Inhibition of mitochondrial aldehyde dehydrogenase and acetaldehyde oxidation by the glutathione-depleting agents diethylmaleate and phorone

Effect of the glutathione-depleting agents diethylmaleate, phorone and buthionine sulfoximine on biliary copper excretion in rats

The involvement of glutathione (GSH) in the biliary excretion of Cu was investigated in bile-cannulated inbred WAG/Rij and BN rats, pretreated with diethylmaleate (DEM), phorone or buthionine sulfoximine (BSO) and injected with Cu doses of 10 or 30 micrograms/100 g body wt. DEM reduced liver GSH to 27-56% and biliary GSH excretion to 18-38%; phorone reduced GSH in the liver to 55% and increased it in the bile (113%) followed by a slight decrease (79%); BSO reduced liver GSH to 50% and bile GSH to 20%. After injection of Cu to control rats a profile of biliary Cu excretion was found, composed of a slowly (SCuE) and a rapidly (RCuE) disappearing component, the latter only present after the dose of 30 micrograms Cu. DEM had no effect on SCuE after a 10 micrograms dose and a temporary effect on SCuE after a 30 micrograms dose in both WAG/Rij and BN rats. Phorone reduced SCuE after both Cu doses to 50%. Both agents abolished RCuE and reduced endogenous biliary Cu excretion to less than 50%. Release of injected Cu from plasma and uptake by the liver was inhibited by DEM and phorone in both rat strains; in BN rats basal plasma Cu level of DEM-treated rats was increased as well. BSO reduced SCuE after both Cu doses but had no influence on RCuE. Endogenous Cu excretion was reduced by BSO in BN rats but not in WAG/Rij rats. The results show that biliary Cu excretion proceeds by a pattern, the components of which can be affected differently by the various drugs. They also indicate that GSH is not directly involved in biliary Cu excretion but suggest that it may play a role in the metabolism of Cu in the liver.

Effect of glutathione depletion on aminopyrine and formaldehyde metabolism

In previous studies, diethylmaleate (DEM)- and phorone-induced hepatic glutathione (GSH) depletion in rats was accompanied by impaired evolution of 14CO2 from the N-14C-labeled methyl groups of aminopyrine, which in turn was attributed to impaired generation of formaldehyde, its subsequent oxidation to formate, or to some combination of both. In the present study, l-buthionine sulfoximine (BSO)-induced hepatic GSH depletion was also accompanied by decreased evolution of CO2 from aminopyrine, but the extent of the fall in CO2 was less than that induced by DEM or phorone, even though the decrease in hepatic GSH was comparable with all three GSH-lowering compounds. Incubation of freshly prepared normal hepatic microsomes in vitro with the GSH-lowering agents resulted in impaired aminopyrine-N-demethylase (APDM) activity with inhibition by phorone greater than DEM greater than BSO. By contrast, hepatic microsomes prepared from rats pretreated with these compounds had normal APDM activity. 14CO2 evolution from i.p. administered [14C]formaldehyde was not impaired by any of the GSH-lowering compounds. Thus, assessment of APDM activity and formaldehyde metabolism did not unequivocally establish the mechanism(s) by which CO2 evolution from aminopyrine is depressed by DEM, phorone and BSO, although low GSH is likely to impair metabolism of formaldehyde formed in liver after demethylation of aminopyrine. Quantitative differences in the degree of depression of CO2 evolution suggest that at least DEM and phorone exert an additional inhibitory effect by a GSH-independent mechanism. This may involve inhibition of aminopyrine-N-demethylase activity.

Depletion of hepatic glutathione by ethanol occurs independently of ethanol metabolism

The mechanism of ethanol-induced depletion of hepatic glutathione (GSH) was studied in vivo and in isolated hepatocytes. Neither inhibition of ethanol metabolism with 4-methylpyrazole, nor a 10-fold elevation of acetaldehyde levels by inhibition of aldehyde dehydrogenase with cyanamide or disulfiram, affected the magnitude of the GSH depletion observed in vivo. The rate of intracellular GSH accumulation by isolated hepatocytes incubated with cysteine or methionine was not inhibited by the addition of 80 mM ethanol. A significantly decreased rate of GSH accumulation was, however, found in hepatocytes isolated from ethanol-intoxicated animals. Neither the in vivo pretreatment with ethanol nor its in vitro addition to isolated hepatocytes affected the rate of GSH efflux. The results suggest that ethanol itself, rather than its metabolic products, causes depletion of liver GSH, and that events occurring in vivo are required for such an effect to be exerted.

