Aminopyrine and biphenyl metabolism in cultured hepatocytes. Induction by alcohols

Immunochemical detection of different isoenzymes of cytochrome P-450 induced in chick hepatocyte cultures

This study investigated whether the same cytochrome P-450 (P-450) isoenzymes were inducible in cultures of chick-embryo hepatocytes as in the liver of chicken embryos. We purified two isoenzymes of cytochrome P-450 from the livers of 17-day-old-chick embryos: one of molecular mass approx. 50 kDa induced in vivo by the phenobarbital-like inducer glutethimide, and the second of approx. 57 kDa induced by 3-methylcholanthrene. Rabbit antiserum against the 50 kDa protein inhibited benzphetamine demethylase activity in hepatic microsomes (microsomal fractions) from glutethimide-treated chick embryo. Antiserum to the 57 kDa protein inhibited ethoxyresorufin de-ethylase activity in hepatic microsomes from methylcholanthrene-treated chick embryo. Cultured chick hepatocytes were treated with chemicals known to induce isoenzymes of P-450 in rodent liver. The induced P-450s were quantified spectrophotometrically and characterized by immunoblotting and enzyme assays. From these studies, chemical inducers were classified into three groups: (i) chemicals that induced a P-450 isoenzyme of 50 kDa and increased benzphetamine demethylase activity: glutethimide, phenobarbital, metyrapone, mephenytoin, ethanol, isopentanol, isobutanol, lindane, lysodren; (ii) chemicals that induced a P-450 isoenzyme of 57 kDa and increased ethoxyresorufin de-ethylase activity: 3-methylcholanthrene and 3,3',4,4'-tetrachlorobiphenyl; and (iii) the mono-alpha-substituted 2,3',4,4',5-pentabromobiphenyl, which induced both proteins and both activities. The immunochemical data showed that chick-embryo hepatocytes in culture retain the inducibility of glutethimide- and methylcholanthrene-induced isoenzymes of P-450 that are inducible in the liver of the chicken embryo.

Comparison of the form(s) of cytochrome P-450 induced by ethanol and glutethimide in cultured chick hepatocytes

In this study, using a combination of immunological and enzymatic characterizations, we compared the forms of cytochrome P-450 induced by ethanol and glutethimide in primary cultures of chicken embryo hepatocytes. Recently we purified a cytochrome P-450 of 50K molecular weight from chicken embryo liver using glutethimide as a prototypic inducer. Antibodies to both this chicken cytochrome P-450 and to rabbit cytochrome P-450 form 3a from the IIE subfamily detected microsomal proteins of 50K induced by either ethanol or glutethimide in cultured chick embryo hepatocytes, indicating the antigenic homology of these subfamilies of cytochromes P-450 among different animal species. However, the antibody to glutethimide-induced chick cytochrome P-450 of 50K inhibited p-nitrophenol hydroxylase and benzphetamine demethylase activities 85-90% in microsomes from both ethanol- and glutethimide-treated cells, indicating similar epitopes whose integrity is required for catalytic activity. In contrast, antibodies to rabbit cytochrome P-450 form 3a had little to no effect on these same microsomal activities. Both ethanol and glutethimide induced microsomal p-nitrophenol and aniline hydroxylase activities in cultured chick embryo hepatocytes. In microsomes from ethanol-treated cells, the turnover of p-nitrophenol per cytochrome P-450 was 2-fold greater than that induced by glutethimide treatment, suggesting that ethanol is inducing a form of cytochrome P-450 that has greater catalytic activity with this substrate than glutethimide-induced forms. Thus, in cultured chick embryo hepatocytes, ethanol may induce cytochromes P-450 from both the IIB and IIE subfamilies.

A high-performance liquid chromatography assay for measuring integrated biphenyl metabolism by intact cells: its use with rat liver and human liver and kidney

A rapid, sensitive high-performance liquid chromatography assay with fluorescence detection for measuring biphenyl metabolism by intact cells has been developed. The assay does not require organic solvent extraction or enzymatic digestion for the measurement of hydroxybiphenyl conjugates. The lower limit of detectability for 4-hydroxybiphenyl is 5 pmol injected. Rat hepatocytes incubated with biphenyl form predominantly 4-hydroxybiphenyl sulfate with lesser amounts of 4-hydroxybiphenyl glucuronide and free hydroxybiphenyls, and small amounts of 3-hydroxybiphenyl sulfate and 3-hydroxybiphenyl glucuronide. Slices of fresh human liver incubated with biphenyl form predominantly 4-hydroxybiphenyl glucuronide with some free hydroxybiphenyl and small amounts of 4-hydroxybiphenyl sulfate. 4-Hydroxybiphenyl glucuronide formation by human liver shows a lag time that is not abolished by preincubating the liver without substrate. Human kidney slices incubated with biphenyl form 4-hydroxybiphenyl glucuronide and 4-hydroxybiphenyl sulfate at rates less than one-tenth those seen with human liver. Human kidney slices do not form detectable free hydroxybiphenyl. There is wide intersubject variability in the rates of hydroxybiphenyl metabolite formation by human liver and kidney.

