O-phthalic acid, a dead-end product in one of the two pathways of phenanthrene degradation in Pseudomonas sp. strain PP2.

Phenanthrene is degraded via either o-phthalic acid or 1, 2-dihydroxynaphthalene in bacteria. A soil isolate Pseudomonas sp. strain PP2 degrades phenanthrene as the sole source of carbon, but failed to utilize naphthalene [Prabhu and Phale (2003) Appl Microbiol Biotechnol 61:342-351]. Analysis of the phenanthrene-grown culture spent media of this strain by gas chromatography-mass spectrometry (GC-MS) showed accumulation of o-phthalic acid. The cell-free extract prepared from this strain showed activity of 1-hydroxy-2-naphthoic acid dioxygenase (1-H-2-NADO). The extract showed conversion of 1-hydroxy-2-naphthoic acid and 2-carboxybenzaldehyde to o-phthalic acid, as analyzed by thin layer chromatography and GC-MS. However, it failed to grow or respire on o-phthalic acid. These results suggest that besides 1, 2-dihydroxynaphthalene pathway, the strain has a truncated o-phthalic acid pathway for phenanthrene metabolism and excretes o-phthalic acid as a dead-end product, indicating the co-existence of two pathways. 1-H-2-NADO, the key enzyme of o-phthalic acid pathway is inducible, has pH optima of 7.5, does not require external addition of Fe(II) as a co-factor and is completely inhibited by 1,10-phenanthroline. Absence of product formation under anaerobic condition and stoichiometric consumption of 0.82 moles of O2 per mole of product formed confirmed the dioxygenase nature of the enzyme.