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

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Abstract
The question whether m-tyrosine can give rise to catechols in vivo was investigated using labeled precursor. DL-[2-14C]m-tyrosine (38 .mu.Ci/mmol) was synthesized from [2-14C]glycine. Radioactive catechols in rat brain, liver and kidneys were examined 15 min after i.p. 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 dopa, 22% as 3,4-dihydroxyphenylacetic acid and 56% as dopamine. When the animals were pretreated with dopa decarboxylase inhibitor, labeled catechols were also observed in liver and brain, and dopa accounted for over 95% of the catechols formed in all 3 organs examined. Thus it is clear that m-tyrosine can be 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.