Search

Contradictory reports on the behaviour of hydroquinone as a tyrosinase substrate are reconciled in terms of the ability of the initially formed ortho-quinone to tautomerise to the thermodynamically more stable para-quinone isomer. Oxidation of phenols by native tyrosinase requires activation by in situ formation of a catechol formed via an enzyme generated ortho-quinone. In the special case of hydroquinone, catechol formation is precluded by rapid tautomerisation of the ortho-quinone precursor to catechol formation., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

A set of 26 substituted phenols, 10 of which were synthesised in our laboratories, were tested for their rate of oxidation by mushroom tyrosinase in vitro as determined by oximetry and spectrophotometry and for their cytotoxic action in a model system. With one exception (4-hydroxybenzoic acid) all the agents tested were oxidised to the corresponding ortho-quinones. The maximum rates of oxidation varied between 15.1 +/- 0.59 nmoles oxygen consumed per minute (4-(2-thioethylthio)phenol) and 372.9 +/- 5.61 nmoles O2/ min. (4-(2-Hydroxyethylthio)phenol) in a reaction system comprising 300 units tyrosinase and 200 microM substrate. The rates of generation of quinone were in close agreement with these oximetric data. Some anomalies in oxygen stoichiometry were observed due to reoxidation of reaction products. Four categories of compounds were tested: those known to undergo side-chain cyclisation (such as tyrosine) (Group A), alkylphenols of increasing chain length with or without terminal hydroxyl groups (Group B), compounds with charged or bulky side-chains (Group C) and agents with oxy-, thio- and selenyl-ether side-chains (Groups D, E and F). In the majority of cases, the cytotoxicity, measured by the reduction of thymidine incorporation in cells exposed for 30 min to the agent in the presence of tyrosinase, reflected the rate of oxidation and is ascribed to the toxic action of the derived ortho-quinone. Tyrosinase-dependent cytotoxicity was absent in cyclising (Group A) and in Group C compounds. Toxicity, expressed by comparison with 4-hydroxyanisole (4HA) (IC50 = 11.7 microM), ranged between 0.36 (4-hydroxybenzyl alcohol) and 1.07 (3-(4-hydroxyphenyl)propanol) for Group B compounds, and be-tween 0.83 (4-ethoxyphenol) and 2.08 (4-(2-hydroxyethylthio)phenol) for groups D, E and F. Addition of glutathione to the toxicity assay system abrogated the cytotoxic action and, on the basis of spectrophotometric data, this is ascribed to the prevention of cellular thiol depletion by the ortho-quinone products of tyrosinase oxidation of the phenolic substrates. The lack of toxicity of the group C compounds may be due to the inability of their derived quinones to gain access to the cells. Addition of catalase or deferoxamine to the incubation medium was without effect on tyrosinase-dependent toxicity.