Inactivation of rat liver cytochrome P450 (P450) by N,N-dimethylformamide and N,N-dimethylacetamide

N,N-dimethylformamide (DMF), an organic solvent widely used in industry, is bioactivated by cytochrome P450 (P450) to reactive metabolites which are believed to be responsible for the hepatotoxicity observed in animals and humans. A decrease of the activating enzyme has been reported in rats treated with DMF, although the specific P450 isoform(s) involved and the nature of the reactive species responsible for this and the other toxic effects are still being investigated. In the present work, the effect of DMF and of the structurally related N,N-dimethylacetamide (DMAc) on the activating enzyme and the nature of the reactive species involved in the mechanism of P450 inactivation by the two chemicals were investigated in vitro. Incubation of liver microsomes from pyridine-induced rats with either substrate resulted in a dose-dependent (0-20 mM) loss of P450 (up to 28 and 24% for DMF and DMAc, respectively), microsomal haem (up to 24 and 20% for DMF and DMAc, respectively), but not protoporphyrin IX content. Moreover, bubbling of CO through the incubation mixture gave almost complete protection against substrate-dependent P450 inactivation, and the spin trapping agent N-tert-butyl-alpha-phenylnitrone, but neither glutathione nor vitamin C, provided a significant protection against DMF- or DMAc-dependent haem loss. Finally, electron spin resonance analysis of microsomal incubations in presence of DMF or DMAc showed spectral evidence for a carbon centered radical intermediate. The results indicate, overall, that both compounds are metabolized in vitro by P450, probably CYP2E1, to free radical metabolites which attack the haem prosthetic group, leading to suicidal enzyme inactivation.

Bioactivation and toxicity in vitro of HCFC-123 and HCFC-141b: role of cytochrome P450

The bioactivation and cytotoxicity in vitro of 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123) and 1,1-dichloro-1-fluoroethane (HCFC-141b), two replacements for some ozone-depleting chlorofluorocarbons (CFC), were investigated in rat liver microsomes and isolated rat hepatocytes. Both compounds were activated by cytochrome P450 to reactive metabolites, as indicated by: (i) the depletion of exogenous and cellular glutathione, (ii) the increased LDH release from hepatocytes, (iii) the loss of microsomal P450 content and activities, and (iv) the formation of free radical species observed in the presence of the two compounds. Moreover, the formation of two stable metabolites and an increased production of conjugated dienes, a marker of lipid peroxidation, were observed for both HCFC-123 and HCFC-141b. The biotransformation of both compounds by pyridine- and phenobarbital-induced rat liver microsomes and the inhibition of LDH release by 4-methylpyrazole and troleandomycin indicate that P450 2E1, 2B and, possibly, also 3A are the isoforms involved in the bioactivation and toxicity of HCFC-123 and HCFC-141b in the rat.

Bioactivation and cytotoxicity of 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123) in isolated rat hepatocytes

The bioactivation and cytotoxicity of 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123), a replacement for some ozone-depleting chlorofluorocarbons, were investigated using freshly isolated hepatocytes from non-induced male rats. A time- and concentration-dependent increase in the leakage of lactate dehydrogenase and a concentration-dependent loss of total cellular glutathione were observed in cells incubated with 1, 5 and 10 mM HCFC-123 under normoxic or hypoxic (about 4% O2) conditions. Lactate dehydrogenase leakage was completely prevented by pretreating the cell suspension with the free radical trapper N-t-butyl-alpha-phenylnitrone. The aspecific cytochrome P450 (P450) inhibitor, metyrapone, totally prevented the lactate dehydrogenase leakage from hepatocytes, while two isoform-specific P450 inhibitors, 4-methylpyrazole and troleandomycin (a P450 2E1 and a P450 3A inhibitor, respectively), provided a partial protection against HCFC-123 cytotoxicity. Interestingly, pretreatment of cells with glutathione depletors, such as phorone and diethylmaleate, did not enhance the HCFC-123-dependent lactate dehydrogenase leakage. Two stable metabolites of HCFC-123, 1-chloro-2,2,2-trifluoroethane and 1-chloro-2,2-difluoroethene, were detected by gas chromatography/mass spectrometry analysis of the head space of the hepatocyte incubations carried out under hypoxic and, although at a lower level, also normoxic conditions, indicating that reductive metabolism of HCFC-123 by hepatocytes had occurred. The results overall indicate that HCFC-123 is cytotoxic to rat hepatocytes under both normoxic and hypoxic conditions, due to its bioactivation to reactive metabolites, probably free radicals, and that P450 2E1 and, to a lower extent, P450 3A, are involved in the process.

Bioactivation to free radicals and cytotoxicity of 1,1-dichloro-1-fluoroethane (HCFC-141b)

1. The in vitro bioactivation by rat liver microsomes and the cytotoxicity in rat hepatocytes of 1,1-dichloro-1-fluoroethane (HCFC-141b), a replacement for some ozone depleting chlorofluorocarbons (CFC), have been investigated. 2. Anaerobic incubations of liver microsomes from pyridine-induced rats with HCFC-141b in the presence of the spin-trapping agent N-t-butyl-alpha-phenylnitrone (PBN) resulted in the formation of a typical ESR radical signal. 3. In the presence of HCFC-141b, a dose-dependent formation of conjugated dienes was observed that was partially inhibited by PBN, glutathione (GSH) and vitamin C. Moreover, HCFC-141b increased the release of lactate dehydrogenase (LDH) and the depletion of cellular glutathione in isolated rat hepatocytes under both normoxic and hypoxic conditions. 4. HCFC-141b-dependent cytotoxicity was completely prevented by PBN under both conditions and it was partially prevented under normoxic conditions by the broad-spectrum P450 inhibitor metyrapone, the P4502E1 specific inhibitor 4-methylpyrazole and the P4503A-specific inhibitor troleandomycin. Interestingly, HCFC-141b-dependent glutathione depletion was not prevented by PBN, metyrapone, 4-methylpyrazole or troleandomycin, whereas two glutathione depletors, 2,6-dimethyl-2,5-heptadien-4-one (phorone) and diethylmaleate, partially prevented LDH release. 5. The present results indicate that HCFC-141b is reductively metabolized in vitro to free radical intermediates by P450, in particular by the CYP2E1 and, to a lower extent, CYP3A isoforms, leading to peroxidative membrane damage and glutathione-independent cytotoxicity.