Synthesis and Anti-cancer applications of Benzimidazole derivatives – Recent Studies

Abstract:

The heterocyclic molecules like benzimidazole occupy a central position in searching for novel and effective anti-cancer drugs. The medicinal chemists designed and synthesized several benzimidazole derivatives and conjugates to evaluate them as potential anti-cancer agents. This compilation of literature covers the progress of benzimidazole-based anti-cancer agents, their synthesis, and their evaluation for cancer disease treatment. The compounds like benzimidazole derivatives of dehydroabietic acid, piperidyl benzimidazole carboxamide, benzimidazole-quinazolinone hybrids, benzimidazole-thiazole conjugate, benzimidazole pendant cyanopyrimidine derivatives have been discussed in detail. This review article will help the medicinal chemists to design and synthesize benzimidazole-based molecules and evaluate them as anti-cancer agent.

Assessment of Dimensional Stability, Biodegradability, and Fracture Energy of Bio-Composites Reinforced with Novel Pine Cone

In this investigation, biodegradable composites were fabricated with polycaprolactone (PCL) matrix reinforced with pine cone powder (15%, 30%, and 45% by weight) and compatibilized with graphite powder (0%, 5%, 10%, and 15% by weight) in polycaprolactone matrix by compression molding technique. The samples were prepared as per ASTM standard and tested for dimensional stability, biodegradability, and fracture energy with scanning electron micrographs. Water-absorption and thickness-swelling were performed to examine the dimensional stability and tests were performed at 23 °C and 50% humidity. Results revealed that the composites with 15 wt % of pine cone powder (PCP) have shown higher dimensional stability as compared to other composites. Bio-composites containing 15-45 wt % of PCP with low graphite content have shown higher disintegration rate than neat PCL. Fracture energy for crack initiation in bio-composites was increased by 68% with 30% PCP. Scanning electron microscopy (SEM) of the composites have shown evenly-distributed PCP particles throughout PCL-matrix at significantly high-degrees or quantities of reinforcing.

Inhibition of Amyloid Fibrillation of HEWL by 4-Methylcoumarin and 4-Methylthiocoumarin Derivatives

BACKGROUND

Several human diseases like Parkinson's, Alzheimer's disease, and systemic amyloidosis are associated with the misfolding and aggregation of protein molecules.

OBJECTIVE

The present study demonstrated the comparison of 4-methyl coumarin and 4-methylthiocoumarin derivative for their anti-amyloidogenic and disaggregation activities. The hen egg-white lysozyme is used as a model system to study protein aggregation and disaggregation under in vitro conditions.

METHODS

Techniques used in the study were Thioflavin T fluorescence assay, intrinsic fluorescence assay, circular dichroism, transmission electron microscopy, and molecular dynamics.

RESULTS

Fifteen compounds were screened for their anti-amyloidogenic and disaggregation potential. Six compounds significantly inhibited the fibril formation, whereas ten compounds showed disaggregation property of pre-formed fibrils. Under in vitro conditions, the compound C3 and C7 showed significant inhibition of fibril formation in a concentration-dependent manner as compared to control. C3 and C7 demonstrated 93% and 76% inhibition of fibril formation, respectively. Furthermore, C3 and C7 exhibited 83% and 76% disaggregation activity, respectively, of pre-formed HEWL fibrils at their highest concentration. These anti-amyloidogenic and disaggregation potential of C3 and C7 were validated by intrinsic fluorescence, CD, molecular dynamics, and TEM study.

DISCUSSION

4-methylthiocoumarins derivatives have shown better anti-amyloidogenic activity as compared to 4-methylcoumarin derivatives for both amyloid formation as well as disaggregation of preformed amyloid fibrils. Structurally, the derivatives of 4-methylthiocoumarins (C3 and C7) contain thio group on 2nd position that might be responsible for anti-amyloidogenic activity as compared to 4- methylcoumarin derivatives (C2 and C4).

CONCLUSION

C3 and C7 are novel 4-methylthiocoumarin derivatives that can be used as a lead for alleviation and symptoms associated with protein aggregation disorders.

