by differences among various animals to absorb, distribute, biotransform, and 

 excrete individual PCB isomers (Hansen et al . 1983). 



Biological activities of PCB isomers differ substantially. Among three 

 symmetrical hexachlorobiphenyl (HCBP) isomers, 3,4,5,3' ,4' ,5'-HCBP was the 

 most toxic in dietary lethal studies to domestic chickens ( Gallus spp.), 

 2,4,5,2',4',5'-HCBP was least toxic, and 2,4,6,2' ,4' ,6'-HCBP intermediate in 

 toxicity (Ayer 1976). A tetrachlorobiphenyl (3, 4, 3', 4') was more than 1000 

 times more potent in producing effects in chicks than 2,4,5,2' ,4' ,5'-HCBP both 

 as an hepatotoxin and as an inducer of cytochrome P mediated mixed function 

 oxidases (Rifkind et al . 1984). The biological half-life of intravenously 

 administered PCB formulations in liver of rat ( Rattus spp.) increased with 

 increasing chlorination: half-lives of 86 hours were determined for 

 4-monochlorobiphenyl , 99 hours for 4,4'-dichlorobiphenyl , 193 hours for 

 2,2',4,5,5'-pentachlorobiphenyl , and 1,308 hours for 2,2 ' ,4,4 ' ,5,5'- 

 hexachlorobi phenyl (Menzie 1978). 



PCBs are usually taken up by animals and stored in lipids under 

 circumstances of increasing lipid content in organs. However, recent studies 

 with marine fishes indicate that PCB components remain mobilizable from organs 

 whose lipid contents increased (Boon et al. 1984). The degree of mobilization 

 in codfish ( Gadus morhua ) and sole ( Solea solea ) appeared to be related to 

 polar lipid components such as phospholipids and glycerols. In other aquatic 

 species, the role of diet and tissue specific sites are important. The 

 patterns of pentachloro BP and higher chlorinated BP (not present in seawater) 

 were equal in marine clams, worms, and sediments, and strongly indicate that 

 uptake was via the diet or from sediments (Duinker et al. 1983). In 

 freshwater fishes, direct partitioning across the gill membrane of the 

 blood:water interface controls PCB accumulation; however, dietary PCBs may 

 significantly affect accumulation and exchange rates at the gill membrane 

 (Rohrer et al . 1982). 



Biological availability and uptake of individual PCBs from aqueous 

 solution are influenced primarily by two factors: the partition coefficient 

 (Kow) based on the solubilities of compounds in N-octanol /water; and steric 

 factors resulting from different patterns of chlorine substitution. Log Kow 

 values for various isomers of Aroclors 1242, 1254, and 1260 are high, varying 

 from 4.0 to 9.35, indicating high biological uptake potential. Steric effect 

 coefficients are based on the number of chlorine atoms in the biphenyl 

 molecule and their arrangement (Shaw and Connell 1982). For example, three 

 chlorines in the ortho positions were assigned a steric effect coefficient of 

 0.3; four chlorines in the ortho postions 0.2; three or four adjacent 

 chlorines on one ring 0.6, and on both rings 0.3; chlorines in the meta 

 position on one ring 0.8, and on both rings 0.6. The product of log Kow and 

 the steric effect coefficient seem to be directly related to bioaccumulation 

 (Shaw and Connell 1982). Thus, maximum uptake was found with penta- and 

 hexachlorobiphenyls predominant in Aroclor 1254, which have high values for 

 log Kow and for steric effect coefficients. Comparatively less uptake was 

 found with di-, tri-, and tetrachlorobiphenyls, typical of Aroclor 1242, which 

 have lower values for log Kow, and with hepta- and octachlorobiphenyls. 



