X = Excretion A = Absorption r
Relative excretion _
CM CM CO CO
Bioaccumulation - up to 70 times higher concentrations than in fish!
R = Relatively resistant to metabolism o m cn cn cn
can be excreted from animals slowly in the feces and urine (or through the gills in aquatic species). Hydroxylated PCBs, and some reactive intermediates formed during hydroxylation, can be conjugated with water-soluble derivatives (e.g., glucuronide) and are excreted in the urine and to some extent in the feces (via bile). Intermediate products in the hydroxylation of PCBs can be sulfonated, which reduces rather than increases their water solubility, and these can be stored within adipose tissues - for example, methyl-sulfonyl PCBs are nonpolar and persistent.
Since metabolism is easiest at adjacent unsubstituted positions, the less-chlorinated PCBs are most easily metabolized - as the level of chlorination increases fewer and fewer PCBs have the unsubstituted positions needed for efficient metabolism. Thus there are specific pattern changes that can generally be seen through foodchains, with the less-chlorinated PCBs becoming relatively reduced in concentration, and the least easily metabolized PCB congeners becoming dominant. In general the higher animals have more capacity to metabolize PCBs than the lower animals, and some specific species have unusually high or low metabolic capacity (e.g., cetaceans metabolize PCBs relatively slowly). It should be noted that within a species there are likely to be P450 polymorphisms (inherited genetic differences in the efficiency of certain P450 enzymes), which may lead some groups of individuals to have unusually high or low capacities for the metabolism of PCBs. It is also important to note that there can be wide variation in the relative efficiencies of specific metabolic pathways between species.
In Figure 2 a comparison of PCB patterns in the diet (fish), feces, and blubber of a gray seal from the North Sea (UK coast) in 2001 is shown. The relative loss of the more easily metabolized PCB congeners can easily be seen by comparing the fish PCB congener pattern with the blubber pattern. This is most notable for most of the less-chlorinated (lower numbered) PCBs (below hexa-chlorobiphenyl), but also affects some highly chlorinated PCBs that have chlorine-substitution patterns that allow effective metabolism. It is also evident that some less-chlorinated PCBs (e.g., PCB 99) are relatively resistant to metabolism. The PCBs that are relatively resistant to metabolism in the gray seal are marked 'R' on the blubber PCB pattern.
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