Pseudo-recalcitrance of chlorophenoxyalkanoate herbicides - Correlation tothe availability of alpha-ketoglutarate

Cleavage of the ether bond of chlorophenoxyalkanoate herbicides is catalyze d by an alpha -ketoglutarate-linked dioxygenase (TfdA). In this step, alpha -ketoglutarate is decarboxylated to succinate and must be regenerated for continual substrate cleavage. Limitations in herbicide degradation are to b e expected in the case of a shortage of alpha -ketoglutarate. Such a situat ion was simulated and studied with Delftia (formerly Comamonas) acidovorans MC1 and Rhodoferax sp. P230, which constitutively express etherolytic diox ygenase activity by excreting 2,4-dichlorophenol (DCP) as a dead-end produc t. The results showed that 2,4-dichlorophenoxyacetate (2,4-D) could hardly be cleaved under these conditions which is attributed to the inability to r egenerate alpha -ketoglutarate from the cleavage. products, i.e. succinate and glyoxylate [1]. With pyruvate, in contrast, liberated as the oxidized a lkanoic acid from the cleavage of (RS)-2-(2,4-dichlorophenoxy)propionate (2 ,4-DP), the regeneration of alpha -ketoglutarate seems to be guaranteed fro m succinate as resulted from the utilization of 2,4-DP to a considerable am ount under these conditions. The extent was limited, however, which was app arently caused by the accumulation of DCP. Continual cleavage of 2,4-DP cou ld be demonstrated in the presence of Ochrobactrum sp. K2-14, which functio ns as a DCP-consuming strain. Addition of extra metabolites, i.e. alpha -ke toglutarate or other readily metabolizable substrates, improved the cleavag e 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 deplet ed by starvation of the pool of metabolites. Again, this deficit could be r estored by adding alpha -ketoglutarate. The limitations in utilizing phenox yalkanoate herbicides are discussed in terms of pseudo-recalcitrance owing to deficits in metabolites (alpha -ketoglutarate) rather than enzyme activi ty (TfdA).