Catalytic oxidation of 2-aminophenols and ortho hydroxylation of aromatic amines by tyrosinase.
The usual substrates of tyrosinase, a copper-containing monooxygenase (EC 1.14.18.1), are monophenols and o-diphenols which are both converted to o-quinones. In this paper, we studied the reaction of this enzyme with two new classes of substrates: aromatic amines and o-aminophenols, structural analogues of monophenols and o-diphenols, respectively. They undergo the same catalytic reactions (ortho hydroxylation and oxidation), as documented by product analysis and kinetic studies. In the presence of tyrosinase, arylamines and o-aminophenols are converted to o-quinone imines, which are isolated as quinone anils or phenoxazones. As an example, in the presence of tyrosinase, 2-amino-3-hydroxybenzoic acid (an o-aminophenol) is converted to cinnabarinic acid, a well-known phenoxazone, while p-aminotoluene (an aromatic amine) gives rise to the formation of 5-amino-2-methyl-1,4-benzoquinone 1-(4-methylanil). Kinetic studies using an oxygen electrode show that arylamines and the corresponding monophenols exhibit similar Michaelis constants (Km = 0.11-0.49 mM). In contrast, the reaction rates observed for aromatic amines are relatively slow (Kcat = 1-3 min-1) as compared to monophenols (1320-6960 min-1). The enzymatic conversion of arylamines by tyrosinase is different from the typical ones: N-oxidation and ring hydroxylation without further oxidation. This difference originates from the regiospecific hydroxylation (ortho position) and subsequent oxidation of the intermediate o-aminophenol to the corresponding o-quinone imine. Finally, the well-known monooxygenase activity of tyrosinase was also confirmed for the aromatic amine p-aminotoluene, with 18O2.(ABSTRACT TRUNCATED AT 250 WORDS)
Toussaint
O
O
Biochemisches Institut, Universität Zürich-Irchel, Switzerland.
Lerch
K
K
D000588:Amines / Q000378:metabolism*; D004156:Catechol Oxidase / Q000378:metabolism*; D006900:Hydroxylation; D007700:Kinetics; D014442:Monophenol Monooxygenase / Q000378:metabolism*; D009492:Neurospora crassa / Q000201:enzymology; D010084:Oxidation-Reduction; D010636:Phenols / Q000378:metabolism*; D013379:Substrate Specificity
D000588:Amines; D010636:Phenols; D004156:Catechol Oxidase; D014442:Monophenol Monooxygenase