Synthesis, characterisation and self-assembly studies of dendron-based novel non-ionic amphiphiles

A novel series of dendron-based non-ionic amphiphiles that aggregate to form supramolecular structures have been designed and synthesized using biocompatible starting materials.

Ellagic acid peracetate is superior to ellagic acid in the prevention of genotoxicity due to aflatoxin B1in bone marrow and lung cells

Earlier observations carried out in our laboratory highlighted the mode of action of acetoxy 4-methylcoumarins and quercetin pentaacetate in preventing the genotoxicity of aflatoxin B1 (AFB1). We have extended the observation to an acetoxy biscoumarin i.e. ellagic acid peracetate (EAPA), which unlike ellagic acid (EA) has demonstrated time-dependent inhibition of liver microsomes catalysed AFB1-epoxidation as measured by AFB1 binding to DNA. EAPA was more potent than EA in preventing bone marrow and lung cells from AFB1-induced genotoxicity. EAPA was acted upon by microsomal acetoxy drug:protein transacetylase (TAase) leading to modulation of the catalytic activity of certain functional proteins (cytochrome P450, NADPH cytochrome c reductase and glutathione S-transferase), possibly by way of protein acetylation.

Calreticulin transacylase: genesis, mechanism of action and biological applications

Our earlier investigations have identified a unique enzyme in the endoplasmic reticulum (ER) termed Acetoxy Drug: Protein Transacetylase (TAase) catalyzing the transfer of acetyl group from polyphenolic acetates (PA) to certain receptor proteins (RP). An elegant assay procedure for TAase was developed based on the inhibition of glutathione S-transferase (GST) due to acetylation by a model acetoxycoumarin, 7, 8-Diacetoxy-4-methylcoumarin (DAMC). TAase purified from various mammalian tissue microsomes to homogeneity exhibited a molecular weight (M.wt) of 55kDa. Further, by N-terminal sequencing TAase was identified as Calreticulin (CR), a multifunctional Ca2+-binding protein in ER lumen. The identity of TAase with CR was evidenced by proteomics studies such as immunoreactivity with anti-CR antibody and mass spectrometry. This function of CR was termed Calreticulin transacetylase (CRTAase). CRTAase was also found to mediate the transfer of acetyl group from DAMC to RP such as NADPH Cytochrome c Reductase (CYPR) and Nitric Oxide Synthase (NOS). The autoacetylation of purified human placental CRTAase concomitant with the acetylation of RP by DAMC was observed. CRTAase activity was found to be inhibited by Ca2+. Our investigations on the individual domains (N, P and C) of CR from a nematode Haemonchus contortus revealed that the P-domain alone was found to possess CRTAase activity. Based on the observation that the autoacetylated CR was a stable intermediate in the CRTAase catalyzed protein acetylation by PA, a putative mechanism was proposed. Further, CRTAase was also found capable of transferring propionyl group from a propoxy derivative of polyphenol, 7,8-Dipropoxy-4-methylcoumarin (DPMC) to RP and concomitant autopropionylation of CR was encountered. Hence, CRTAase was assigned the general term Calreticulin Transacylase. Also, CRTAase was found to act upon the biological acyl group donors, acetyl CoA and propionyl CoA. CRTAase mediated modulation of specific functional proteins by way of acylation was exploited to elicit the biological applications of PA.

Specificities of acetoxy derivatives of coumarins, biscoumarins, chromones, flavones, isoflavones and xanthones for acetoxy drug: Protein transacetylase

The earlier work carried out in our laboratory led to the identification of a novel rat liver microsomal enzyme termed as acetoxy drug: protein transacetylase (TAase), catalyzing the transfer of acetyl group from polyphenolic acetates (PA) to functional proteins. In this paper, we have reported the comparison of the specificities of acetoxy derivatives of coumarins, biscoumarins, chromones, flavones, isoflavones and xanthones with special reference to the phenyl moiety/bulky group on the pyran ring of PA. The results clearly indicated that compounds having phenyl moieties, when used as the substrates, resulted in a significant reduction of TAase catalyzed activity. The alteration in TAase catalyzed activation of NADPH cytochrome c reductase and inhibition of benzene-induced micronuclei in bone marrow cells by PA were in tune with their specificities to TAase.

