OH-PCBs

Detoxification of PCBs by Phase I conjugation produces hydroxylated PCBs (OH-PCBs) through either direct hydroxide insertion or via arene oxide intermediates [7]. The latter can... 

Ueno, D. Darling, C. Alaee, M. Campbell, L. Pacepavicius, G. Teixeira, C. Muir, D. Detection of hydroxylated polychlorinated biphenyls (OH-PCBs) in the abiotic environment Surface water and precipitation from Ontario, Canada Environ. Sci. Technol. 2007, 41, 1841-1848. 

Fig. 1. The chemical numbering system for individual MeS02-PCB and OH-PCB congeners. The PCB congener number is based on the PCB numbering system of Ballschmiter et al. [45]...

Fig. 4. Metabolism scheme for PCBs as exemplified by 2,2, 4,5,5 -pentachlorobipenyl (CB-101). The formation of MeS02-PCB and OH-PCB metabolites are shown. CB-101 is one of several PCBs known to form persistent MeS02-PCB metabolites in biota. These precursor PCBs possess at least one chlorine-unsubstituted meta-para position, facilitating the formation of a 3,4-arene oxide...

PCB arene oxides can also generate dihydrodiols via a hydrolytic pathway mediated by microsomal epoxide hydrolase, although metabolism to monohydroxy-metabolites is more commonly observed [74,75]. OH-PCBs are susceptible to further metabolism, i. e. conjugation reaction with glucuronic acid or sulfate, which increases the water solubility and facilitates excretion. Glucuronic acid and sulfate conjugates of several PCB congeners have been determined in bile and urine from experimental animals exposed to the PCBs [50,76,77]. Biliary excretion is the preferred pathway for PCB metabolites, whereas only a small portion is excreted via the urine [77]. 

OH-PCB compounds are much more labile than the MeS02-PCBs. OH-PCBs may undergo oxidation if exposed to air for a prolonged period of time. Most oxidation products are as yet unidentified, although catechols ((OH)2-PCB) and quinone-type products are known to be formed [116]. A number of OH-PCBs, analyzed as their corresponding MeO-PCB derivatives, have so far been identified by comparison to the authentic standards listed in Table 2. 

GC separation of the isolated phenolic compounds occurs prior to identification and quantification using ECD- and MS-coupled detectors (Sect. 4.3). However, phenolic compounds are polar, and derivatization gives better GC peak shapes in the most commonly used types of GC columns. Hitherto the standard method of derivatization of OH-PCBs retained in blood has been methylation by diazomethane to form the corresponding methoxy-PCB (MeO-PCB) [39,43, 135]. OH-PCB methylation by ion-pair alkylation with methyl iodide is an alternative to the diazomethane technique [43,140]. Acetylation has been shown to give comparable recoveries of OH-PCB derivatization as the methyl iodide and diazomethane approaches to OH-PCB methylation [139]. Silylation reactions have also been applied for the derivatization of various OH-PCBs [137]. 

GC separations have mainly been performed on columns such as 5%-phenylmethyl-derivatized column (e.g., DB-5 or XTI-5) [39,42]. However, coelution of MeO-PCBs occurs, which may require separation on two columns of different polarity. A cyano-derivatized column (e.g., SP-2331) has been used for this reason to facilitate full identification/separation of the congeners present in the OH-PCB envelope of elution [39]. GC(ECD) and GC/MS(EI) [39,42], and GC/MS(ECNI) [43] have been used to detect and quantify the MeO-PCBs. The OH-PCB congeners so far identified in wildlife and human blood are listed in Table 2. 

The present review on patterns and levels of OH-PCBs in biota concentrates on those present in blood since the majority of OH-PCBs formed in vivo are readily excreted. Most OH-PCBs are therefore anticipated to be present at very low and transient levels. A number of OH-PCBs have been identified or at least indicated in the blood of humans and biota during the last 5 to 10 years (Table 2, Table 5, Fig. 6). However, reports on the identity, presence and levels of OH-PCBs in biota remain very limited in comparison to the MeS02-PCBs. This situation is expected to change in the near future as a consequence of more scientific interest in this area. 

Up to approximately 30 OH-PCB and a few (OH)2-PCB congeners have been detected in human blood (Fig. 6). As co-elution of OH-PCB congeners is known to occur, the number of OH-PCB congeners present in the samples may still be... 

Fig. 6A, B. GC/ECD chromatograms of the phenolic fraction obtained from A a women from the Faeroe islands with a relatively high OHS contamination level in her blood (A. Bergman et al., unpublished data), and B albatross [42] plasma. The structures of the identified congeners are labeled on the peaks. The number labels refer to the corresponding OH-PCB congeners listed in Table 2. The I.S. is 2,3,3, 4, 5,5, 6-heptachloro-4-biphenylol (4-OH-CB193)...

The structures of the OH-CBs are given in Table 2. b These OH-PCBs co-elute and are therefore presented together. 

The parent PCBs of most of the retained OH-PCBs have been identified and are listed in Table 5. The majority of retained OH-PCBs are formed via an arene oxide followed by a 1,2-shift. However, direct insertion of a OH-group may also occur (see Sect. 2.2). 

In the first report on phenolic PCB metabolites in blood from rats after exposure to Aroclor 1254, and from environmentally exposed Baltic grey seals and humans only a few OH-PCBs were structurally identified [39]. Since then, additional metabolites have been identified and are shown in the chromatogram of a human blood sample taken from a Faeroe Island woman (Fig. 6A). All the identified metabolites have chlorine atoms attached to the carbons ortho to the oxidized carbon. With a few exceptions, the hydroxy-group is attached to one of the two para-positions of the PCB molecule. These structural elements are also found in 3,3, 4, 5-tetraiodo-L-thyronine, or thyroxine, the natural substrate of TTR [44]. The affinities of the retained OH-PCB congeners are up to 10 times greater than for thyroxine [44,134]. The structural similarity to thyroxine was... 

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