Peter Garratt - Academia.edu (original) (raw)

Papers by Peter Garratt

Biochemical …, Jan 1, 1998

When 3,4-dihydroxybenzylcyanide (DBC) is oxidized by mushroom tyrosinase, the first visible produ... more 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.

Chem. Commun., Jan 1, 1998

Novel heterocyclic betaines relevant to the mechanism of tyrosinase-catalysed oxidation of phenol... more Novel heterocyclic betaines relevant to the mechanism of tyrosinase-catalysed oxidation of phenols ... John Clews,a Christopher J. Cooksey,b Peter J. Garratt,b Edward J. Land,c Christopher A. Ramsden*a and Patrick A. Rileyd a Department of Chemistry, Keele University, Keele, ...

Journal of Biological …, Jan 1, 1997

Tyrosinase (EC 1.14.18.1) exhibits unusual kinetic properties in the oxidation of monohydric phen... more Tyrosinase (EC 1.14.18.1) exhibits unusual kinetic properties in the oxidation of monohydric phenol substrates consisting of a lag period that increases with increasing substrate concentration. The cause of this is an autocatalytic process dependent on the generation of a dihydric phenol substrate, which acts as an activator of the enzyme. Experiments with N-substituted dihydric phenol substrates (N-methyldopamine, N-acetyldopamine) demonstrate that oxygen consumption is retarded in the N-acetyl substituted material due to a diminished rate of cyclization. The oxygen uptake exhibited a similar pattern when N-acetyltyramine was oxidized, and this was reflected by a prolongation of the lag period. N,N-Dipropyldopamine was oxidized with normal kinetics but with an oxygen stoichiometry of 0.5 mol of oxygen/mol of substrate. We show that this is the result of the formation of a stable indoliumolate product with oxidation-reduction properties that prevent the formation of dopaminochrome, thus blocking further stages in the tyrosinase-catalyzed oxidation.

Journal of the American Chemical Society, 1984

to a reduced catalytic viability of the complex.

Journal of Medicinal Chemistry, 2006

A series of -substituted and , -disubstituted N-acyl 5-methoxy-1-methyltryptamines and 5-methoxyt... more A series of -substituted and , -disubstituted N-acyl 5-methoxy-1-methyltryptamines and 5-methoxytryptamines have been prepared as melatonin analogues to investigate the nature of the binding site of the melatonin receptor. The affinity of analogues was determined in a radioligand binding assay using cloned human MT 1 and MT 2 receptor subtypes expressed in NIH 3T3 cells. Agonist and antagonist potency of all analogues was measured using the pigment aggregation response of a clonal line of Xenopus laeVis melanophores. -Methylmelatonin (17a) and , -dimethylmelatonin (17b), though showing a slight decrease in binding at human receptors, show an increase in potency on Xenopus. N-Butanoyl 5-methoxy-1-methyl-, -trimethylenetryptamine (12c) is an antagonist at human MT 1 receptors but an agonist at MT 2 , while N-butanoyl 5-methoxy-1-methyl-, -tetramethylenetryptamine (13c) is an antagonist at MT 1 but had no action at MT 2 and is one of the first examples of an MT 1 selective antagonist.

Biochemical …, Jan 1, 1998

When 3,4-dihydroxybenzylcyanide (DBC) is oxidized by mushroom tyrosinase, the first visible produ... more 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.

Chem. Commun., Jan 1, 1998

Novel heterocyclic betaines relevant to the mechanism of tyrosinase-catalysed oxidation of phenol... more Novel heterocyclic betaines relevant to the mechanism of tyrosinase-catalysed oxidation of phenols ... John Clews,a Christopher J. Cooksey,b Peter J. Garratt,b Edward J. Land,c Christopher A. Ramsden*a and Patrick A. Rileyd a Department of Chemistry, Keele University, Keele, ...

Journal of Biological …, Jan 1, 1997

Tyrosinase (EC 1.14.18.1) exhibits unusual kinetic properties in the oxidation of monohydric phen... more Tyrosinase (EC 1.14.18.1) exhibits unusual kinetic properties in the oxidation of monohydric phenol substrates consisting of a lag period that increases with increasing substrate concentration. The cause of this is an autocatalytic process dependent on the generation of a dihydric phenol substrate, which acts as an activator of the enzyme. Experiments with N-substituted dihydric phenol substrates (N-methyldopamine, N-acetyldopamine) demonstrate that oxygen consumption is retarded in the N-acetyl substituted material due to a diminished rate of cyclization. The oxygen uptake exhibited a similar pattern when N-acetyltyramine was oxidized, and this was reflected by a prolongation of the lag period. N,N-Dipropyldopamine was oxidized with normal kinetics but with an oxygen stoichiometry of 0.5 mol of oxygen/mol of substrate. We show that this is the result of the formation of a stable indoliumolate product with oxidation-reduction properties that prevent the formation of dopaminochrome, thus blocking further stages in the tyrosinase-catalyzed oxidation.

Journal of the American Chemical Society, 1984

to a reduced catalytic viability of the complex.

Journal of Medicinal Chemistry, 2006

A series of -substituted and , -disubstituted N-acyl 5-methoxy-1-methyltryptamines and 5-methoxyt... more A series of -substituted and , -disubstituted N-acyl 5-methoxy-1-methyltryptamines and 5-methoxytryptamines have been prepared as melatonin analogues to investigate the nature of the binding site of the melatonin receptor. The affinity of analogues was determined in a radioligand binding assay using cloned human MT 1 and MT 2 receptor subtypes expressed in NIH 3T3 cells. Agonist and antagonist potency of all analogues was measured using the pigment aggregation response of a clonal line of Xenopus laeVis melanophores. -Methylmelatonin (17a) and , -dimethylmelatonin (17b), though showing a slight decrease in binding at human receptors, show an increase in potency on Xenopus. N-Butanoyl 5-methoxy-1-methyl-, -trimethylenetryptamine (12c) is an antagonist at human MT 1 receptors but an agonist at MT 2 , while N-butanoyl 5-methoxy-1-methyl-, -tetramethylenetryptamine (13c) is an antagonist at MT 1 but had no action at MT 2 and is one of the first examples of an MT 1 selective antagonist.