p-hydroxyphenylacetic acid and-3,4-dihydroxyphenylacetic acid as substrates for mushroom tyrosinase

The hydroxylation of p-hydroxyphenylacetic acid (PHPAC) and the oxidation o f 3,4-dihydroxyphenylacetic acid (DOPAC) by mushroom tyrosinase are illustr ated DOPAC quinone (4-carboxy-methyl-o-benzoquinone (lambda max = 400+/-10 nm) is the initial pigmented product formed when DOPAC is oxidized by the e nzyme. DOPAC quinone is very unstable, and, once formed, is converted rapid ly to further oxidation product(s). The relationships between the rate of p-dihydroxyphenylacetic acid hydroxyl ation and 3,4-dihydroxyphenylacetic acid oxidation as a function of various concentrations of each substrate and of mushroom tyrosinase, are described . The effect of the addition of various chemicals that can potentially conjug ate otherwise affect DOPAC quinone was studied The Km value of 3,4-dihydrox yphenylacetic acid for mushroom tyrosinase was estimated to be 4.0 mM.