Polychlorinated Biphenyl Congener and Hexachlorobenzene by Vitamin B12 NADA ASSAF-ANID, LORING

The concern about synthetic chemical compounds such as polychlorinated biphenyls (PCBs) and hexachlorobenzene (HCB) as a threat to living organisms (21) has fostered research on innovative treatment technologies for their destruction. Because of their hydrophobic properties, these compounds accumulate in fatty tissues of all organisms (21). In the environment, they often partition into aquatic sediments, where they decompose very slowly by reductive dechlorination (4, 18). Reductive dechlorination (R-Cl + H+ + 2e -R-H + Cl-) of a chemical compound requires the transfer of two electrons to R-Cl, the release of chlorine as chloride (19, 22), and the scavenging of a proton from solution. A better understanding of the factors that enhance this important and major anaerobic transformation of PCBs is crucial for the design of any biological or chemical process concerned with their destruction. Laboratory studies have shown that highly chlorinated isomers of PCBs and HCB are extremely resistant to microbial oxidation; however, bacterially mediated reductive dechlorination results in less-chlorinated analogs, which are more susceptible to oxidation and often less toxic (3, 9, 18). No anaerobic microbial pure culture capable of dechlorinating these compounds has been isolated yet, and the mechanism of reductive dechlorination by anaerobic mixed consortia is still unknown. Attempts to chemically reduce chlorinated aromatic compounds with the transition metal coenzymes vitamin B12 and hematin have been reported only for the monoaromatics HCB, pentachlorobenzene, and (2,4,5-trichlorophenoxy)acetic acid (10). These attempts and others on a range of aliphatic compounds (1, 5, 10, 11, 13), on aliphatic moieties of aromatic compounds (24), and on alicyclic compounds (2, 15), combined with knowledge about the existence of the coenzyme catalysts in anaerobes (23), support the hypothesis that these catalysts are involved in nonspecific microbial dechlorination (10, 22). However, until this work, PCBs have not been shown to undergo these abiotic biomimetic reactions. The reductive dechlorination of 2,3,4,5,6-pentachlorobiphenyl (AccuStandard, New Haven, Conn.) and HCB (Accu Standard) was assayed by using a buffered, anoxic water1,4-dioxane medium. Dithiothreitol (DTT; Sigma Chemical Co.) was used as the reducing agent, and vitamin B12 (cyanocobalamin; purity, ca. 99%; Sigma Chemical Co.) was