Tyrosinase kinetics: failure of the auto-activation mechanism of monohydric phenol oxidation by rapid formation of a quinomethane intermediate.
When 3,4-dihydroxybenzylcyanide (DBC) is oxidized by mushroom tyrosinase, the first visible product, identified as the corresponding quinomethane, exhibits an absorption maximum at 480 nm. Pulse-radiolysis experiments, in which the o-quinone is formed by disproportionation of semiquinone radicals generated by single-electron oxidation of DBC, showed that the quinomethane (A480 6440 M-1.cm-1) is formed through the intermediacy of the o-quinone with a rate constant at neutral pH of 7.5 s-1. The oxygen stoichiometry of the formation of the quinomethane by tyrosinase-catalysed oxidation of DBC was 0.5:1. On the basis of oxygen utilization rates the calculated Vmax was 4900 nmol.min-1 and the apparent Km was 374 microM. The corresponding monohydric phenol, 4-hydroxybenzylcyanide (HBC), was not oxidized by tyrosinase unless the enzyme was pre-exposed to DBC, the maximum acceleration of HBC oxidation being obtained by approximately equimolar addition of DBC. These results are consistent with tyrosinase auto-activation on the basis of the indirect formation of the dihydric phenol-activating cofactor. The rapid conversion of the o-quinone to the quinomethane prevents the formation of the catechol by reduction of the o-quinone product of monohydric phenol oxidation from occurring in the case of the compounds studied. In the absence of auto-activation, the kinetic parameters for HBC oxidation by tyrosinase were estimated as Vmax 70 nmol.min-1 and Km 309 microM. The quinomethane was found to decay with a rate constant of 2k 38 M-1.s-1, as determined both by pulse-radiolysis and tyrosinase experiments. The second-order kinetics indicate that a dimer is formed. In the presence of tyrosinase, but not in the pulse-radiolysis experiments, the quinomethane decay was accompanied by a steady-state oxygen uptake concurrently with the generation of a melanoid product measured by its A650, which is ascribed to the formation of an oligomer incorporating the oxidized dimer.
Cooksey
C J
CJ
Department of Chemistry, Christopher Ingold Laboratories, UCL, 20 Gordon Street, London WC1H 0AJ, UK.
Garratt
P J
PJ
Land
E J
EJ
Ramsden
C A
CA
Riley
P A
PA
D000097:Acetonitriles / Q000737:chemistry*; D002396:Catechols / Q000737:chemistry; D004789:Enzyme Activation; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008875:Middle Aged; D014442:Monophenol Monooxygenase / Q000737:chemistry*; D009570:Nitriles / Q000138:chemical synthesis* / Q000378:metabolism*; D010084:Oxidation-Reduction; D011809:Quinones / Q000737:chemistry*; D013053:Spectrophotometry
C114328:3,4-dihydroxybenzylcyanide; D000097:Acetonitriles; D002396:Catechols; D009570:Nitriles; D011809:Quinones; C026914:4-hydroxybenzylcyanide; D014442:Monophenol Monooxygenase; C034221:catechol