In vivo studies on the conversion of m-tyrosine to 3,4-dihydroxyphenylalanine in the rat.

The question whether m-tyrosine can give rise to catechols in vivo has been investigated using labelled precursor. DL-[2-14C]m-tyrosine (38 muCi/mmol (1 Ci = 37 GBq)) was synthesized from [2-14C]glycine. Radioactive catechols in rat brain, liver, and kidneys were examined 15 min after intraperitoneal administration of DL-[2-14C]m-tyrosine (100 mg/kg). The kidney was the only organ which showed demonstrable amounts of radioactive catechols, and about 14% of the catechols formed was identified as 3,4-dihydroxyphenylalanine (dopa), 22% as 3,4-dihydroxyphenylacetic acid, and 56% as dopamine. However, when the animals were pretreated with dopa decarboxylase inhibitor, labelled catechols were also observed in liver and brain, and dopa accounted for over 95% of the catechols formed in all three organs examined. Thus it is clear that m-tyrosine can by hydroxylated in vivo. Results from experiments using [2-14C]m-tyrosine enantiomers and specific enzyme inhibitors suggest that phenylalanine hydroxylase could be the enzyme catalyzing this reaction.