Modulation of mitomycin C resistance by glutathione transferase inhibitor ethacrynic acid

Enhanced cytotoxicity of bioreductive antitumor agents with dimethyl fumarate in human glioblastoma cells

We compared the cytotoxicity of the bioreductive antitumor agents mitomycin C (MMC) and streptonigrin (SN) with or without the DT-diaphorase (DTD) inducer dimethyl fumarate (DMF) in four human glioblastoma cell lines with the conventional chemotherapeutic agent, 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). We also examined four other types of cancer cells to compare with glioblastoma cells. Cytotoxicity was measured with the sulforhodamine B (SRB) assay and was represented by 50% inhibition concentration (IC50). Enzymatic activities of DTD, cytochrome b5 reductase and glutathione-S-transferase (GST) in cells were measured spectrophotometrically. IC50 for BCNU was in a range of 28-300 microM in the glioblastoma cell lines. Glioblastoma cells were more sensitive to MMC or SN than to BCNU. Pretreatment with DMF significantly increased cytotoxicity of MMC and SN in glioblastoma cell lines and the NCI-H1299 lung cancer cell line, but had no effect on BCNU cytotoxicity. DMF significantly increased DTD and cytochrome b5 reductase activity, and decreased GST in three of four glioblastoma cell lines. Addition of the DTD inhibitor, dicumarol, significantly inhibited cytotoxicity of MMC and SN, and reversed the increased cytotoxicity seen when DMF was combined with either MMC or SN in all glioblastoma cell lines. Combining inducers of DTD and cytochrome b5 reductase with bioreductive agents may be a potential therapeutic strategy for glioblastoma.

Non-cross resistance of ZD0473 in acquired cisplatin-resistant lung cancer cell lines

ZD0473 is a new generation platinum agent that, in preclinical studies, shows evidence of an extended spectrum of anti-tumor activity and overcomes platinum resistance mechanisms. The drug contains a bulky methylpyridine ligand at its platinum center, which is responsible for its ability to overcome platinum resistance. We examined the growth inhibitory effects of ZD0473 in human lung cancer cell lines resistant to cisplatin in vitro. Four cisplatin resistant human lung cancer cell lines (PC-14/CDDP, SBC-3/CDDP, PC-9/CDDP, H69/CDDP) showed the expected resistance to cisplatin but were non-cross, or much less, resistant to ZD0473, as determined by an MTT assay. A reduction in the intracellular accumulation of cisplatin, but not of ZD0473, was observed in the PC-14/CDDP cells compared with the levels in PC-14 parental cells. The reduction in cisplatin accumulation is considered a major mechanism of the acquired cisplatin resistance in PC-14/CDDP cells. Therefore, the increase in platinum accumulation is considered a possible mechanism underlying the activity of ZD0473 in cisplatin-resistant cells. Glutathione-mediated resistance to cisplatin was also overcome by ZD0473 in PC-14/CDDP cells. In addition, we showed that the intraperitoneal administration of ZD0473 at its maximum tolerable dose in mice produced a marked in vivo antitumor activity against cisplatin-resistant PC-14/CDDP tumors. These results suggest that ZD0473 may be a potent agent in human lung cancer cells with multifactorial cisplatin resistance.

Expression of paclitaxel-inactivating CYP3A activity in human colorectal cancer: implications for drug therapy

Cytochrome P450 3A is a drug-metabolising enzyme activity due to CYP3A4 and CYP3A5 gene products, that is involved in the inactivation of anticancer drugs. This study analyses the potential of cytochrome P450 3A enzyme in human colorectal cancer to impact anticancer therapy with drugs that are cytochrome P450 3A substrates. Enzyme activity, variability and properties, and the ability to inactivate paclitaxel (taxol) were analysed in human colorectal cancer and healthy colorectal epithelium. Cytochrome P450 3A enzyme activity is present in healthy and tumoral samples, with a nearly 10-fold interindividual variability. Nifedipine oxidation activity+/-s.d. for colorectal cancer microsomes was 67.8+/-36.6 pmol min(-1) mg(-1). The K(m) of the tumoral enzyme (42+/-8 microM) is similar to that in healthy colorectal epithelium (36+/-8 microM) and the human liver enzyme. Colorectal cancer microsomes metabolised the anticancer drug paclitaxel with a mean activity was 3.1+/-1.2 pmol min(-1) mg(-1). The main metabolic pathway is carried out by cytochrome P450 3A, and it is inhibited by the cytochrome P450 3A-specific inhibitor ketoconazole with a K(I) value of 31 nM. This study demonstrates the occurrence of cytochrome P450 3A-dependent metabolism in colorectal cancer tissue. The metabolic activity confers to cancer cells the ability to inactivate cytochrome P450 3A substrates and may modulate tumour sensitivity to anticancer drugs.

