Reaction mixture containing 20 μl of L-DOPA (L-dihydroxyphenyl alanine) (10 mM), 20 μl of tyrosinase enzyme (specific activity: 200 nmol/mg protein) and 160 μl of the 50 mM phosphate buffer, pH 6.8, (with or without the sample) was added to a 96-well microplate.