Acta Med. 2000, 43: 15-17

Metabolism of Tryptophan in the Liver: Interference with Decarboxylation of Other Aromatic Amino Acids

Jaroslav Dršataa, Eliška Marklováb

aCharles University in Prague, Faculty of Pharmacy, Department of Biochemical Sciences, Hradec Králové, Czech Republic
bCharles University in Prague, Faculty of Medicine in Hradec Králové, Department of Pediatrics, Hradec Králové, Czech Republic

Received December 1, 1999
Accepted March 1, 2000

Decarboxylation of aromatic amino acid in mammalian tissues is catalyzed by aromatic amino acid decarboxylase (EC., AAD). The enzyme differs in its affinity to individual aromatic amino acids, the best substrates being 3,4-dihydroxyphenylalanine (dopa) and 5-hydroxytryptophan. Surprisingly, AAD is abundant in the liver, where the substrates with rather low affinity to AAD as tryptophan, phenylalanine, and tyrosine are offered to decarboxylation. In the present paper, the possibility of interference of tryptophan with decarboxylation of phenylalanine, tyrosine as well as dopa in the liver was investigated. The AAD activity was measured radiometrically with 1-14C-labeled aromatic amino acid substrates using the rat liver enzyme. The influence of tryptophan on decarboxylation of tyrosine was formally competitive with Ki = 9.2 x 10-3 M, while the inhibition of decarboxylation of phenylalanine by tryptophan was non-competitive with Ki at 2.75 x 10-2 M. The effect of tryptophan on decarboxylation of dopa was small and it could not be expressed in terms of inhibition kinetics and inhibition constant. At physiological concentrations of aromatic amino acids in plasma, tryptophan does not seem to have remarkable effects on decarboxylation of phenylalanine, tyrosine, and dopa in the liver.


This work was supported by the Czech National Research Council of Ministry of Health, Grant No. 4097-3.


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