The degradation of L-tyrosine to phenol and benzoate in pig manure

Abstract

The formation of odorous compounds in piggery wastes was investigated. Phenol and para-cresol are generally encountered in these typically anaerobic environments. They are produced from L-tyrosine by microbial metabolism. Phenol is further converted to benzoate via para-carboxylation. The biochemical pathways were studied by feeding manure with miscellaneous metabolites at concentration between 5 and 20 mM. Metabolites were analyzed by gas chromatography (GC) and high-performance liquid chromatography (HPLC). Experiments were carried out at room temperature. The degradation of L-tyrosine to phenol, benzoate, and para-cresol was confirmed. 4HPPyrA and 4HPAA are not intermediate compounds in phenol production. It was shown that phenol was converted to benzoate without any production of 4HBA. Other experiments showed that 4HBA was decarboxylated to phenol, but not dehydroxylated to benzoate.When phenol was added in presence of benzoate (5 mM each) or alone at higher concentrations (10 or 20 mM), transient small amounts of 4HBA were observed (about 0.02 mM). Our experiments show that 4HBA is not an intermediate metabolite in the conversion of phenol to benzoate. The decarboxylation of 4HBA to phenol is probably the last step of another degradation pathway. This reaction is proposed to have a weakly reversible property, explaining 4HBA production.