Calreticulin transacetylase (CRTAase): Identification of novel substrates and CRTAase-mediated modification of protein kinase C (PKC) activity in lymphocytes of asthmatic patients by polyphenolic acetates

Earlier reports from our laboratory established the acetyl transferase function of calreticulin (CRT), enabling CRT to transfer acetyl groups from the acetoxy groups of polyphenolic acetates (PAs) to certain receptor proteins. We have in this paper documented the ability of CRT to catalyze the possible transfer of acetyl moiety from 7-acetamido-4-methylcoumarin (7-N-AMC) to the proteins, glutathione S-transferase (GST), and NADPH cytochrome c reductase, leading to the modification of their catalytic activities. 7-Acetoxy-4-methylthiocoumarin (7-AMTC) compared to 7-acetoxy-4-methylcoumarin (7-AMC) when used as a substrate for calreticulin transacetylase (CRTAase) yielded significantly higher catalytic activity. PM3-optimized geometries suggested that the availability of electrons on the sulfur atom of the thiocarbonyl group of the thiocoumarin may render the substrate binding more favorable to the active site of the enzyme as compared to its oxygen analog. Further CRTAase activity was characterized in the human blood lymphocytes. There was no appreciable difference in CRTAase activity of lymphocytes of asthmatic patients as compared to those of normal subjects. The results presented here highlight for the first time the irreversible inhibition of human blood lymphocytes protein kinase C (PKC) by 7,8-diacetoxy-4-methylcoumarin (DAMC) possibly by way of acetylation. The activity of PKC in lymphocytes of asthmatic patients was found to proportionally increase with the severity of the disease. When PA was incubated with lymphocytes of normal patients, PKC was inhibited marginally. On the other hand, lymphocyte PKC of severe asthmatic patients was inhibited drastically. Several PAs inhibited PKC of asthmatic patients in tune with their specificity to CRTAase. DAMC was found to exert maximum inhibitory action on PKC, while 7,8-dihydroxy-4-methylcoumarin (DHMC), the deacetylated product of DAMC, failed to inhibit PKC. These observations clearly describe DAMC as the novel irreversible inhibitor of PKC, and DAMC may be found useful in the control of inflammation and may serve as a potential drug candidate in the therapy of asthma.

Characterization of protein transacetylase from human placenta as a signaling molecule calreticulin using polyphenolic peracetates as the acetyl group donors

We have earlier shown that a unique membrane-bound enzyme mediates the transfer of acetyl group(s) from polyphenolic peracetates (PA) to functional proteins, which was termed acetoxy drug: protein transacetylase (TAase) because it acted upon several classes of PA. Here, we report the purification of TAase from human placental microsomes to homogeneity with molecular mass of 60 kDa, exhibiting varying degrees of specificity to several classes of PA confirming the structure-activity relationship for the microsome-bound TAase. The TAase catalyzed protein acetylation by a model acetoxy drug, 7,8-diacetoxy-4-methyl coumarin (DAMC) was established by the demonstration of immunoreactivity of the acetylated target protein with anti-acetyl lysine antibody. TAase activity was severely inhibited in calcium-aggregated microsomes as well as when Ca2+ was added to purified TAase, suggesting that TAase could be a calcium binding protein. Furthermore, the N-terminal sequence analysis of purified TAase (EPAVYFKEQFLD) using Swiss Prot Database perfectly matched with calreticulin (CRT), a major microsomal calcium binding protein of the endoplasmic reticulum (ER). The identity of TAase with CRT was substantiated by the observation that the purified TAase avidly reacted with commercially available antibody raised against the C-terminus of human CRT (13 residues peptide, DEEDATGQAKDEL). Purified TAase also showed Ca2+ binding and acted as a substrate for phosphorylation catalyzed by protein kinase C (PKC), which are hallmark characteristics of CRT. Further, purified placental CRT as well as the commercially procured pure CRT yielded significant TAase catalytic activity and were also found effective in mediating the acetylation of the target protein NADPH cytochrome P-450 reductase by DAMC as detected by Western blot using anti-acetyl lysine antibody. These observations for the first time convincingly attribute the transacetylase function to CRT. Hence, this transacetylase function of CRT is designated calreticulin transacetylase (CRTAase). We envisage that CRTAase plays an important role in protein modification by way of acetylation independent of Acetyl CoA.

