THE ROLE OF FORMALDEHYDE AND S-CHLOROMETHYLGLUTATHIONE IN THE BACTERIAL MUTAGENICITY OF METHYLENE-CHLORIDE

Methylene chloride was less mutagenic in Salmonella typhimurium TA100/ NG-11 (glutathione-deficient) compared to TA100, indicating that gluta thione is involved in the activation of methylene chloride to a mutage n in bacteria. In rodents, the pathway of methylene chloride metabolis m utilizing glutathione produces formaldehyde via a postulated S-chlor omethylglutathione conjugate (GSCH(2)Cl). Formaldehyde is known to cau se DNA-protein cross-links, and GSCH(2)Cl may act as a monofunctional DNA alkylator by analogy with the glutathione conjugates of 1,2-dihalo alkanes. The lack of sensitivity of Salmonella TA100 towards formaldeh yde (Schmid et al., Mutagenesis, 1 (1986) No. 6, 427-431) suggests tha t GSCH(2)Cl is responsible for methylene chloride mutagenicity in Salm onella. In Escherichia coli K12 (AB1157), formaldehyde was mutagenic o nly in the wild-type, a characteristic shared with cross-linking agent s, whereas 1,2-dibromoethane (1,2-DBE) was more mutagenic in uvrA cell s (AB1886). Methylene chloride, activated by S9 from mouse liver, was mutagenic only in wild-type cells, suggesting a mutagenic role for met abolically derived formaldehyde in E. coli. Mouse-liver S9 also enhanc ed the cell-killing effect of methylene chloride in the uvrA, and a re cA/uvrA double mutant (AB2480) which is very sensitive to DNA damage. This pattern was consistent with formaldehyde damage. However, a mutag enic role in bacteria for the glutathione conjugate of methylene chlor ide cannot be ruled out by these E. coli experiments because S9 fracti ons did not increase 1,2-DBE mutagenicity, suggesting lack of cell wal l penetration by this reactive species. Rat-liver S9 did not activate methylene chloride to a bacterial mutagen or enhance methylene chlorid e-induced cell-killing, which is consistent with the carcinogenicity d ifference between the species.