Biosynthesis of violacein: intact incorporation of the tryptophan molecule on the oxindole side, with intramolecular rearrangement of the indole ring on the 5-hydroxyindole side.

Feeding experiments with a mixture of [2-13C]- and [indole-3-13C]tryptophans, of [3-13C]- and [indole-3-13 C]tryptophans (1:1 molar ratio) and of others have proved that the 1,2-shift of the indole ring occurred via an intramolecular process for formation of the left part (5-hydroxyindole side) of the violacein skeleton and demonstrated that the C-C bond from C2 of the indole ring to C2 of the side chain was completely retained for formation of the right part (oxindole side) during the entire biosynthetic process. Due to the involvement of transaminase, it has remained unresolved whether indolylpyruvic acid is the biosynthetic intermediate and/or from where the nitrogen atom of the pyrrolidone ring originates. An incorporation experiment with a mixture of [2-13C]- and [alpha-15N]tryptophans (1:1 molar ratio) verified that the nitrogen atom in the central ring was exclusively derived from the right-side tryptophan. Thus, all the carbon and nitrogen atoms in the right part of the violacein skeleton were constructed by intact incorporation of the tryptophan molecule, with decarboxylation probably occurring at a later biosynthetic stage.