Earlier studies in our laboratory showed that hydroxylated metabolites of polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), and dibenzofurans (PCDFs) competitively inhibit thyroxine (T4) binding to transthyretin (TTR) and type I deiodinase (D1) activity. In this study, we investigated the possible inhibitory effects of hydroxylated metabolites of polyhalogenated aromatic hydrocarbons (PHAHs) on iodothyronine sulfotransferase activity. Rat liver cytosol was used as a source of sulfotransferase enzyme in an in vitro assay with 125I-labeled 3,3'-diiodothyronine (T2) as a model substrate. Increasing amounts of hydroxylated PCBs, PCDDs, or PCDFs or extracts from incubation mixtures of PHAHs and induced liver microsomes were added as potential inhibitors of T2 sulfotransferase activity. Hydroxylated metabolites of PCBs, PCDDs, and PCDFs were found to be potent inhibitors of T2 sulfotransferase activity in vitro with IC50 values in the low micromolar range (0.2-3.8 microM). The most potent inhibitor of T2 sulfotransferase activity in our experiments was the PCB metabolite 3-hydroxy-2,3',4, 4',5-pentachlorobiphenyl with an IC50 value of 0.2 microM. A hydroxyl group in the para or meta position appeared to be an important structural requirement for T2 sulfotransferase inhibition by PCB metabolites. Ortho hydroxy PCBs were much less potent, and none of the parent PHAHs was capable of inhibiting T2 sulfotransferase activity. In addition, the formation of T2 sulfotransferase-inhibiting metabolites of individual brominated diphenyl ethers and nitrofen as well as from some commercial PHAH mixtures (e.g., Bromkal, Clophen A50, and Aroclor 1254) was also demonstrated. These results indicate that hydroxylated PHAHs are potent inhibitors of thyroid hormone sulfation. Since thyroid hormone sulfation may play an important role in regulating free hormone levels in the fetus, and PCB metabolites are known to accumulate in fetal tissues after maternal exposure to PCBs, these observations may have implications for fetal thyroid hormone homeostasis and development.

A novel flavoprotein that catalyses the NADPH-dependent oxidation of 4-hydroxyacetophenone to 4-hydroxyphenyl acetate, was purified to homogeneity from Pseudomonas fluorescens ACB. Characterization of the purified enzyme showed that 4-hydroxyacetophenone monooxygenase (HAPMO) is a homodimer of < 140 kDa with each subunit containing a noncovalently bound FAD molecule. HAPMO displays a tight coupling between NADPH oxidation and substrate oxygenation. Besides 4-hydroxyacetophenone a wide range of other acetophenones are readily converted via a Baeyer±Villiger rearrangement reaction into the corresponding phenyl acetates. The P. fluorescens HAPMO gene (hapE) was characterized. It encoded a 640 aminoacid protein with a deduced mass of 71 884 Da. Except for an N-terminal extension of < 135 residues, the sequence of HAPMO shares significant similarity with two known types of Baeyer±Villiger monooxygenases: cyclohexanone monooxygenase (27±33% sequence identity) and steroid monooxygenase (33% sequence identity). The HAPMO sequence contains several sequence motifs indicative for the presence of two Rossman fold domains involved in FAD and NADPH binding. The functional role of a recently identified flavoprotein sequence motif (ATG) was explored by site-directed mutagenesis. Replacement of the strictly conserved glycine (G490) resulted in a dramatic effect on catalysis. From a kinetic analysis of the G490A mutant it is concluded that the observed sequence motif serves a structural function which is of importance for NADPH binding.

