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R G Smyth

Publications and source records attributed to R G Smyth.

5 recordsLinked to original sources
PubMed1977

Studies on the depletion of brain amines by m-tyrosine.

The biochemical basis of the well-known physiological and pharmacological actions of m-tyrosine was examined by a detailed study of its effect on the brain biogenic amines. m-Tyrosine was injected i.p. and rat brain monoamine levels were measured. Endogenous levels of dopamine, norepinephrine and serotonin all showed approximately 50% reductions 1 h after the administration of L-m-tyrosine at 150 mg/kg. These actions of L-m-tyrosine could be blocked by the inhibition of the central dopa decarboxylase. Depletion of brain monoamines was also observed with the D-isomer of m-tyrosine, although this effect was less pronounced than that of the L-isomer. In vitro experiments with rat brain homogenates showed that L-m-tyrosine, m-tyramine and m-octopamine enhanced in efflux of exogenous labeled monamines from brain particles, whereas D-m-tyrosine was completely ineffective. From these results it is concluded that the observed decreased in brain monamine levels by L-m-tyrosine may be due to a m-tyramine-enhanced release of the amines which are quickly metabolized in vivo.

Animals↗
PubMed1977

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.

Animals↗