by Ito et al. reproduced the catalytic
activity of Cu(II) on dopachrome conversion, and also showed that Cu(II) promoted
the oxidative polymerization to form eumelanin with higher ratios of DHICA [7].
Since Cu(II) is relatively abundant in melanosomes [11, 12], the DHI/DHICA ratio
of eumelanin may be significantly affected by not only DCT but also Cu(II). Palumbo
conducted a comparative experiment for the two catalytic factors, DCT and Cu(II),
on dopachrome conversion [13]. Their results show that DCT had higher catalytic
activity than Cu(II).
From the above experimental results, the main factors that control dopachrome
conversion are pH (of melanosomes) and the DCT activity, while Cu(II) may also act
as secondary factor. It was also reported that correlation between the DCT activity
and the DHI/DHICA ratio could be not straightforward [14, 15]. Commo et al.
showed that human follicular melanocytes from elderly individuals (aged older than
45 years old) have scarcely detectable DCT proteins. Nevertheless, melanin from
these samples showed relatively high DHICA content (33–45%), clearly demonstrating the existence of an alternative mechanism to convert dopachrome to DHICA
that does not rely on DCT [15, 16]. In other words, Cu(II) ions in melanosomes are
likely to be a complementary factor, which promote DHICA production even in the
absence of DCT activity.
In the case of DCT-catalyzed reaction, the selective DHICA production may be
explained by a relatively straightforward mechanism. From the reported high stereospecificity, DCT presumably has a site capable of recognizing carboxyl group of
dopachrome [17]. (Note that this carboxyl group is located at a chiral carbon.) Therefore, the formation of DHI would be suppressed by the inhibition of decarboxylation due to the interaction with the carboxyl-recognizing site of DCT. In contrast,
it is not clear how (non-protein bound) metal ions such as Cu(II) alone catalyzes
selective formation of DHICA. Although the metal ion-catalyzed DHICA formation was thought to occur through chelation of metal ions at the quinonoid site of
dopachrome, mechanistic significance of such metal-dopachrome complexes on the
DHICA formation is unclear.
pH is another factor affecting dopachrome conversion, although its mechanistic
roles have not yet been clarified. The reported slower conversion at acidic pH would
be due to the suppression of the rate-limiting deprotonation of dopachrome. Vavricka
et al. showed that the conversion rate was also correlated with the concentration of
buffer solution even at the same pH [18]. Interestingly, some types of buffer solution
promoted DHICA production. From these results, there would be various factors
besides pH affecting proton exchange processes between dopachrome and solvent
molecules. Furthermore, the reported preference of DHICA formation under strongly
acidic and basic conditions are also not clear.
Dopachrome conversion is a reaction that proceeds through proton rearrangements
and forms a transient unstable species. There have been experimental difficulties in
investigating the behavior of protons at the level of elementary reactions. To elucidate the conversion mechanism, Sugumaran et al. prepared esterified dopachrome to