The effect of GSH depletion on thermal radiosensitization

Depletion of intracellular glutathione (GSH) increased aerobic thermal radiosensitization in Chinese hamster ovary (CHO) cells gamma irradiated and heated at 42 degrees C. The GSH concentration was decreased to various stable levels by exposure to increasing concentrations of diethylmaleate (DEM). Analysis of dose-response curves indicated that GSH depletion affected thermal sensitization and thermal radiosensitization at 42 degrees when greater than 95% of the GSH had been depleted. GSH depletion also increased the fixation of radiation damage. For example, survival after 10 Gy decreased from 0.012 to 0.006 if CHO cells were incubated in 100 microM DEM at 37 degrees for 2 hrs after irradiation. The results show that GSH might be an important agent for the protection of cells against thermal enhancement of radiation damage.

Inhibition of the low-Km mitochondrial aldehyde dehydrogenase by diethyl maleate and phorone in vivo and in vitro. Implications for formaldehyde metabolism

Formaldehyde can be oxidized primarily by two different enzymes, the low-Km mitochondrial aldehyde dehydrogenase and the cytosolic GSH-dependent formaldehyde dehydrogenase. Experiments were carried out to evaluate the effects of diethyl maleate or phorone, agents that deplete GSH from the liver, on the oxidation of formaldehyde. The addition of diethyl maleate or phorone to intact mitochondria or to disrupted mitochondrial fractions produced inhibition of formaldehyde oxidation. The kinetics of inhibition of the low-Km mitochondrial aldehyde dehydrogenase were mixed. Mitochondria isolated from rats treated in vivo with diethyl maleate or phorone had a decreased capacity to oxidize either formaldehyde or acetaldehyde. The activity of the low-Km, but not the high-Km, mitochondrial aldehyde dehydrogenase was also inhibited. The production of CO2 plus formate from 0.2 mM-[14C]formaldehyde by isolated hepatocytes was only slightly inhibited (15-30%) by incubation with diethyl maleate or addition of cyanamide, suggesting oxidation primarily via formaldehyde dehydrogenase. However, the production of CO2 plus formate was increased 2.5-fold when the concentration of [14C]formaldehyde was raised to 1 mM. This increase in product formation at higher formaldehyde concentrations was much more sensitive to inhibition by diethyl maleate or cyanamide, suggesting an important contribution by mitochondrial aldehyde dehydrogenase. Thus diethyl maleate and phorone, besides depleting GSH, can also serve as effective inhibitors in vivo or in vitro of the low-Km mitochondrial aldehyde dehydrogenase. Inhibition of formaldehyde oxidation by these agents could be due to impairment of both enzyme systems known to be capable of oxidizing formaldehyde. It would appear that a critical amount of GSH, e.g. 90%, must be depleted before the activity of formaldehyde dehydrogenase becomes impaired.

Inhibition of CO2 production from aminopyrine or methanol by cyanamide or crotonaldehyde and the role of mitochondrial aldehyde dehydrogenase in formaldehyde oxidation

Previous results have shown that cyanamide or crotonaldehyde are effective inhibitors of the oxidation of formaldehyde by the low-Km mitochondrial aldehyde dehydrogenase, but do not affect the activity of the glutathione-dependent formaldehyde dehydrogenase. These compounds were used to evaluate the enzyme pathways responsible for the oxidation of formaldehyde generated during the metabolism of aminopyrine or methanol by isolated hepatocytes. Both cyanamide and crotonaldehyde inhibited the production of 14CO2 from 14C-labeled aminopyrine by 30-40%. These agents caused an accumulation of formaldehyde which was identical to the loss in CO2 production, indicating that the inhibition of CO2 production reflected an inhibition of formaldehyde oxidation. The oxidation of methanol was stimulated by the addition of glyoxylic acid, which increases the rate of H2O2 generation. Crotonaldehyde inhibited CO2 production from methanol, but caused a corresponding increase in formaldehyde accumulation. The partial sensitivity of CO2 production to inhibition by cyanamide or crotonaldehyde suggests that both the mitochondrial aldehyde dehydrogenase and formaldehyde dehydrogenase contribute towards the metabolism of formaldehyde which is generated from mixed-function oxidase activity or from methanol, just as both enzyme systems contribute towards the metabolism of exogenously added formaldehyde.