| Abstract: | Cleavage of the ether bond of chlorophenoxyalkanoate herbicides is catalyzed by an α‐ketoglutarate‐linked dioxygenase (TfdA). In this step, α‐ketoglutarate is decarboxylated to succinate and must be regenerated for continual substrate cleavage. Limitations in herbicide degradation are to be expected in the case of a shortage of α‐ketoglutarate. Such a situation was simulated and studied with Delftia (formerly Comamonas) acidovorans MC1 and Rhodoferax sp. P230, which constitutively express etherolytic dioxygenase activity by excreting 2,4‐dichlorophenol (DCP) as a dead‐end product. The results showed that 2,4‐dichlorophenoxyacetate (2,4‐D) could hardly be cleaved under these conditions which is attributed to the inability to regenerate α‐ketoglutarate from the cleavage products, i.e. succinate and glyoxylate 1 ]. With pyruvate, in contrast, liberated as the oxidized alkanoic acid from the cleavage of (RS )‐2‐(2,4‐dichlorophenoxy)propionate (2,4‐DP), the regeneration of α ‐ketoglutarate seems to be guaranteed from succinate as resulted from the utilization of 2,4‐DP to a considerable amount under these conditions. The extent was limited, however, which was apparently caused by the accumulation of DCP. Continual cleavage of 2,4‐DP could be demonstrated in the presence of Ochrobactrum sp. K2‐14, which functions as a DCP‐consuming strain. Addition of extra metabolites, i.e. α‐ketoglutarate or other readily metabolizable substrates, improved the cleavage of the herbicides. This was most pronounced with 2,4‐D that was found now to be also utilized to a considerable extent. Conversely, the cleavage of the herbicides (2,4‐DP) was reduced and ultimately ceased with cells depleted by starvation of the pool of metabolites. Again, this deficit could be restored by adding α‐ketoglutarate. The limitations in utilizing phenoxyalkanoate herbicides are discussed in terms of pseudo‐recalcitrance owing to deficits in metabolites (α‐ketoglutarate) rather than enzyme activity (TfdA). |