Nitrotriazole AK-2123 enhances mitomycin C activity in mice bearing multidrug-resistant tumors

The often observed cross resistance of multidrug-resistant (MDR) tumors to mitomycin C (MMC) is surprising, as these tumors are, as a rule, sensitive to alkylating drugs, and the mechanism of MMC activity is connected to alkylation of DNA. This study shows that nitrotriazole AK-2123 significantly enhances the sensitivity of MDR-strains of P388 mouse leukemia (developed and characterized by authors previously) to mitomycin C. The modulating effect is dependent on the initial sensitivity of resistant tumors to MMC which is correlated with the existence or absence of sorcin (cytosole Ca2+-binding protein) gene coamplification in MDR-amplicon. In agreement with authors' previous data about AK-2123 influence on active Ca2+-transport, it is supposed that the modulatory effect of radiosensitizer is at least partially dependent on this capacity. AK-2123 has no own antitumor effect on investigated tumors and cannot modify the sensitivity of the parent tumor P388 to MMC.

P-glycoprotein-expressing tumor cells are resistant to anticancer drugs in human gastrointestinal cancer

The resistance to doxorubicin (DOX) by some tumor cells is mainly due to the effect of P-glycoprotein encoded by the multidrug resistance-1 (mdr1) gene. We tried to prove the correlations between P-glycoprotein expression and the sensitivity for anticancer drugs including DOX and other cytotoxic drugs that are currently used for gastrointestinal cancer patients. We quantified the P-glycoprotein expression by flow cytometry techniques, and the sensitivity for anticancer drugs using a tetrazolium salt, 3-(4,5-di-methylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT), assay in highly purified fresh human tumor cells obtained from 25 cancer patients. The inhibition rates were the lowest in DOX and mitomycin C (MMC), compared with other drugs. The most significant correlation between DOX and MMC was seen in the inhibition rates. A significant correlation was also seen between the inhibition rates for DOX and P-glycoprotein expression, whereas only a slight correlation between the sensitivity for MMC and P-glycoprotein expression was observed. We should therefore pay close attention to the effect of P-glycoprotein when treating cancer patients, especially if both the inhibition rates of DOX and MMC are low based on the findings of an MTT assay.

Regulatory network of mitomycin C action in human colon cancer cells

A network composed of activation and inactivation pathways to regulate mitomycin C (MMC) action is suggested to exist in human cancer cells. COLO201 colon cancer cells were stably transfected with human NQO1 cDNA that encodes NAD(P)H:quinone oxidoreductase (DT-diaphorase, DTD), and a clonal cell line with about 57-fold elevated DTD activity was obtained. Northern analysis revealed that expression of the NADPH:cytochrome P450 reductase (P450 reductase) gene was decreased in the transfectant, COLO201/NQO1, associated with the increase of NQO1 expression. Biochemical characterization of the cells showed a significant increase of the glutathione (GSH) content concomitantly with the decrease of the P450 reductase activity. As a result of these coordinated modulations, sensitivity of COLO201/NQO1 to MMC was not increased as compared to the parent cells. Analyses of inhibition by specific inhibitors of DTD, P450 reductase and glutathione S-transferase (GST) in 5 human colon cancer cell lines including the transfectant showed that DTD and P450 reductase play significant roles in MMC activation in cells with sufficiently high DTD activity and with marginal DTD activity, respectively. In contrast, GST appeared to participate in MMC inactivation in cells with a high level of GST activity. These results indicated that DTD, P450 reductase, GSH and GST may act together compensatively or competitively, depending on their levels in cells, to determine the cellular sensitivity to MMC.

