INHIBITION OF TRYPTOPHAN-HYDROXYLASE BY DOPAMINE AND THE PRECURSOR AMINO-ACIDS

Effects of dopamine and its precursor amino acids on the activity of t ryptophan hydroxylase were examined. They inhibited the enzyme activit y prepared from mastocytoma cells in terms of the biopterin cofactor a nd the substrate L-tryptophan. In relation to the biopterin, tryptopha n hydroxylase was found to have two different kinetics, and dopamine i nhibited the activity in a non-competitive way to both the components. Dopamine had the highest affinity to the enzyme, followed by L-DOPA a nd L-tyrosine, while D-tyrosine did not inhibit the activity. In terms of L-tryptophan, L-tyrosine, L-DOPA and dopamine inhibited the enzyme non-competitively and their affinity to the enzyme was in this order. These results indicate that the indoleamine metabolism may be regulat ed by catecholamines and their related amino acids in the brain.