Acetoxy drug: Protein transacetylase catalyzed activation of human platelet nitric oxide synthase by polyphenolic peracetates

An enhanced intracellular level of Nitric oxide (NO) is essential to ameliorate several pathological conditions of heart and vasculature necessitating the activation of NOS. We have projected in this report the acetylation of eNOS by polyphenolic peracetates (PA) catalyzed by the novel enzyme acetoxy drug: protein transacetylase (TAase) discovered in our laboratory as an unambiguous way of activating NOS which results in the manifestation of physiological action. The human platelet was chosen as the experimental system in order to validate the aforementioned proposition. PA caused profound irreversible activation of platelet NADPH cytochrome c reductase mediated by TAase. The convincing biochemical evidences are presented to show that PA could cause acetylation of the reductase domain of NOS leading to the activation of eNOS in tune with their specificities to platelet TAase. As a result, the enhanced level of NO due to activation of platelet eNOS by PA was found to inhibit the ADP-induced platelet aggregation. The present studies highlight for the first time the role of PA as the novel potent agent for enhancing the intracellular NO levels.

Synthesis of novel amino and acetyl amino-4-methylcoumarins and evaluation of their antioxidant activity

The six novel 4-methylcoumarins bearing different functionalities such as amino, hydroxy, N-acetyl, acetoxy and nitro have been synthesized and confirmed on the basis of their spectral data (1H-, 13C-NMR, UV, IR and EI mass). They were examined for the first time for their effect on NADPH dependent liver microsomal lipid peroxidation in vitro, and the results were compared with other model 4-methylcoumarin derivatives to establish the structure-activity relationship. Our studies demonstrated that amino group is an effective substitute for the hydroxyl group for antioxidant property and produced a dramatic inhibition of lipid peroxidation. Ortho dihydroxy and ortho hydroxy-amino coumarins were found to possess highest antioxidant and radical scavenging activities.

Acetoxy drug: protein transacetylase of buffalo liver—characterization and mass spectrometry of the acetylated protein product

The purification and characterization of the buffalo liver microsomal transacetylase (TAase) catalyzing the transfer of acetyl groups from a model acetoxy drug: 7,8-diacetoxy-4-methylcoumarin (DAMC) to GST3-3 has been described here. The enzyme was routinely assayed using DAMC and cytosolic GST as the substrates and was partially purified from microsomes of the buffalo liver. The enzyme was found to have approximate molecular of weight 65 kDa. The action of TAase and DAMC on liver cytosolic GST resulted in the formation of monoacetoxymonohydroxy-4-methylcoumarin (MAMHC) and 7,8-dihydroxy-4-methylcoumarin (DHMC), although the former was the major metabolite. The buffalo liver microsomal TAase exhibited hyperbolic kinetics and yielded K(m) (1667 microM) and V(max) (192 units) when the concentration of DAMC was varied keeping the concentration of GST constant. After having characterized the nature of the substrates and a product of the TAase-catalyzed reaction, we set out to identify the acetylated protein which is another product of the reaction. GST3-3 was used as a model protein substrate for the action of TAase using DAMC as the acetyl donor. The subunit of control and modified GST3-3 were separated by SDS-polyacrylamide gel electrophoresis (PAGE) and digested with trypsin. The tryptic peptides were extracted from the gel pieces and analyzed by matrix assisted laser desorption/ionization-time of flight-mass spectrometry (MALDI-TOFMS). The data search for calibrated and labeled mass peaks of peptides was performed on the Matrix Science Server using the search engine Mascot. The peptide maps so obtained covered 97% of the GST3-3 sequence. On comparison of MALDI peptide maps of modified and control GST, seven new peaks were recognized corresponding to the potentially acetylated peptides in peptide map. The mass value of each of them was 42 Da higher than the theoretical mass of a non-modified GST3-3 tryptic peptide, strongly suggesting acetylation. By examining the fragmentation patterns and by comparing experimental and predicted values for MS/MS daughter ions, the identity of the seven acetylated GST tryptic peptides could be confirmed by the application of LC/MS/MS. In the modified GST, N-terminal proline and six lysines (Lys(51), Lys(82), Lys(123), Lsy(181), Lys(191) and Lys(210)) were found to be acetylated. The structure of acetylated GST revealed that the lysines that underwent acetylation were peripheral in positions.

