Calculating molar absorptivities for quinones: application to the measurement of tyrosinase activity

The molar absorptivities of the quinones produced from different o-diphenols, triphenols, and flavonoids were calculated by generating the respective quinones through oxidation with an excess of periodate. Oxidation of these substrates by this reagent was analogous to oxidation by tyrosinase with molecular oxygen, although the procedure showed several advantages over the enzymatic method in that oxidation took place almost immediately and quinone stability was favored because no substrate remained. The o-diphenols studied were pyrocatechol, 4-methylcatechol, 4-tert-butylcatechol, 3,4-dihydroxyphenylalanine, 3,4-dihydroxyphenylethylamine, 3,4-dihydroxyphenylacetic acid, 3,4-dihydroxyphenylpropionic acid, and caffeic acid; the triphenols studied were pyrogallol, 1,2,4-benzenetriol, 6-hydroxydopa, and 6-hydroxydopamine; and the flavonoids studied were (+)catechin, (-)epicatechin, and quercetin. In addition, the stability of the quinones generated by oxidation of the compounds by periodate]0/substrate]0 < 1 was studied. Taking the findings into account, tyrosinase could be measured by following o-quinone formation in rapid kinetic studies using the stopped-flow method. However, measuring o-quinone formation could not be useful for steady-state studies. Therefore, several methods for following tyrosinase activity are proposed, and a kinetic characterization of the enzyme's action on these substrates is made.