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Figure 3<br />

The main metabolic routes of dopamine and noradrenaline in<br />

the brain. COMT, Catechol O-methyltransferase; MAO, monoamino<br />

oxidase; DDC, dopa decarboxylase; DBH, dopamine<br />

β-hydroxylase; 3-OMD, 3-methoxytyrosine; Dopac,<br />

dihydroxyphenyl acetic acid.<br />

Page 346<br />

enzyme [13]. The existence of a thermolabile low-activity and a thermostable high-activity COMT in<br />

human population has been reported [18]. Interestingly, the two published sequences of human soluble<br />

COMT differ in only one amino acid. Recent kinetic studies have shown that this difference affects<br />

unambiguously the thermostability of the enzyme [19].<br />

C. Kinetics of Human COMT<br />

The kinetic mechanism of the methylation reaction of human COMT has been studied exhaustively<br />

using recombinant enzymes [19]. The mechanism is sequential ordered: AdoMet binding first, then<br />

Mg 2+ and the catechol substrate as the last ligand. Human S-COMT and MB-COMT have similar kinetic<br />

properties. The main difference is the one-order lower K m value of MB-COMT for dopamine as<br />

substrate (S-COMT 207 μM and MB-COMT 15 μM). The COMT enzyme is a rather slow enzyme with<br />

a low catalytic number. At saturating substrate levels S-COMT has a double efficiency compared with<br />

MB-COMT (k cat=37 and k cat =17, respectively). At low substrate concentrations (

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