Modulation of mitomycin C-induced sister chromatid exchanges and cell cycle delay by buthionine sulfoximine and reduced glutathione in mouse bone marrow cells in vivo

Mitomycin C (MMC) is a bifunctional alkylating agent and a chemotherapeutic drug, covalently binds to the DNA of cells and produces monoadducts and DNA-DNA, DNA-protein crosslinks, induces damages at chromosomal level and slows down the rate of cell proliferation. Reduced glutathione (GSH), a major non-protein thiol substance plays an important role in detoxification of cells against the toxic effect of exogenous compounds. In order to understand the role of factor which affects MMC sensitivity, we have made an attempt to establish a relationship between MMC-induced DNA damages and the endogenous GSH-status of the cells. MMC was treated to normal and buthionine sulfoximine (BSO; GSH-depleting agent)-treated mice. Exogenous GSH was also added to MMC-treated normal mice. Cells were fixed at 24 h and sister chromatid exchanges (SCEs) and average generation time (AGT) were scored. MMC-induced SCEs and cell cycle delay significantly with respect to control and the frequency of SCEs was increased considerably while MMC treatment combined with either GSH or BSO. The induction of cell cycle delay by MMC was reduced significantly when GSH or BSO was present along with MMC. These observations indicate that the factor responsible for inducing delay in cell cycle after MMC treatment may not be relevant for SCE-induction.

Haloenol lactone: a new synergist of chemotherapy in vitro

Over-expression of glutathione S-transferases (GST) has been found to play a significant role in multiple drug resistance in cancer chemotherapy. To combat GST-mediated drug resistance, GST inhibitors are being studied as potential synergists for effective cancer chemotherapy. We have designed and synthesized a haloenol lactone derivative as a mechanism-based inactivator of GST-pi isozyme. In the current study, we examined the inhibitory effect of the haloenol lactone compound on GST of a human renal carcinoma cell line UOK130 and found that this compound shows time-dependent GST inhibition in these cancer cells. The enzyme activity lost upon incubation with the haloenol lactone could not be restored by extensive dialysis against buffer. Pretreatment of the cancer cells with 1.0 microM of haloenol lactone increased cytotoxicity induced by cisplatin in the UOK130 cell line. This report further supports the possibility of synergizing alkylating agents in cancer chemotherapy by use of selective GST inhibitors.

Molecular targeting of mitomycin C chemotherapy

In 10 human cancer cell lines, the activity of mitomycin C (MMC) was found to be determined by an interplay between activation by DT-diaphorase (DTD) and inactivation by glutathione S-transferase (GST). NADPH/cytochrome P-450 reductase was not responsible for MMC activation and expression of MDRI (Mr 170,000 P-glycoprotein), and MRP (multidrug resistance-associated protein) genes did not relate to MMC resistance. Gene expression analysis for NQO1 (DTD gene) and GSTpi predicted which enzyme activity predominated in a cell line, except K562 and K562/DOX. For tumors with DTD activity only, MMC given by itself was most active. In cell lines in which DTD action was predominant, tumor selectivity was achieved by enhancing DTD-mediated activation with m-iodobenzylguanidine and hyperglycemia, which reduced the intra-tumoral pH. KW2149, a novel MMC analogue activated by glutathione, was most active against tumors in which GSTpi predominated. These various enzyme-specific effects could be observed even in cell lines derived from tumors with multidrug resistance. Such MMC treatment based on cell enzymology may enhance significantly MMC efficacy, helping to overcome multidrug resistance.

Purification and characterization of class alpha and Mu glutathione S-transferases from porcine liver

Six cytosolic GSTs from porcine liver were purified by a combination of glutathione affinity chromatography and ion-exchange HPLC. The isoenzymes were characterized by SDS-PAGE, gel filtration, isoelectric focusing, immunoblotting analysis and determination of substrate specificities and inhibition characteristics. The purified GSTs belong to the alpha and mu classes, respectively. No class pi isoenzyme was isolated or detected. The class alpha GST pA1-1* exists as a homodimer (M(r) = 25.3 kDa), whereas GST pA2-3* consists of two subunits with different M(r) values (27.0 and 25.3 kDa). The estimated pI values were 9.5 and 8.8, respectively. Furthermore, four class mu porcine GSTs, pM1-1*, pM1-2*, pM3-?* and pM4-?*, were isolated. The isoenzyme pM1-1* possesses a relative molecular mass of 27.2 kDa and a pI value of 6.2. Additional pM1 isoenzymes hybridize with the subunit pM2* (M(r) = 25.2) to furnish a heterodimer, which shows a pI value of 5.8. The other class mu isoenzymes are heterodimers with pI values of 5.45 and 5.05. Substrate specificities and inhibition characteristics correlate very well with those of the corresponding human isoenzymes. The results are discussed with regard to the usefulness of porcine GSTs as an in vitro testing model.

