The meta Cleavage of Catechol by Azotobacter Species

Catechol was metabolized through 2‐hydroxymuconic semialdehyde by cell‐free extracts of benzoate‐grown Azotobacter Strains. Some properties of catechol 2,3 oxygenase preparations from Azotobacter vinelandii 206 are described. Two different enzymatic activities able to attack 2‐hydroxymuconic semialdehyde have been found in crude extracts from benzoate‐grown cells; one catalyses a hydrolytic release of formate from the semialdehyde and the other a dehydrogenation of this compound to 4‐oxalocrotonate. However, the low, non‐inducible levels of 2‐hydroxymuconic semialdehyde hydrolase activity appear negligible for metabolic purposes and the semialdehyde seems to be dissimilated almost exclusively via 4‐oxalocrotonate, by the action of a NAD + ‐dependent dehydrogenase, in Azotobacter strains. A tautomerase activity responsible for the interconversion of the enol and keto forms of 4‐oxalocrotonic acid was found in extracts from benzoate‐grown cells. 4‐Oxalocrotonate was stoicheiometrically converted to CO 2 and 4‐hydroxy‐2‐oxovalerate by a partially purified extract, with the transient formation of a compound that appears to be 2‐oxopent‐4‐enoic acid. The 4‐oxalocrotonate decarboxylase activity was stimulated by Mg 2+ or Mn 2+ ions and was inhibited by EDTA. Cell‐free extracts from Azotobacter strains converted synthetic 4‐hydroxy‐2‐oxovalerate to acetaldehyde and pyruvate. A reaction sequence, termed the 4‐oxalocrotonate pathway, for the dissimiation of catechol to acetaldehyde and pyruvate by Azotobacter species is presented. All the enzymes operative in this pathway were inducible, except the 4‐hydroxy‐2‐oxovalerate aldolase. The findings described here are discussed in connection with the two previously reported meta cleavage pathways for the oxidation of catechol in Pseudomonas strains.