Ans: Step 1, tyrosine hydroxylase, tetrahydrobiopterin; Step 2, L-aromatic amino acid decarboxylase, pyridoxal phosphate; Step 3, Dopamine b-hydroxylase, ascorbic acid.

b. Explain why tolcapone and entacapone will potentiate the central effect of a-methyl L-tyrosine or a-methyldopa, but not a-methyldopamine?

Ans: Since only a-methyl L-tyrosine and a-methyldopa can be transported into the brain by the L-aromatic acid carriers, tolcapone and entacapone will potentiate their central effect, i.e., prevent their inactivation by the enzyme, COMT. On the other hand, a-methyldopamine is too water soluble to gain entry into the brain (i.e., there is No carrier for dopamine or norepinephrine)