Mechanism of biochemical action of substituted 4-methylbenzopyran-2-ones. Part 10: identification of inhibitors for the liver microsomal acetoxycoumarin: protein transacetylase

The quantitative structure-activity relationship (QSAR) studies conducted by us earlier revealed the cardinal role of the pyran ring carbonyl group in the acetoxy polyphenolic compounds for the acetoxy polyphenol:protein transacetylase (TAase) activity. Hence, an attempt was made to examine whether such substrate analogues of benzopyran acetates which lack in the pyran ring carbonyl group, such as 7-acetoxy-2,3-dihydro-2,2-dimethylbenzopyran (BPA), cetachin pentaacetate (CPA) and hematoxylin pentaacetate (HPA) could inhibit the 7,8-diacetoxy-4-methylcoumarin (DAMC):protein (glutathione-S-transferase) transacetylase activity. These compounds were indeed found to remarkably inhibit the TAase activity in a concentration dependent manner and exerted their inhibitory action very rapidly. Further BPA, CPA and HPA were found to abolish the TAase mediated activation of NADPH cytochrome C reductase as well as the inhibition of liver microsome catalyzed aflatoxin B(1) (AFB(1))-DNA binding by DAMC very effectively. These results strongly suggest that the acetoxybenzopyrans merit as potent inhibitors of TAase.

Mechanism of biochemical action of substituted 4-methylbenzopyran-2-ones. Part 9: comparison of acetoxy 4-methylcoumarins and other polyphenolic acetates reveal the specificity to acetoxy drug: protein transacetylase for pyran carbonyl group in proximity to the oxygen heteroatom

The evidences for the possible enzymatic transfer of acetyl groups (catalyzed by a transacetylase localized in microsomes) from an acetylated compound (acetoxy-4-methylcoumarins) to enzyme proteins leading to profound modulation of their catalytic activities was cited in our earlier publications in this series. The investigations on the specificity for transacetylase (TA) with respect to the number and positions of acetoxy groups on the benzenoid ring of coumarin molecule revealed that acetoxy groups in proximity to the oxygen heteroatom (at C-7 and C-8 positions) demonstrate a high degree of specificity to TA. These studies were extended to the action of TA on acetates of other polyphenols, such as flavonoids and catechin with a view to establish the importance of pyran carbonyl group for the catalytic activity. The absolute requirement of the carbonyl group in the pyran ring of the substrate for TA to function was established by the observation that TA activity was hardly discernible when catechin pentacetate and 7-acetoxy-3,4-dihydro-2,2-dimethylbenzopyran (both lacking pyran ring carbonyl group) were used as the substrates. Further, the TA activity with flavonoid acetates was remarkably lower than that with acetoxycoumarins, thus suggesting the specificity for pyran carbonyl group in proximity to the oxygen heteroatom. The biochemical properties of flavonoid acetates, such as irreversible activation of NADPH cytochrome C reductase and microsome-catalyzed aflatoxin B(1) binding to DNA in vitro were found to be in tune with their specificity to TA.

