Dopamine, norepinephrine and epinephrine are well known as physiologically active compounds. Melanin formation from dopamine is also known. In the present review, these two path-ways are referred to as the first and second ways of dopamine, respectively. If the side-chain of dopamine is N-acylated, however, such N-acyldopamines are neither physiologically active nor melanogenic. Instead, N-acyldopamines work as cross-linking agents of insect cuticular proteins and also constitute some insect pigments such as Papiliochromes. This is the third way of dopamine in the present review. When N-acyldopamines are oxidized to their o-quinones by phenoloxidase, amino or imino groups of some amino acids are bonded to either the benzene ring or the side-chain of the o-quinones. The former is the case for the quinone-tanning in insect sclerotization and the latter is the case for β-sclerotization mechanism. In the β-sclerotization, o-quinones of N-acyl-dopamines are known to take p-quinone methide structure, to the β-carbon or α,β-carbons of which amino or imino groups are bonded. This mechanism is also applicable to Papiliochrome II synthesis. The stereochemistry of β-substituted dopamine derivatives is also discussed. The third way of dopamine is important from a comparative biochemical point of view.
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