Several aerobic microorganisms are capable of utilizing acetophenones for their growth (16,17,(30)(31)(32). However, relatively little is known about the oxidative enzymes involved in acetophenone mineralization (45,47). The catabolism of 4-hydroxyacetophenone in Pseudomonas fluorescens ACB proceeds through the initial formation of 4-hydroxyphenyl acetate and hydroquinone (31,37,47). The latter compound is further degraded via 4-hydroxymuconic semialdehyde and maleylacetate to ␤-ketoadipate (46). We have purified HapA, the enzyme responsible for the Baeyer-Villiger oxidation of 4-hydroxyacetophenone, and expressed its gene in Escherichia coli (37). Moreover, we established that this flavin adenine dinucleotide-containing monooxygenase is useful for the production of phenols and catechols, which are valuable intermediates in the synthesis of pharmaceuticals, agricultural chemicals, and material products (36,38,48).In the accompanying paper (46), we showed that the genes encoding 4-hydroxyacetophenone monooxygenase (hapA), 4-hydroxyphenylacetate esterase (hapB), 4-hydroxymuconic semialdehyde dehydrogenase (hapE), and maleylacetate reductase (hapF) belong to a gene cluster (hapCDEFGHIBA) involved in 4-hydroxyacetophenone utilization. Based on biochemical data and sequence analysis, we proposed that the function of the hapC and hapD genes is linked to the conversion of hydroquinone to 4-hydroxymuconic semialdehyde.Several ring cleavage enzymes acting on substituted hydroquinones have been described. These include intradiol dioxygenases acting on hydroxyhydroquinone (4,20,22,35,39,42,55,66) and extradiol dioxygenases that are active with (homo-)gentisate (3, 28) or chlorohydroquinone (10,44,51). However, enzymes that use hydroquinone as the physiological ring cleavage substrate have not been characterized. Here we report on the purification and properties of hydroquinone dioxygenase (HQDO) from P. fluorescens ACB. It is shown that the heterotetrameric enzyme, encoded by the hapC and hapD genes, is a novel member of the family of nonheme-iron(II)-dependent dioxygenases. The present results confirm that the hapG gene, encoding an intradiol dioxygenase (46), is not involved in 4-hydroxyacetophenone degradation. This finding has important implications for the function of related genes involved in the catabolism of other aromatic compounds.

The catabolism of 4-hydroxyacetophenone in Pseudomonas fluorescens ACB is known to proceed through the intermediate formation of hydroquinone. Here, we provide evidence that hydroquinone is further degraded through 4-hydroxymuconic semialdehyde and maleylacetate to ␤-ketoadipate. The P. fluorescens ACB genes involved in 4-hydroxyacetophenone utilization were cloned and characterized. Sequence analysis of a 15-kb DNA fragment showed the presence of 14 open reading frames containing a gene cluster (hapCDEFGHIBA) of which at least four encoded enzymes are involved in 4-hydroxyacetophenone degradation: 4-hydroxyacetophenone monooxygenase (hapA), 4-hydroxyphenyl acetate hydrolase (hapB), 4-hydroxymuconic semialdehyde dehydrogenase (hapE), and maleylacetate reductase (hapF). In between hapF and hapB, three genes encoding a putative intradiol dioxygenase (hapG), a protein of the Yci1 family (hapH), and a [2Fe-2S] ferredoxin (hapI) were found. Downstream of the hap genes, five open reading frames are situated encoding three putative regulatory proteins (orf10, orf12, and orf13) and two proteins possibly involved in a membrane efflux pump (orf11 and orf14). Upstream of hapE, two genes (hapC and hapD) were present that showed weak similarity with several iron(II)-dependent extradiol dioxygenases. Based on these findings and additional biochemical evidence, it is proposed that the hapC and hapD gene products are involved in the ring cleavage of hydroquinone.

The biocatalytic potential of the NADHdependent p-hydroxybenzoate hydroxylases (PHBH) from Rhodococcus rhodnii 135 and Rhodococcus opacus 557 was investigated. Monofluorinated 4-hydroxybenzoates were efficiently hydroxylated, albeit at different rates. 2-Fluoro-4-hydroxybenzoate was a true substrate for PHBH from R. rhodnii 135 but a substrate inhibitor for PHBH from R. opacus 557. Monochlorinated 4-hydroxybenzoates also acted as PHBH substrates, but with these compounds strong uncoupling of hydroxylation (formation of hydrogen peroxide) occurred. PHBH from R. rhodnii 135 preferred the 5'-hydroxylation of 2-chloro-4-hydroxybenzoate but the enzyme from R. opacus 557 favored the formation of 2-chloro-3,4-dihydroxybenzoate. Conversely, PHBH from R. rhodnii 135 regioselectively hydroxylated 2-fluoro-4-hydroxybenzoate to 2-fluoro-3,4-dihydroxybenzoate whereas the enzyme from R. opacus 557 also produced significant amounts of 2-fluoro-4,5-dihydroxybenzoate. At high NADH/substrate ratio, both 2-fluorodihydroxybenzoate products were further converted to 2-fluoro-3,4,5-trihydroxybenzoate. PHBH from R. rhodnii 135 and R. opacus 557 preferred the 5'-hydroxylation of 3-chloro-4-hydroxybenzoate. However, conversion of 3-fluoro-4-hydroxybenzoate involved considerable dehalogenation affording nearly equal amounts of 3,4-dihydroxybenzoate and 5-fluoro-3,4-dihydroxybenzoate. At high NADH/substrate ratio, the latter compound was further converted to 3,4,5-trihydroxybenzoate. The results are discussed in relation to the properties of the NADPH-specific PHBH from Pseudomonas fluorescens.