Establishment of the enzymatic protein acetylation independent of acetyl CoA: recombinant glutathione S-transferase 3-3 is acetylated by a novel membrane-bound transacetylase using 7,8-diacetoxy-4-methyl coumarin as the acetyl donor

The current knowledge on biological protein acetylation is confined to acetyl CoA-dependent acetylation of protein catalyzed by specific acetyl transferases and the non-enzymatic acetylation of protein by acetylated xenobiotics such as aspirin. We have discovered a membrane-bound enzyme catalyzing the transfer of acetyl groups from the acetyl donor 7,8-diacetoxy-4-methyl coumarin (DAMC) to glutathione S-transferase 3-3 (GST3-3), termed DAMC:protein transacetylase (TAase). The purified enzyme was incubated with recombinant GST3-3 subunit and DAMC, the modified protein was isolated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) in gel digested with trypsin and the tryptic digest was analyzed by mass spectrometry. The N-terminus and six lysines, Lys-51, -82, -124, -181, -191 and -210, were found to be acetylated. The acetylation of GST3-3 described above was not observed in the absence of either DAMC or TAase. These results clearly establish the phenomenon of protein acetylation independent of acetyl CoA catalyzed by a hitherto unknown enzyme (TAase) utilizing a certain xenobiotic acetate (DAMC) as the active acetyl donor.

Microsomal acetoxy drug: protein transacetylase of placenta: part 1. Characterization of DAMC: GST transacetylase

We have earlier established in tissues of several mammalian animal species the existence of a novel membrane bound enzyme termed 7,8-diacetoxy-4-methylcoumarin (DAMC): protein transacetylase (TAase) that possibly transfers acetyl groups from the model acetoxy drug (DAMC) to certain enzyme protein viz. glutathione S-transferase (GST), cytochrome P-450 and NADPH cytochrome C reductase leading to the drastic modulation of their catalytic activities. We have in this report extended the studies to human tissue and characterized TAase from placenta. For this purpose placental microsomes were preincubated with DAMC along with the receptor protein (cytosolic GST) followed by the addition of the substrates of GST in order to quantify the catalytic activity of GST, the extent of inhibition of GST served as a measure of TAase. Placental TAase was also found to irreversibly activate NADPH cytochrome C reductase by DAMC. Placental enzyme activated the reductase even at very low concentration of DAMC. Iodoacetamide nearly abolished the placental TAase suggesting the presence of active thiol group in the enzyme and the TAase demonstrated hyperbolic kinetics. Kinetic constants obtained by varying the concentrations of either of the substrates DAMC or cytosolic GST characterized TAase catalysed reaction as the bimolecular reaction. Further studies are in progress to delineate the physiological importance of TAase in placenta.

Acetoxy-4-methylcoumarins confer differential protection from aflatoxin B(1)-induced micronuclei and apoptosis in lung and bone marrow cells

