l-tyrosine, and also l-dopa. Ascorbate activates, and diethyldithiocarbamate is inhibitory [A541].
Berberis stolonifera phenoloxidase, molecular weight 60 000 and optimum pH 6.0, oxidizes both l-tyrosine and tyramine, as well as some phenolic tetrahydroisoquinoline natural products. Both ascorbate and oxygen are required for activity [F855].
Horseradish peroxidase and peroxide act on mixtures of dopa and phenols; the phenols accelerate the oxidation of dopa, although when the phenol used is tyrosine there is no indication of tyrosine hydroxylation. Peroxidase forms dopa from tyrosine in the presence of oxygen and dihydrofumarate, however [A2458].
Enzyme from Mucuna pruriens, in which one of the highest naturally-occurring concentrations of l-dopa is found (rendering its fruit toxic) acts on a range of phenols including l-tyrosine, generating catechols [E769]. This is the basis of a synthetic process for making l-dopa.
Papaver somniferum tyrosinase acts on tyrosine [A171].
Of the 17 phenoloxidase isozymes detected in potato, five act on tyrosine as well as l-dopa [A3190].
Portulaca grandiflora tyrosinase, molecular weight 53 000 and optimum pH 5.7, acts on l-tyrosine, but d-tyrosine is a poor although significant substrate. The dopa formed is further oxidized via dopaquinone to cyclo -dopa, an intermediate in the formation of betanidin [K93].
In wheat the enzyme that hydroxylates tyrosine can be separated electrophoretically from the enzyme that forms dopaquinone from l-dopa [A176], suggesting that it is a different class of enzyme from most tyrosinases, which catalyse these reactions sequentially with a single enzyme.
Agoricus (presumably a misprint for Agaricus) bisporus tyrosinase oxidizes N-acetyltyramine with a lag period (typical for phenols), but the lag for hordenine is indefinitely long. The lag is eliminated by the addition of a trace of a catechol. The normal lag period is terminated by the autocatalytic oxidation of phenol to catechol. N,N-Dipropyldopamine oxidation yields as final product N,N-dipropylindoliumolate [J460]. Mushroom (presumably A. bisporus) tyrosinase, molecular weight 122500, has a Stokes radius stated to be 42.75 x 10-8 cm2 sec-1 (units meant to be cm?) [B489]. It is not affected by superoxide dismutase or by superoxide scavengers [D2].
Aspergillus oryzae enzyme is activated at pH 2 5, and the resultant tyrosinase activity has an optimum at pH 6.0 [A1131].
Vibrio tyrosinaticus is composed of two tyrosinases, molecular weights 41 000 and 38 500; it does not cross-react with antiserum to hamster melanoma tyrosinase. It acts on l-tyrosine and slightly on the d- isomer and m-tyrosine, but catechol and l-phenylalanine are not substrates. It is inactivated by diethyldithiocarbamate, and this is reversed by Cu2 + , Mn2 + , Cd2+ or Fe2 + [A141].
This reaction has been detected in nematode, locust, Thalictrum, avocado, Mucuna and mushroom [A2572, A2804, A3975, B765, E769, F112, F855, H242, H907]. At least in some instances, the reaction is probably catalysed by the same enzyme that forms l-dopa from l-tyrosine.
p -Hydroxyphenylpyruvate oxidase;
(4-hydroxyphenylpyruvate dioxygenase, E.C. 126.96.36.199)
^-Hydroxyphenylpyruvate + O2 0 CO2 + homogentisate
This is a key vertebrate enzyme, and is involved the catabolism of tyrosine. Tyrosine is a major dietary amino acid, and it is also formed from dietary phenylalanine, quantitatively greatly in excess of amounts required for the formation of protein and hormones, and a mechanism is required for its disposal. A deficiency of activity is found in premature infants and in the inherited diseases classified as Tyrosinosis. In man mutations in the gene controlling this enzyme lead to tyrosinaemia type III and hawkinsinuria [K665].
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