In normal animals and an, the rate-limiting step in the biosynthesis of the catecholamine neurotransmitters dopamine and norepinephrine is

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The rate-limiting step in the biosynthesis of dopamine and norepinephrine is the hydroxylation of L-tyrosine by the enzyme tyrosine hydroxylase. This step is the first and most important step that determines the rate of catecholamine synthesis. The other choices listed are also involved in the biosynthesis of dopamine and norepinephrine, but they are not the rate-limiting step.

In Normal Animals and An, the Rate-Limiting Step in the Biosynthesis