PCBs, congeners

PCB congeners exhibit considerable variations that are dependent on the specific response and the test species. ... 

Fig. 2. Toxic PCB congeners coplanar (a) and monoortho coplanar compounds (b). See Table 2.

There is some evidence for chemically mediated endocrine disruption in amphibians. The egg yolk protein, vitellogenin, is inducible in amphibians by exposure to DDT. " Males of the short clawed toad Xenopus laevis given 250 fig/g or 1 fig/g o,p -DDT for seven days have been shown to produce vitellogenin, although the induction was less than that achieved by treatment with 1 fig/g of either 17/1-oestradiol or diethylstilboestrol. Research has also shown that endocrine disrupting chemicals can alter sex ratios in wild populations of certain species PCB congeners and organochlorine compounds have been linked with male domination of sex ratios in polluted compared to unpolluted sites. ... 

G. M. Frame, A collaborative study of 209 PCB congeners and 6 Aroclor s on 20 different HRGC columns. Part 2. Semi-quantitative Aroclor congener distributions . Anal. Chem. 70 714-722(1997). 

The chirality of the PCB congeners has also been taken into account in this separation. From a total of 209 congeners, 78 are axially chiral in their nonplanar... 

Figure 13.1 Monitor (FID) (a) and analytical (ECD) (b) channel responses for PCB congeners in Aroclor 1254, showing selection of the six heart-cut events Frr-st columns, HT8 second columns, BPX5. Reprinted from Journal of High Resolution Chromatography, 19, R. M. Kinghorn et al., Multidimensional capillar-y gas chr omatography of polychlorinated biphenyl marker compounds , pp. 622-626, 1996, with per-mission from Wiley-VCH.

Y. V. Gankin, A. E. Gorshteyn and A. Robbat-Jr, Identification of PCB congeners by gas chromatography electi on capture detection employing a quantitative sti ucture-retention model , Aim/. Chem. 67 2548-2555 (1995). 

Some PCB congeners have coplanar structures (see, e.g., 3,4,3, 4 -tetrachloro-biphenyl in Figure 6.1). The coplanar conformation is taken up when there is no chlorine substitution in ortho positions. If there is substitution of chlorine in only one ortho position, the molecule may still be close to coplanarity, because of only limited interaction between Cl and H on adjoining rings. Substitution of chlorines in... 

In a study with captive male American kestrels (Drouillard et al. 2001), birds were dosed with Aroclor-contaminated diet and the toxicokinetics of 42 PCB congeners contained therein was stndied. Those congeners that were most rapidly cleared contained vicinal meta-para hydrogen substituents on at least one phenyl ring. This provides further evidence for the importance of open (i.e., not substituted by chlorine) meta-para positions for metabolic attack, an issue that will be returned to in the next section (Section 6.2.3). 

An early indication of the tendency for certain PCB congeners to be biomagnihed came from stndies on the Great Lakes of North America. The concentration of total PCBs in the food chain was fonnd to be as follows ... 

The enrichment index, which is PCB congener as a percentage of total PCB in seabird depot fat/PCB as a percentage of total PCB in Aroclor 1260. 

Among the 14 PCB congeners found at die highest concentration in eggs from die Mediterranean Sea and die Atlantic Ocean (Renzoni et al. 1986). These audiors also found numbers 156, 172, and 183. Numbers 172 and 183 were also reported in Borlakoglu et al. (1988). 

The acute toxicity of PCB mixtures to vertebrates tends to be low, typically, 1-10 g/ kg to rats. The concern is about sublethal and chronic toxicity. Different PCB congeners show different modes of toxic action, so it is not surprising that their mixtures can produce a disconcerting range of toxic effects (Robertson and Hansen 2001). 

Analysis of chlorobenzoates in sediments, which had been contaminated with PCBs, was used to demonstrate that the lower PCB congeners that had initially been produced by anaerobic dechlorination were subsequently degraded under aerobic conditions. The chlorobenzoates were transient metabolites and their concentrations were extremely low since bacteria that could successfully degrade them were present in the sediment samples (Elanagan and May 1993). 

A range of organisms able to degrade PCB congeners has been isolated, generally from biphenyl enrichments. These include... 

Whole-cell studies using oxygen uptake by Burkholderia (Pseudomonas) sp. strain LB400 had clearly shown the versatility of this organism in the degradation of PCB congeners containing up... 

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