ANAEROBIC DEGRADATION OF CRESOLS BY DENITRIFYING BACTERIA

The initial reactions in anaerobic metabolism of methylphenols (cresols) and dimethylphenols were studied with dentrifying bacteria. A newly isolated strain, possibly a Paracoccus sp., was able to grow on o- or p-cresol as sole organic substrate with a generation time of 11 h; o- or p-cresol was completely oxidized to CO2 with nitrate being reduced to N2. A dentrifying Pseudomonas-like strain oxidized m- or p-cresol as the sole organic growth substrate completely to CO2 with a generation time of 14 h. Demonstration of intermediates and/or in vitro measurement of enzyme activities suggest the following enzymatic steps: (1) p-Cresol was metabolized by both strains via benzoyl-CoA as central intermediate as follows: p-cresol --> 4-OH-benzaldehyde --> 4-OH-benzoate --> 4-OH-benzoyl-CoA --> benzoyl-CoA. Oxidation of the methyl group to 4-OH-benzaldehyde was catalyzed by p-cresol methylhydroxylase. After oxidation of the aldehyde to 4-OH-benzoate, 4-OH-benzoyl-CoA is formed by 4-OH-benzoyl-CoA synthetase; subsequent reductive dehydroxylation of 4-OH-benzoyl-CoA to benzoyl-CoA is catalyzed by 4-OH-benzoyl-CoA reductase (dehydroxylating). (2) o-Cresol was metabolized in the Paracoccus-like strain via 3-CH3-benzoyl-CoA as central intermediate as follows: o-cresol --> 4-OH-3-CH3-benzoate --> 4-OH-3-CH3-benzoyl-CoA --> 3-CH3-benzoyl-CoA. The following enzymes were demonstrated: (a) An enzyme catalyzing an isototope exchange reaction between (CO2)-C-14 and the carboxyl of 4-OH-3-CH3-benzoate; this activity is thought to be a partial reaction catalyzed by an o-cresol carboxylase. (b) 4-OH-3-CH3-benzoyl-CoA synthetase (AMP-forming) activating the carboxylation product 4-OH-3-CH3-benzoate to its coenzyme A thioester. (c) 4-OH-3-CH3-benzoyl-CoA reductase (dehydroxylating) catalyzing the reductive dehydroxylation of the 4-hydroxyl group with reduced benzyl viologen as electron donor to yield 3-CH3-benzoyl-CoA. This thioester may also be formed by action of a coenzyme A ligase when 3-CH3-benzoate is metabolized. 2,4-Dimethylphenol was metabolized via 4-OH-3-CH3-benzoate and further to 3-CH3-benzoyl-CoA. (3) The initial reactions of anaerobic metabolism of m-cresol in the Pseudomonas-like strain were not resolved. No indication for the oxidation of the methyl group nor for the carboxylation of m-cresol was found. In contrast, 2,4- and 3,4-dimethylphenol were oxidized to 4-OH-3-CH3- and 4-OH-2-CH3-benzoate, respectively, probably initiated by p-cresol methylhydroxylase; however, these compounds were not metabolized further.