Establishment by adriamycin exposure of multidrug-resistant rat ascites hepatoma AH130 cells showing low DT-diaphorase activity and high cross resistance to mitomycins

A resistant subline (AH130/5A) selected from rat hepatoma AH130 cells after exposure to adriamycin (ADM) showed remarkable resistance to multiple antitumor drugs, including mitomycin C (MMC) and porfiromycin (PFM). PFM, vinblastine (VLB), and ADM accumulated in AH130/5A far less than in the parent AH130 (AH130/P) cells. AH130/5A cells showed overexpression of P-glycoprotein (PGP), an increase in glutathione S-transferase activity, and a decrease in DT-diaphorase and glutathione peroxidase activity. The resistance to MMC and VLB of AH130/5A cells was partly reversed by H-87, an inhibitor of PGP. Buthionine sulfoximine, an inhibitor of glutathione synthase, did not affect the action of MMC. tert-Butylhydroquinone induced DT-diaphorase activity, increased PFM uptake, and enhanced the growth-inhibitory action of MMC in AH130/5A cells. Dicumarol, an inhibitor of DT-diaphorase, decreased PFM uptake and reduced the growth-inhibitory action of MMC in AH130/P cells. These results indicated that the adriamycin treatment of hepatoma cells caused multifactorial multidrug resistance involving a decrease in DT-diaphorase activity.

Inhibition of glutathione S-transferase activity in human melanoma cells by alpha,beta-unsaturated carbonyl derivatives. Effects of acrolein, cinnamaldehyde, citral, crotonaldehyde, curcumin, ethacrynic acid, and trans-2-hexenal

The glutathione S-transferase (GST) activity towards 1-chloro-2,4-dinitrobenzene in intact human IGR-39 melanoma cells was determined by the quantification by HPLC-analysis of the excreted glutathione (GSH) conjugate (S-(2,4-dinitrophenyl)glutathione; DNPSG). The major GST subunit expressed in these melanoma cells is the pi-class GST subunit P1. Using this system, the effect of exposure for 1 h to a series of alpha, beta-unsaturated carbonyl compounds at non-toxic concentrations was studied. Curcumin was the most potent inhibitor (96% inhibition at 25 microM), while 67 and 61% inhibition at 25 microM was observed for ethacrynic acid and trans-2-hexenal, respectively. Moderate inhibition was observed for cinnamaldehyde and crotonaldehyde, while no inhibition was found for citral. The reactive acrolein did not inhibit the DNPSG-excretion at 2.5 microM, the highest non-toxic concentration. Up to about 50% GSH-depletion was found after treatment with crotonaldehyde, curcumin and ethacrynic acid, however the consequences for GST conjugation are presumably small. Reversible inhibition of GST was the major mechanism of inhibition of DNPSG-excretion in melanoma cells, except in the cases of curcumin and ethacrynic acid, which compounds also inactivated GSTP1-1 by covalent modification. This was clear from the fact that depending on the dose between 30 and 80% inhibition was still observed after lysis of the cells, under which conditions reversible inhibition was is absent. Intracellular levels of DNPSG remained relatively high in the case of ethacrynic acid. It is possible that ethacrynic acid also inhibits the transport of DNPSG by inhibition of the multidrug resistance-associated protein gene encoding glutathione conjugate export pump (MRP/GS-X pump) in some way.

Biochemical characterization of a mitomycin C-resistant human bladder cancer cell line

This study describes characteristics of a mitomycin C (MMC)-resistant human bladder cancer cell line, J82/MMC-2, which was established by repeated in vitro exposures of a 6-fold MMC-resistant variant (J82/MMC) to 18 nM MMC. A 9.6-fold higher concentration of MMC was required to kill 50% of the J82/MMC-2 sub-line compared with parental cells (J82/WT). NADPH cytochrome P450 reductase and DT-diaphorase activities were significantly lower in J82/MMC-2 cells compared with J82/WT, suggesting that reduced sensitivity of J82/MMC-2 cells to MMC resulted from impaired drug activation. Consistent with this hypothesis, the formation of MMC-alkylating metabolites was significantly lower in J82/MMC-2 cells compared with J82/WT. Furthermore, DT-diaphorase activity in J82/MMC-2 cells was significantly lower compared with the 6-fold MMC-resistant variant. Glutathione (GSH) levels were comparable in all 3 cell lines. Although GSH transferase (GST) activity was significantly higher in the J82/MMC-2 cells compared with J82/WT, this enzyme activity did not differ between 6- and 9.6-fold MMC-resistant variants. Whereas DNA polymerase alpha mRNA expression was comparable in these cell lines, levels of DNA ligase I mRNA were slightly lower in both MMC-resistant variants relative to J82/WT. However, the DNA polymerase beta mRNA level was markedly higher in the J82/MMC-2 cell line compared with either J82/WT or J82/MMC. Thus, emergence of a higher level of resistance to MMC in J82/MMC-2 cells compared with J82/MMC may be attributed to (i) impaired drug activation through further reduction in DT-diaphorase activity and (ii) enhanced DNA repair through over-expression of DNA polymerase beta.

