The rate limiting step for norepinephrine synthesis is:: (A) Conversion of phenylalanine to tyrosine, (B) Conversion of tyrosine to DOPA, (C) Conversion of DOPA to dopamine, (D) Conversion of dopamine to norepinephrine
Answer is B. Ans. (B) Conversion of tyrosine to DOPA(Ref: Katzung 10/e p78; KDT 8th/e p136)Rate limiting enzyme in norepinephrine synthesis is tyrosine hydroxylase that catalyses the conversion of tyrosine to dopa. In ACh synthesis, rate limiting step is uptake of choline by the neurons.