The rate-limiting enzyme in the synthesis of dopamine is which of the following?: (A) Tyrosine hydroxylase, (B) Dopa decarboxylase, (C) Homovanillic acid, (D) Dopamine beta-hydroxylase
Answer is A. The first step in the chemical synthesis of dopamine, epinephrine, and norepinephrine is the conversion of L-tyrosine to L-hydroxyphenylalanine, which is catalyzed by tyrosine hydroxylase. This is the rate-limiting step.