Modulation of sensitivity to mitomycin C and a dithiol analogue by tempol in non-small-cell lung cancer cell lines under hypoxia

We examined the mechanisms involved in the bioactivation of mitomycin C (MMC) and a newly developed MMC analogue: 7-N-(2-([2-(gamma-L-glutamylamino)ethyl]dithio)ethyl)mitomycin C, KW-2149, in non-small-cell lung cancer (NSCLC) cell lines under aerobic and hypoxic conditions. To investigate these mechanisms, we used MMC-resistant non-small-cell lung cancer cell lines (PC-9/MC4) that had been established in our laboratory from the parent PC-9 cell line by continuous exposure to MMC. We previously reported that the MMC-resistant cell line (PC-9/MC4) was poor in NAD(P)H dehydrogenase (quinone) activity and approximately 6-fold more resistant than the parent cells (PC-9) to MMC on 2-h exposure under aerobic conditions. In this study, the subline PC-9/MC4 was 6.7-fold more resistant to MMC than PC-9, the parent cell line, under aerobic conditions, and 5.2-fold more resistant under hypoxic conditions after 2-h exposure to MMC. However, on co-incubation with tempol, an inhibitor of the one-electron reduction pathway, the sensitivity of PC-9/MC4 to MMC was impaired under hypoxic conditions, but the impairment was not evident under aerobic conditions. KW-2149, the newly developed MMC analogue, was cytotoxic for both PC-9/MC4 and PC-9 cells, and the sensitivity of both cell lines to KW-2149 was not changed by exposure to hypoxic conditions or by coincubation with tempol. There were no significant differences in the intracellular uptake of MMC and the activities of cytosolic detoxification enzymes between the PC-9 and PC-9/MC4 cell lines. These results support the hypothesis that the one-electron reduction pathway plays a partial role in the bioactivation of MMC, but not of KW-2149, and that KW-2149 is excellent at circumventing resistance to MMC in NSCLC.

Characterization of a human bladder cancer cell line selected for resistance to BMY 25067, a novel analogue of mitomycin C

This study describes characteristics of a human bladder cancer cell line, SCaBER/R, selected for resistance to a mitomycin C (MMC) analogue BMY 25067. The SCaBER/R cell line was isolated by repeated 24 h exposures of the parental cells to 0.09 microM BMY 25067 (IC90, 24 h drug exposure) over a period of about 180 days. Approximately 2.2-fold higher concentration of BMY 25067 was required to kill 50% of the SCaBER/R cell line compared with parental cells (p < 0.001). The IC20 and IC90 values for BMY 25067 were also significantly higher in the SCaBER/R cell line than in SCaBER. Unlike most MMC resistant cell lines, the SCaBER/R cell line displayed a marked cross-resistance to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and lacked cross-resistance to cisplatin, doxorubicin or VP-16. The SCaBER/R cell line also displayed a marked cross-resistance to the parent drug (MMC) and BMY 25282, another analogue of MMC. NADPH cytochrome P450 reductase activity, an enzyme implicated in bio-reductive activation of MMC, did not differ significantly in these cells. DT-diaphorase activity, another MMC activation enzyme, was significantly lower in the SCaBER/R cell line when compared to the SCaBER cells. These results suggest that relatively lower sensitivity of SCaBER/R cell line to MMC and BMY 25067 may result from impaired drug activation. Cellular levels of glutathione (GSH) and GSH-transferase (GST), which have been suggested to affect the cytotoxicity of MMC, were comparable in SCaBER and SCaBER/R cell lines. BMY 25067 induced DNA interstrand cross-links (DNA-ISC) could not be detected in either of the cell lines even at drug concentrations which produced a significant cell kill. These findings suggest that (a) cellular resistance to BMY 25067 in the SCaBER/R cell line may be due to impaired drug activation, and (b) the nature of the cytotoxic produced by BMY 25067 may be different from that of MMC.

