Drsata J, Marklová E
Department of Biochemical Sciences, Faculty of Pharmacy, Charles University, Prague.
Acta Medica (Hradec Kralove). 2000;43(1):15-7.
Decarboxylation of aromatic amino acid in mammalian tissues is catalyzed by aromatic amino acid decarboxylase (EC. 4.1.1.28, AAD). The enzyme differs in its affinity to individual aromatic amino acids, the best substrates being 3,4-dihydroxyphenylalanine (dopa) and 5-hydroxytroptophan. 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.
哺乳动物组织中芳香族氨基酸的脱羧作用由芳香族氨基酸脱羧酶（EC. 4.1.1.28，AAD）催化。该酶对不同芳香族氨基酸的亲和力不同，最佳底物是3,4 - 二羟基苯丙氨酸（多巴）和5 - 羟基色氨酸。令人惊讶的是，AAD在肝脏中含量丰富，而肝脏中提供给脱羧反应的色氨酸、苯丙氨酸和酪氨酸等对AAD亲和力相当低的底物。在本文中，研究了色氨酸干扰肝脏中苯丙氨酸、酪氨酸以及多巴脱羧反应的可能性。使用大鼠肝脏酶，以1 - 14C标记的芳香族氨基酸底物通过放射性测量法测定AAD活性。色氨酸对酪氨酸脱羧反应的影响呈形式上的竞争性，Ki = 9.2×10（-3）M，而色氨酸对苯丙氨酸脱羧反应的抑制作用是非竞争性的，Ki为2.75×10（-2）M。色氨酸对多巴脱羧反应的影响较小，无法用抑制动力学和抑制常数来表示。在血浆中芳香族氨基酸的生理浓度下，色氨酸似乎对肝脏中苯丙氨酸、酪氨酸和多巴的脱羧反应没有显著影响。