Uroporphyrin accumulation produced by halogenated biphenyls in chick-embryo hepatocytes. Reversal of the accumulation by piperonyl butoxide

Cultures of chick-embryo hepatocytes were used to study the mechanism by which 3,4,3',4'-tetrachlorobiphenyl and 2,4,5,3',4'-pentabromobiphenyl cause accumulation of uroporphyrin. In a previous paper, an isoenzyme of cytochrome P-450 induced by 3-methylcholanthrene had been implicated in this process [Sinclair, Bement, Bonkovsky & Sinclair (1984) Biochem. J. 222, 737-748]. Cells treated with 3,4,3',4'-tetrachlorobiphenyl and 5-aminolaevulinate accumulated uroporphyrin and heptacarboxyporphyrin, whereas similarly treated cells accumulated protoporphyrin immediately after piperonyl butoxide was added. Piperonyl butoxide also restored haem synthesis as detected by incorporation of radioactive 5-aminolaevulinate into haem, and decrease in drug-induced 5-aminolaevulinate synthase activity. The restoration of synthesis of protoporphyrin and haem by piperonyl butoxide was not affected by addition of cycloheximide, indicating recovery was probably not due to protein synthesis de novo. Piperonyl butoxide also reversed uroporphyrin accumulation caused by 3,4,5,3',4',5'-hexachlorobiphenyl, mixtures of other halogenated biphenyls, lindane, parathion, nifedipine and verapamil. The effect of piperonyl butoxide was probably not due to inhibition of metabolism of these compounds, since the hexachlorobiphenyl was scarcely metabolized. Other methylenedioxyphenyl compounds, as well as ellipticine and acetylaminofluorene, also reversed the uroporphyrin accumulation caused by 3,4,3',4'-tetrachlorobiphenyl. SKF-525A (2-dimethylaminoethyl-2,2-diphenyl valerate) did not reverse the uroporphyrin accumulation caused by the halogenated biphenyls, but did reverse that caused by phenobarbital and propylisopropylacetamide. We conclude that the mechanism of the uroporphyrin accumulation cannot be due to covalent binding of activated metabolites of halogenated compounds to uroporphyrinogen decarboxylase.

Induction of 5-aminolaevulinate synthase by two- to five-carbon alcohols in cultured chick-embryo hepatocytes. Relationship to induction of cytochrome P-450

The induction of 5-aminolaevulinate synthase and of cytochrome P-450 by short-chain aliphatic alcohols was compared in primary cultures of chicken-embryo hepatocytes. Isopropyl alcohol, isobutanol, pentan-1-ol and isopentanol alone caused up to a 4-fold increase in 5-aminolaevulinate synthase, whereas ethanol and propan-1-ol did not. Induction of the synthase by isopentanol was maximal at 8 h, and reached a plateau thereafter, whereas the activity induced by 2-propyl-2-isopropylacetamide continued to increase for 20 h. In the presence of 3,4,3',4'-tetrachlorobiphenyl, an inhibitor of haem synthesis at the uroporphyrinogen decarboxylase step, synergistic induction of 5-aminolaevulinate synthase was observed with all the alcohols except ethanol. Ethanol, but not isopentanol, decreased the extent of induction of 5-aminolaevulinate synthase by 2-propyl-2-isopropylacetamide and 3,4,3',4'-tetrachlorobiphenyl (50% decrease at 112 mM-ethanol). Total protein synthesis was not inhibited by ethanol in these cells. The composition of porphyrins was determined after treatment of cells with ethanol, isopentanol or 2-propyl-2-isopropylacetamide. Untreated cells, when incubated with 5-aminolaevulinate for 6 h, accumulated mainly protoporphyrin. However, when cells were pretreated with ethanol, isopentanol or 2-propyl-2-isopropylacetamide for 20 h, and 5-aminolaevulinate was added, 8- and 7-carboxyporphyrins increased, whereas protoporphyrin decreased. The dose responses for induction of either 5-aminolaevulinate synthase or cytochrome P-450 after a 20 h exposure to 3- to 5-carbon alcohols were identical. The results indicate that: simple alcohols can induce both enzymes; hydrophobicity increases their effectiveness; and induction of both enzymes are probably mediated by a common mechanism.

Modification of chromium(VI)-induced DNA damage by glutathione and cytochromes P-450 in chicken embryo hepatocytes

The role of glutathione and cytochrome P-450 in the production of DNA damage by chromium(VI) was examined in chicken embryo hepatocytes by the alkaline elution technique. Cellular levels of glutathione and cytochrome P-450 were altered by treating the hepatocytes with N-acetyl-L-cysteine, buthionine sulfoximine, isopentanol, or beta-naphthoflavone. A dramatic increase in chromium(VI)-induced DNA strand breaks was observed after increasing glutathione levels in the cells. Chromium(VI)-induced DNA strand breaks were even more numerous when the level of cytochrome P-450 was also increased. Upon depletion of glutathione levels and induction of cytochrome P-450 or cytochrome P-448, little or no DNA strand breaks or DNA interstrand cross-links were observed after chromium(VI) treatment. Chromium(VI)-induced DNA-protein cross-links generally decreased after either increases or decreases in cellular levels of glutathione or cytochrome P-450 or P-448. These results suggest that glutathione enhances chromium(VI)-induced DNA damage through metabolic activation of chromium(VI). The possible production of reactive chromium species upon metabolism by glutathione and cytochrome P-450 or P-448 and their involvement in DNA damage is discussed.