Although the mechanism of pigment formation from phenolic substrates is not understood completely, the general course of pigmentation is known to involve enzymatic oxidation, nonenzymatic oxidation, nonoxidative transformations, and polymerizations. The o-quinones formed from phenolic compounds by PPOs are the precursors of the brown color. The o-quinones themselves possess little color, but they are among the most reactive intermediates occurring in plants. They take part in a secondary reaction, bringing about the formation of more intensely colored secondary products. The most important secondary reactions are coupled oxidation of substrates oxidized, complexing with amino compounds and proteins, and condensation and polymerization. The principal reaction of o-quinones in browning reaction is the one leading to the formation of the unstable hydroxyquinones. The hydroxyquinones polymerize readily and are easily further oxidized non-enzymatically to a dark brown pigment.
As a typical oxidation reaction, tyrosine is oxidized to dihydroxyphenylalanine (dopa) quinone by PPO and then proceeds to melanin (Scheme 1). Tyrosine is first converted into dopa, which is then oxidized to the corresponding dopa quinone. The dopa quinone, on intramolecular rearrangement, is converted into 5,6-dihydroxy indole-2-carboxylic acid, which on further oxidation is converted into red 5,6-quinone indole carboxylic acid; then finally the black melanins are formed. Quinones also react readily with simple amines, such as o-Benzoquinone + glycine ->■ 4-JV-glycyl-o-benzoquinone
This reaction product is the intermediate responsible for the deamination of glycine with the concomitant formation of deeply colored pigments. o-Quinones also react with proteins, sulfhydryl compounds, such as cysteine, and produce dark-colored, insoluble products.
Dimerization or polymerization of o-quinones that lead to the colored products is common. The formation of dimers or oligomers of o-quinone by condensation of a hydroxy quinone with a quinone was recently demonstrated. The en-zymatically generated caffeoyl tartaric acid o-quinones were shown to oxidize other phenols, such as 2-S-glutathionyl caffeoyl tartaric acid and flavans, by coupled oxidation mechanisms, with reduction of the cafeoyl tartaric acid quinones back to caffeoyl tartaric acid (63). The o-quinones formed by enzymatic or coupled oxidation can also react with a hydroquinones to yield a condensation product (64-66). It is possible to regenerate the original phenolic from an intermediate by reduction provided that oxidation and subsequent transformation had not gone too far. In later stages the browning is no longer reversible.
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