The ability of various acetoxy derivatives of 4-methylcoumarins to inhibit the genotoxic changes due to aflatoxin B(1) (AFB(1)) is reported here. Several 4-methylcoumarins (test compounds), such as 7,8-diacetoxy-4-methylcoumarin (DAMC), monoacetoxy-4-methylcoumarin (MAC), 5-N-acetyl-6-acetoxy-4-methylcoumarin (NAMC) and 7,8-dihydroxy-4-methylcoumarin (DHMC) were separately administered intraperitoneally (i.p.) to male wistar rats followed by AFB(1) administration i.p. or intratracheally (i.t.) (2-8 mg/kg b.wt.) and another dose of the test compound. The animals were sacrificed 26h after AFB(1) administration. From animals receiving AFB(1) i.p., bone marrow (BM) cells were isolated and stained with Mayer's haematoxylin and eosin. Micronuclei (MN) in BM were scored by light microscopy. From animals receiving AFB(1) i.t., bronchoalveolar lavage (BAL) was obtained, lung cells (LG) were isolated and stained with fluorochrome 6-diamidino-2-phenylindole (DAPI) for the analysis of MN, apoptotic bodies (AP) and cell cycle variations. Rats were separately treated with the vehicle DMSO to serve as the proper control. AFB(1) caused significant dose-dependent induction of MN in BM as well as LG. AP were observed in LG of rats receiving AFB(1) and was found to correlate with MN induction. DAMC injection caused significant decrease in AP due to AFB(1) in LG and MN in both BM and LG. The effectiveness of MAC was approximately half that of DAMC, thereby indicating that number of acetoxy groups on the coumarin molecule determine the efficacy. The fact that NAMC had no effect either on MN or AP indicate that neither acetoxy group at C-6 nor the N-acetyl group at C-5 facilitate the transfer of acetyl group to P-450 required for inhibition of AFB(1)-epoxidation. DHMC, the deacetylated product of DAMC had no normalizing effect on the induction of MN and AP. These findings confirm our earlier hypothesis that DAMC-mediated acetylation of microsomal P-450 (catalysing epoxidation of AFB(1)) through the action of microsomal transacetylase is responsible for the protective action of DAMC. The relative number and position of acetoxy groups on the coumarin nucleus determine the specificity to the transacetylase necessary for the chemopreventive action.

Chemoprevention of benzene-induced bone marrow and pulmonary genotoxicity

Our earlier studies documented the ability of 7,8-diacetoxy-4-methylcoumarin (DAMC) to cause irreversible inhibition of cytochrome P-450 linked mixed function oxidases (MFO) mediated by membrane bound DAMC: protein transacetylase. Since P-450 catalyzed oxidation of benzene is crucial to its toxic effects, the action of DAMC and related analogues were considered promising in preventing the genotoxicity due to benzene. For this purpose rats were pretreated with various acetoxy-4-methylcoumarins (test compounds), which was followed by the administration of benzene either intratracheally (IT) or intraperitoneally (IP), and sacrificed 26 h after the injection of benzene. The incidence of micronuclei (MN) in bone marrow (BM) and lung (LG) were assessed by light and fluorescent microscopy, respectively. A dose-dependent induction of MN in BM and LG cells was observed in rats administered with benzene. A significant reduction in benzene-induced MN in BM and LG was observed as a result of DAMC administration to rats; a higher dose of DAMC resulted in greater inhibition of clastogenic action of benzene as revealed by MN incidence. 7,8-dihydroxy-4-methylcoumarin (DHMC), the deacetylated product of DAMC, demonstrated relatively lesser potency to inhibit the clastogenic action of benzene. This observation is consistent with the ability of DAMC to inhibit the formation of benzene oxide as well as to scavenge the oxygen radicals formed during the course of benzene metabolism. The fact that DHMC can only scavenge the oxygen radicals and is ineffective in inhibiting benzene oxidation in vivo explains the reduced capability of dihydroxy coumarin to prevent MN due to benzene. 7-Acetoxy-4-methylcoumarin (MAC) inhibits the MN due to benzene being roughly 50% of that produced by DAMC. DAMC is also effective in normalizing the cell cycle alterations produced by benzene in BM and LG. These observations further substantiate our hypothesis that the biological effects of acetoxy coumarins are mediated by the action of membrane bound transacetylase that catalyzes the acetylation of concerned proteins. Teratogenesis Carcinog. Mutagen. 21:181-187, 2001.

Mechanism of biochemical action of substituted 4-methylbenzopyran-2-ones. Part 7: Assay and characterization of 7,8-diacetoxy-4-methylcoumarin:protein transacetylase from rat liver microsomes based on the irreversible inhibition of cytosolic glutathione S-transferase

