<prompt> The rate limiting step for norepinephrine synthesis is:Conversion of phenylalanine to tyrosineConversion of tyrosine to DOPAConversion of DOPA to dopamineConversion of dopamine to norepinephrine  <response>: Ans. B Conversion of tyrosine to DOPARef: Katzung 10e p78; KDT 8the p136Rate 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. 