... by scintillation counting.The incubations contained no ligand (d), 40 lMH4biopterin (m),40 lMdopamine (n)or1 mML-Phe in combination with either 40 lMH4biopterin or 40 lMdopamine ... [32].Secondly, on binding at the active site, the dopamine main-chain interacts with the autoregulatory sequence at Ser23(by hydrogen bonding) and Ile25 (Fig. 4), which results in an inhibition ... controlled by severalmechanisms in order to maintain the phenylalanine and tyrosine homeostasis in vivo despite great fluctuations in thedietary intake of L-Phe and the overall rate of proteincatabolism....