Cleavage of pyrogallol by non-heme iron-containing dioxygenases

Both intradiol and proximal extradiol dioxygenases are thought to produce the same product, alpha-hydroxymuconic acid, when pyrogallol (3-hydroxycatechol) is used as a substrate. However, when these enzymes were reacted with pyrogallol, they gave different products. A proximal extradiol dioxygenase, metapyrocatechase (catechol:oxygen 2,3-d-oxidoreductase (decyclizing), EC 1.13.11.2), gave a product having an absorption maximum at 290 nm, which was gradually converted to a more stable compound having an absorption maximum at 239 nm. On the other hand, an intradiol dioxygenase, protocatechuate 3,4-dioxygenase (protocatechuate:oxygen 3,4-oxidoreductase (decyclizing), EC 1.13.11.3), gave a product having an absorption maximum at 300 nm. Based on the spectral data and direct comparison with authentic samples, the primary products obtained by the action of the former and the latter enzymes were identified as alpha-hydroxymuconic acid and 2-pyrone-6-carboxylic acid, respectively. While another intradiol dioxygenase, pyrocatechase (catechol:oxygen 1,2-oxidoreductase (decyclizing), EC 1.13.11.1), gave a mixture of nearly equimolar amounts of these two compounds. Isotope labeling experiments indicated that 1 atom of oxygen was incorporated in 2-pyrone-6-carboxylic acid from the atmosphere. Based on these findings, the reaction mechanism for the formation of 2-pyrone-6-carboxylic acid is discussed. This may be the first experimental evidence indicating the presence of a seven-membered lactone intermediate during the oxygenative cleavage of catechols, proposed by Hamilton (Hamilton, G.A. (1974) in Molecular Mechanisms of Oxygen Activation (Hayaishi, O., ed) pp. 405-451, Academic Press, New York).