Metabolism of 14C-labeled polychlorinated biphenyl congeners by wheat cell suspension cultures
The metabolism of [UL-14C]-2,2′,5,5′-tetrachlorobiphenyl (14C-PCB-52), [UL-14C]-2,2′,4,4′,5,5′-hexachlorobiphenyl (14C-PCB-153, and a congeneric mixture of [UL-14C]-labeled polychlorinated biphenyls (14C-PCB-Mix) was studied in cell suspension cultures of wheat (Triticum aestivum L. cv. ‘Heines Koga II’). About 50% of applied 14C-PCB-52 (20 μg/assay) was transformed during 96 h of incubation. While 7.6% on non-extractable residues emerged, turnover of 14C-PCB-52 was mainly due to soluble polar metabolites. These were subjected to chemical glycoside cleavage. In the resulting hydrolysate, four aglycons were identified by GC-EIMS, namely four tetrachloro-hydroxy-biphenyl isomers (C6H6Cl4O, M+· at m/z = 306, 308, 310 and 312), and one trichloro-hydroxy-biphenyl (C6H7Cl3O, M+· at m/z = 272, 274 and 276). Number and character of hydroxylated products pointed to cytochromes P450 as enzymatic catalysts of hydroxylation. 14C-PCB-153 was metabolized by wheat to minor degree if at all. Due to GC-EIMS analysis, of 14C-PCB-Mix consisted of biphenyl, one mono-, four di-, seven tri-, eleven tetra-, and four pentachlorobiphenyls besides traces of further mono- and hexachlorobiphenyls. Among these were PCB-28, PCB-52, PCB 101, and PCB-118 (identified by seven key congeners standard). The mixture resembled industrial products Clophen A30 or Aroclor 1016. Metabolic turnover of applied 14C-PCB-Mix (15 μg/assay) was 30% after 96 h; 8.4% of non-extractable residues emerged. Using DDE (p,p’-dichlorodiphenyl-dichloroethylene) as internal standard it was demonstrated that biphenyl, one monochloro-, two dichloro-, and one trichlorobiphenyl were completely metabolized to polar products. Partial metabolization occurred with one di-, five tri-, and four tetrachlorobiphenyls. Two tri-, four tetra-, and all pentachlorbiphenyls proved to be stable. Due to strong interference by matrix, evaluation of three congeners was not possible. In addition to wheat, results of similar experiments with cell cultures of other species are briefly mentioned.