Specificity of dopachrome tautomerase and inhibition by carboxylated indoles. Considerations on the enzyme active site.
Dopachrome tautomerase (EC 5.3.2.3) catalyses the tautomerization of dopachrome to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) within the melanin-formation pathway. We have analysed a series of substrate analogues and related compounds as possible substrates and inhibitors of tautomerization. The enzyme appears to be highly specific since D-dopachrome, alpha-methyldopachrome, dopaminochrome, adrenochrome methyl ether and deoxyadrenochrome are not substrates. Conversely, dopachrome tautomerase catalyses the tautomerization of dopachrome methyl ester, suggesting that a carboxy group, either free or as a methyl ester, is essential for enzyme recognition. No inhibition of dopachrome tautomerization was observed in the presence of either semiquinonic compounds, such as tropolone and L-mimosine, or pyrrole-2-carboxylic acid and unsubstituted indole. However, a number of indole derivatives, including DHICA, the product of dopachrome tautomerization, and the analogues 5-hydroxyindole-2-carboxylic and indole-2-carboxylic acid were able to inhibit the enzyme. Furthermore, indoles with a side chain at position 3 of the ring and containing a carboxylic group at the gamma-position of this chain, such as L-tryptophan or indole-3-propionic acid, are stronger inhibitors of the enzyme. Indole-3-carboxylic acid, indole-3-acetic acid and indole-3-butyric acid are very weak inhibitors, showing that the carboxylic group needs to be located at an optimal distance from the indole ring to mimic the carboxylic group at position 2 on the authentic substrate.
Aroca
P
P
Departmento Bioquimíca y Biología Molecular, Facultad de Medicina, Universidad de Murcia, Spain.
Solano
F
F
Garcia-Borrón
J C
JC
Lozano
J A
JA
D000645:Ammonium Sulfate; D000818:Animals; D002264:Carboxylic Acids / Q000494:pharmacology*; D002460:Cell Line; D005260:Female; D007211:Indoles / Q000494:pharmacology*; D019746:Intramolecular Oxidoreductases; D007535:Isomerases / Q000037:antagonists & inhibitors / Q000302:isolation & purification / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D008546:Melanoma, Experimental / Q000201:enzymology*; D051379:Mice; D008810:Mice, Inbred C57BL; D008811:Mice, Inbred DBA; D013329:Structure-Activity Relationship; D013379:Substrate Specificity
D002264:Carboxylic Acids; D007211:Indoles; D007535:Isomerases; D019746:Intramolecular Oxidoreductases; C042501:dopachrome isomerase; D000645:Ammonium Sulfate