Encapsulation of mitomycin C in albumin microspheres markedly alters pharmacokinetics, drug quinone reduction in tumour tissue and antitumour activity. Implications for the drugs' in vivo mechanism of action

Pharmacokinetics and metabolism of mitomycin C (MMC) have been studied in NMRI mice bearing MAC 16 colon adenocarcinoma after direct intratumoural injection of either 500 micrograms free MMC or the same dose incorporated in albumin microspheres. Microspheres produced a tumour pharmacokinetic profile of steady state drug levels, avoiding the much higher early peak (20.5 micrograms/tumour vs 98.9 micrograms/tumour) and lower trough of free MMC, and reducing significantly the levels of drug reaching the systemic circulation (AUC 1.8 micrograms/mL x hr for microspheres vs 6.8 micrograms/mL x hr for free drug). 2,7-Diaminomitosene (2,7-DM), a key intermediate in MMC quinone bioreduction, was used as an indicator of drug metabolic activation in tumour tissue. Peak levels were 10-fold higher (11.2 micrograms/tumour vs 1.1 micrograms/tumour) and area under the curve 5-fold higher after free drug. Even taking into account differences in tumour pharmacokinetic profiles of the parent drug, microspheres actively inhibited 2,7-DM formation 3-fold. However, the microspheres generated a completely different pattern of drug metabolism where four previously uncharacterized mitosane metabolites and elevated levels of cis and trans 1-hydroxy 2,7-diaminomitosene were detected. Despite similar parent drug exposure in tumours, free drug was significantly more active (P < 0.05, Student's t-test) against MAC 16. These results suggest that formation of 2,7-DM correlates more closely with antitumour activity than sustained parent drug levels or appearance of other key metabolites. Potentially, they provide the first direct evidence for an in vivo mechanism of action dependent on bioreductive activation and formation of 2,7-DM.

Mechanism of differential sensitivity of human bladder cancer cells to mitomycin C and its analogue

This study was undertaken to elucidate the mechanism(s) of differential sensitivity of human bladder cancer cell lines J82 and SCaBER to mitomycin C (MMC) and its analogue, BMY 25067. The IC50 values for MMC and BMY 25067 in the SCaBER cell line were respectively 5- and 4-fold higher than in J82. BMY 25282 and BMY 25067 were significantly more cytotoxic, on a molar basis, than MMC in both the cell lines. NADPH cytochrome P450 reductase and DT diaphorase activities were significantly higher in the J82 cell line than in SCaBER, suggesting that relatively lower sensitivity of the SCaBER cell line to MMC and BMY 25067 may be due to deficient drug activation. This conclusion was supported by the observation that IC50 values for BMY 25282, which has lower quinone reduction potential than MMC and BMY 25067, did not differ significantly in these cell lines. A correlation between drug sensitivity, oxyradical formation and levels of antioxidative enzymes was not observed. These results suggest that the relatively lower sensitivity of SCaBER cells to MMC or BMY 25067 may be independent of differential oxyradical formation. MMC-induced DNA interstrand cross-link (ISC) formation was markedly lower in the SCaBER cell line than in J82. However, it remains to be seen if the reduced ISC frequency in the SCaBER cell line is a consequence of deficient drug activation or results from increased repair of the damaged DNA.

Ethacrynic acid and its glutathione conjugate as inhibitors of glutathione S-transferases

1. The diuretic drug ethacrynic acid (EA) is a potent reversible inhibitor of rat and human glutathione S-transferases (GST), with I50-values (microM) of 4.6-6.0, 0.3-1.9 and 3.3-4.8 for alpha, mu and pi-class, respectively. 2. The reversible inhibition by the glutathione conjugate of EA is even stronger for alpha and mu-class, with I50-values (microM) of 0.8-2.8 and < 0.1-1.2, respectively, while the I50 for the pi-class is 11. 3. Inhibition of rat and human pi-class GST also occurs by covalent binding of ethacrynic acid. 14C-ethacrynic acid, 0.8 nmol EA per nmol pi-class GST could be incorporated, resulting in 65-93% inhibition of the catalytic activity. 4. Owing to the chemical nature of the covalent binding (Michael addition), this reaction should be reversible. Indeed, full restoration of the catalytic activity of GST P1-1 inactivated by covalently-bound EA was reached in about 125 h by incubation with an excess of glutathione. 5. EA has been used to inhibit GST in biological systems. The reversible covalent binding may very well play a role in the observed inhibition of GST by EA in vivo.