Identification of a serine hydrolase as a key determinant in the microbialdegradation of polychlorinated biphenyls

The ability of 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoate (HOPDA) hydrolase (BphD) of Burkholderia cepacia LB400 to hydrolyze polychlorinated biphenyl (PCB) metabolites was assessed by determining its specificity for monochlor inated HOPDAs. The relative specificities of BphD for HOPDAs bearing chlori ne substituents on the phenyl moiety were 0.28, 0.38, and 1.1 for 8-Cl, 9-C l, and 10-Cl HOPDA, respectively, versus HOPDA (100 mM phosphate, pH 7.5, 2 5 degrees C). In contrast, HOPDAs bearing chlorine substituents on the dien oate moiety were poor substrates for BphD, which hydrolyzed 3-Cl, 4-Cl, and 5-Cl HOPDA at relative maximal rates of 2.1 x 10(-3), 1.4 x 10(-4), and 0. 36, respectively, versus HOPDA. The enzymatic transformation of 3-, 5-, 8-, 9-, and 10-Cl HOPDAs yielded stoichiometric quantities of the correspondin g benzoate, indicating that BphD catalyzes the hydrolysis of these HOPDAs i n the same manner as unchlorinated HOPDA. HOPDAs also underwent a nonenzyma tic transformation to products that included acetophenone. In the case of 4 -Cl HOPDA, this transformation proceeded via the formation of 4-OH HOPDA (t (1/2) = 2.8 h; 100 mM phosphate, pH 7.5, 25 degrees C). 3-Cl HOPDA (t(1/2) = 504 h) was almost 3 times more stable than 4-OH HOPDA. Finally, 3-Cl, 4-C l and 4-OH HOPDAs competitively inhibited the BphD-catalyzed hydrolysis of HOPDA (K-ic values of 0.57 +/- 0.04, 3.6 +/- 0.2, and 0.95 +/- 0.04 mu M, r espectively). These results explain the accumulation of HOPDAs and chloroac etophenones in the microbial degradation of certain PCB congeners. More sig nificantly, they indicate that in the degradation of PCB mixtures, BphD wou ld be inhibited, thereby slowing the mineralization of all congeners. BphD is thus a key determinant in the aerobic microbial degradation of PCBs.