PURPLING REACTION OF SINAPIC ACID MODEL SYSTEMS CONTAINING L-DOPA AND MUSHROOM TYROSINASE

To explain the formation of purple lesions in washed mushrooms during postharvest storage, the reactions of phenolic adds with quinones formed in the oxidation of catechol and L-DOPA by mushroom tyrosinase were investigated using a diode array UV-visible spectrophotometer. Among the phenolic acids tested, only gallic acid induced a purple color in the oxidation of catechol by tyrosinase, while gallic, ferulic, and sinapic acids induced a purple color in the oxidation of L-DOPA by tyrosinase. Sinapic acid was highly reactive with a melanogenic quinone derived from L-DOPA in producing a stable purple pigment with lambda(max) at 320 and 530 nm. This melanochrome analogue, ostensibly with the spectral properties of melanochrome (lambda(max) at 300 and 540 nm), was not produced in the gamma-L-glutaminyl-4-hydroxybenzene tyrosinase-sinapic acid system. Isozymes of commercial and partially purified mushroom tyrosinase, separated by electrophoresis, were all active in inducing purpling in the L-DOPA-sinapic acid system. It is suggested that the formation of purple pigment may be due to the nonenzymatic polymerization of a canonical form of indole-5,6-quinone, derived from L-DOPA, with a mesomeric form of sinapic acid.