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Mechanisms of aluminium neurotoxicity in oxidative stress-induced ...

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INTRODUCTION<br />

Neurochemical and neuropathological features <strong>of</strong> PD<br />

Dopam<strong>in</strong>e<br />

12<br />

Dopam<strong>in</strong>e (DA) plays important roles <strong>in</strong> control movement, motivation and<br />

reward, behaviour and cognition, mood, sleep, attention and learn<strong>in</strong>g. This biogenic<br />

monoam<strong>in</strong>e is synthesized <strong>in</strong> the catecholam<strong>in</strong>ergic neurons from L-tyros<strong>in</strong>e. First,<br />

tyros<strong>in</strong>e hydroxylase (tyros<strong>in</strong>e 3-monooxygenase, TH) converts L-tyros<strong>in</strong>e to 3,4-<br />

dihydroxy-L-phenylalan<strong>in</strong>e (L-DOPA). TH requires 5,6,7,8-tetrahydrobiopter<strong>in</strong> (BioH4)<br />

as a coenzyme (Agid 1991). The second step is catalyzed by the DOPA decarboxylase<br />

(aromatic L-am<strong>in</strong>o acid decarboxylase, AADC), L-DOPA is decarboxylated to DA<br />

(Figure 3). In some neurones, beta-hydroxylase processes DA <strong>in</strong>to noradrenal<strong>in</strong>e, which<br />

is metabolized to adrenal<strong>in</strong>e by phenylethanolam<strong>in</strong>e N-methyltransferase. DA is<br />

degraded by catechol-O-methyl transferase (COMT), aldehyde dehydrogenase (ADH)<br />

and monoam<strong>in</strong>e oxidase (MAO) (Figure 4). MAO-A and -B also degrade other<br />

monoam<strong>in</strong>es <strong>in</strong>clud<strong>in</strong>g seroton<strong>in</strong>, noradrenal<strong>in</strong>e, and adrenal<strong>in</strong>e.<br />

Figure 3: Dopam<strong>in</strong>e synthesis (Méndez-Álvarez and Soto-Otero 2004)<br />

L-tyros<strong>in</strong>e<br />

tyros<strong>in</strong>e<br />

hydroxylase<br />

HO<br />

COO<br />

HO HO<br />

H 2N<br />

HN<br />

N<br />

O<br />

H<br />

N<br />

N<br />

H<br />

NH 3<br />

O 2<br />

CH CH CH3 OH OH<br />

5,6,7,8-tetrahydrobiopter<strong>in</strong><br />

(BioH 4)<br />

NAD<br />

dihydrobiopter<strong>in</strong><br />

reductase<br />

NADH + H<br />

HN<br />

H 2O<br />

NH 3<br />

COO<br />

3,4-dihydroxy-L-phenylalan<strong>in</strong>e<br />

(L-DOPA)<br />

HN<br />

N<br />

O<br />

H<br />

N<br />

N<br />

CH CH CH3 OH OH<br />

qu<strong>in</strong>onoid dihydrobiopter<strong>in</strong><br />

(BioH 2)<br />

CO 2<br />

aromatic L-am<strong>in</strong>o acid<br />

decarboxylase<br />

(pyridoxal phosphate)<br />

HO NH 3<br />

HO<br />

dopam<strong>in</strong>e<br />

(DA)

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