Biodegradation of phenanthrene by Alcaligenes sp. strain PPH: Partial purification and characterization of 1-hydroxy-2-naphthoic acid hydroxylase

摘要

Alcaligenes sp. strain PPH degrades phenanthrene via 1-hydroxy-2-naphthoic acid (1-H2NA), 1,2-dihydroxynaphthalene (1,2-DHN), salicylic acid and catechol. Enzyme activity versus growth profile and heat stability studies suggested the presence of two distinct hydroxylases, namely 1-hydroxy-2-naphthoic acid hydroxylase and salicylate hydroxylase. 1-Hydroxy-2-naphthoic acid hydroxylase was partially purified (yield 48%, fold 81) and found to be a homodimer with a subunit molecular weight of ～34 kDa. The enzyme was yellow in color, showed UV-visible absorption maxima at 274, 375 and 445 nm, and fluorescence emission maxima at 527 nm suggested it to be a flavoprotein. The apoenzyme prepared by the acid-ammonium sulfate (2 M) dialysis method was colorless, inactive and lost the characteristic flavin absorption spectra but regained ～90% activity when reconstituted with FAD. Extraction of the prosthetic group and its analysis by HPLC suggests that the holoenzyme contained FAD. The enzyme was specific for 1-H2NA and failed to show activity with any other hydroxynaphthoic acid analogs or salicylic acid. The K_m for 1-H2NA in the presence of either NADPH or NADH remained unaltered (72 and 75 μM, respectively), suggesting dual specificity for the coenzyme. The K_m for FAD was determined to be 4.7 μM. The enzyme catalyzed the conversion of 1-H2NA to 1,2-DHN only under aerobic conditions. These results suggested that 1-hydroxy-2-naphthoic acid hydroxylase is a flavoprotein monooxygenase specific for 1-H2NA and different from salicylate-1-hydroxylase.