Mutagenic relevance of rat hepatocyte nuclei in the activation and inactivation of xenobiotica. Cyclophosphamide and epichlorohydrin activity on the yeasts S. pombe and S. cerevisiae

The influence on the mutagenicity of cyclophosphamide (Cy) and epichlorohydrin (ECH), of liver nuclei and hepatic post-mitochondrial (S9) preparations from phenobarbital-treated rats, was examined. The study was conducted in vitro, with 2 yeasts, Schizosaccharomyces pombe (P1 strain) which allows the detection of forward mutations, and Saccharomyces cerevisiae (D5 strain), in which the induction of different genetic effects, such as mitotic recombination, can be evaluated. The results indicated that the nuclear fraction has a qualitative capacity for biotransformation of compounds, overlapping that of S9. From a quantitative point of view, the Cy-activating capacity of the nuclear fraction was twice as high as that of S9 whereas the two fractions showed a similar ECH-inactivating ability. The present study strengthens the hypothesis of the relevance of nuclei as a site of metabolic activation and inactivation of exogenous compounds.

Genotoxicity, metabolism and blood kinetics of epichlorohydrin in mice

Epichlorohydrin (ECH), a direct mutagen in vitro, did not induce chromosomal aberrations in bone-marrow cells of CD1 mice given single oral doses of 50 and 200 mg/kg in water. The ECH diol derivative (3-chloro-1,2-propanediol) was tested in vitro by a forward-mutation assay on the yeast Schizosaccharomyces pombe and showed a weak but significant mutagenic effect. The failure of ECH to induce mutagenic effects appears to be due to the rapid metabolic clearance of the compound in vivo. ECH blood kinetics at both doses, and at the same time the concentration of the diol, were determined. ECH rapidly disappeared from mouse blood, being no longer detectable 20 min after treatment. In contrast, 3-chloro-1,2-propanediol was measurable up to 5 h after dosage. No difference was observed in the kinetic and metabolic behavior of ECH after single and repeated doses (50 and 200 mg/kg/day for 7 days). When 3-chloro-1,2-propanediol was tested, neither glutathione depletion nor epoxide hydrolase inhibition (evaluated with both styrene-7,8-oxide and ECH as substrates) could be detected in mouse liver. Finally, no difference in ECH blood kinetics or metabolism were observed in experiments in which the compound was administered (200 mg/kg) intraperitoneally in water or orally as a solution in dimethyl sulfoxide.

Evaluation of epichlorohydrin (ECH) genotoxicity. Microsomal epoxide hydrolase-dependent deactivation of ECH mutagenicity in Schizosaccharomyces pombe in vitro

The mutagenic effect of epichlorohydrin (ECH) on the yeast Schizosaccharomyces pombe was studied in vitro in the presence of mouse-liver S9 mix and microsomal and cytosolic fractions. The incubations were always performed in the absence of NADPH-generating systems. S9 mix and microsomes from phenobarbital-pretreated mice significantly reduced ECH mutagenicity, whereas the cytosol did not result in any deactivating effect. The various protein contents of the subcellular fractions were not involved in any scavenger effect as regards ECH mutagenic activity. Moreover, the addition of reduced glutathione to the incubation mixtures indicated that it did not play an important role, either per se or through the enzyme(s) glutathione-S-epoxide transferase(s), in preventing ECH genotoxicity. Our results suggest that microsomal epoxide hydrolase(s) represents the major step in the detoxifying pathway of ECH. These observations were supported by measurements of the specific epoxide hydrolase activity in the various fractions on the same substrate.

In vivo and in vitro mutagenicity studies of a possible carcinogen, trichloroethylene, and its two stabilizers, epichlorohydrin and 1,2-epoxybutane

In vivo and in vitro methodologies that have employed the yeast Schizosaccharomyces pombe as genetic indicator have been utilized to investigate the mutagenicity of two trichloroethylene (TCE) samples of pure and technical grade. Mutagenicity assays were also performed on two stabilizers contained in the technical grade sample: epichlorohydrin and 1,2-epoxybutane. In the in vitro studies a metabolic conversion system was supplied by liver homogenate (S-9) from mice and rats untreated and pretreated with phenobarbital and/or beta-naphthoflavone. Up to highly toxic doses of TCE were applied to growing and stationary-phase yeast cells. In the in vivo studies two different host-mediated assays, intrasanguineous and intraperitoneal methodologies, were performed on different mice breeds treated by oral administration. Epichlorohydrin and epoxybutane were tested singly or combined in a mixture of the same ratio as in the technical grade TCE sample. Both TCE samples gave negative results for in vivo and in vitro assays, whereas the two contaminants were found mutagenic only in vitro. The high toxicity of the technical TCE sample did not allow us to reach concentrations containing effective levels of its two additives.

Mutagenic action of structurally related alkene oxides on Schizosaccharomyces pombe: the influence, 'in vitro', of mouse-liver metabolizing system

A series of aliphatic epoxides were tested for their ability to induce forward mutations in the yeast Schizosaccharomyces pombe. For all the compounds under study, a linear dose-response relationship was found, and the ranking of the relative specific activity was: epichlorohydrin greater than ethylene oxide greater than glycidol greater than 1,2-epoxybutane greater than 1,1,1-trichloropropylene oxide greater than propylene oxide greater than 2,3-epoxybutane. The influence of the metabolic conversion of the epoxides by the mouse-liver S9 fraction was also investigated. In such conditions, except for the 2,3-epoxybutane, the genetic activity of epoxides seems to be reduced.