Dopamine Biosynthesis From L‐tyrosine and L‐phenylalanine in Rat Brain Synaptosomes: Preferential Use of Newly Accumulated Precursors¹,²

The biosynthesis of dopamine (DA) from L‐tyrosine (Tyr) and L‐phenylalanine (Phe) was investigated using synaptosomes prepared from the striatum and olfactory tubercle of the rat. The formation of 14 CO 2 from either carboxyl labeled precursor occurred exclusively within the synaptosome following hydroxylation and subsequent decarboxylation. The optimum pH for the formation of DA was 6.2 and was independent of precursor and tissue source. As pH increased beyond this optimum, synthesis from Tyr declined more rapidly than that from Phe. Synthesis obeyed Michaelis–Menton kinetics when expressed as a function of the specific activity of precursor in the medium. It was characterized by an overall K m (approx 0.9 μM) which was independent of precursor and tissue source, and was considerably lower than the K t for accumulation of precursor by synaptosomes (15.3 and 13.3 μM for Tyr and Phe, respectively). While each precursor was an uncompetitive inhibitor of DA synthesis from the opposing labeled amino acid, Tyr was a more effective inhibitor of synthesis from Phe ( K i = 1.5 μM) than was Phe an inhibitor of synthesis from Tyr ( K i = 9.2 μM). Tryptophan inhibited synthesis competitively ( K i = 15.2 and 13.2 μM for synthesis from Tyr and Phe, respectively), and DA inhibited non‐competitively ( K i = 1.1 and 0.42 μM for Tyr and Phe, respectively). A model of DA synthesis within the synaptosome is presented which attempts to integrate these data. A major feature of this schema is the proposal that newly accumulated precursor does not mix rapidly with endogenous precursor pools but rather is preferentially converted to DA.