The enzymatic transfer of acetyl groups from acetylated xenobiotics to specific proteins is a relatively grey area in the evergreen field of biotransformation of foreign compounds. In this paper, we have documented evidence for the existence of a transacetylase in liver microsomes that catalyses the transfer of acetyl groups from 7,8-diacetoxy-4-methylcoumarin (DAMC) to glutathione S-transferase (GST), either purified or present in cytosol leading to the irreversible inhibition of GST. A simple procedure is described for the assay of transacetylase by preincubation of DAMC with liver microsomes and pure GST/liver cytosol, followed by the addition of 1-chloro-2,4-dinitrobenzene (CDNB) and reduced glutathione (GSH) in order to quantify GST activity by the conventional procedure. The extent of inhibition of GST by DAMC under the conditions of the assay is indicative of DAMC:protein transacetylase activity. Following the assay procedure described here, the transacetylase was shown to exhibit hyperbolic kinetics. The bimolecular nature of the transacetylase reaction was apparent by the demonstration of Km and vmax values. 7,8-Dihydroxy-4-methylcoumarin (DHMC), one of the products of transacetylase reaction was identified and quantified using the partially purified enzyme. The fact that p-hydroxymercuribenzoate (PHMB) and iodoacetamide abolished irreversible inhibition of GST upon the action of transacetylase on DAMC strongly characterized transacetylase as a protein containing thiol group at the active site. In addition, the relative specificities of acetoxy 4-methylcoumarins to transacetylase have been demonstrated.

Mechanism of biochemical action of substituted 4-methylbenzopyran-2-ones. Part 6: hydrolysis of 7,8-diacetoxy-4-methylcoumarin by a novel deacetylase in rat liver microsomes--a simple method for assay and characterisation

The existence of a novel microsomal deacetylase in rat liver catalysing deacetylation of diacetoxy 4-methylcoumarins has been reported. A simple method is outlined for the enzyme assay based upon the quantification of the dihydroxy derivative by measuring the UV absorption of its complex with ADP and Fe3+ at 600 nm. The enzyme can be routinely assayed using 7,8-diacetoxy-4-methylcoumarin (DAMC) as the substrate and demonstrated hyperbolic kinetics and yielded Km and vmax values of 1250 microM and 500 units, respectively. The pH optima was found to be 7.5 for the enzyme. No DAMC deacetylase activity was found in hepatic cytosol and the enzyme activity was not discernible in extrahepatic tissues.

Mechanism of biochemical action of substituted 4-methylbenzopyran-2-ones. Part 4: hyperbolic activation of rat liver microsomal NADPH-cytochrome C reductase by the novel acetylator 7,8-diacetoxy-4-methylcoumarin

The effect of 7,8-diacetoxy-4-methylcoumarin (DAMC) has been studied on hepatic NADPH cytochrome C reductase-- an enzyme participating in the microsomal electron transport. The preincubation of liver microsomes with DAMC resulted in a time-dependent activation of NADPH cytochrome C reductase. The catalytic activity of the enzyme enhanced nearly 600% by 25 microM concentration of DAMC after 10 min of preincubation. The action of DAMC on the reductase resulted in enhanced v(max) while Km remained constant. A plot of 1/v(max) as a function of DAMC concentration resulted in a non-linear, but rectangular hyperbola indicative of hyperbolic activation. DAMC was also proved to be effective in significantly enhancing the activity of NADPH cytochrome C reductase in vivo. 7,8-Dihydroxy-4-methylcoumarin (DHMC), the deacetylated product of DAMC failed to irreversibly activate the enzyme. The activation effect of DAMC upon the enzyme was abolished by p-hydroxymercury benzoate. The role of a transacetylase in transferring the acetyl group of DAMC to the amino acid(s) of the active site of NADPH cytochrome C reductase causing irreversible enzyme activation is enunciated.

Mechanism of biochemical action of substituted 4-methylbenzopyran-2-ones. Part 3: A novel mechanism for the inhibition of biological membrane lipid peroxidation by dioxygenated 4-methylcoumarins mediated by the formation of a stable ADP-Fe-inhibitor mixed ligand complex

7,8-Dihydroxy-4-methylcoumarin (DHMC) and 7,8-diacetoxy-4-methylcoumarin (DAMC) have been reported to effectively inhibit in-vivo lipid peroxidation in rat tissues induced by CCl4 and paraquat. DHMC was found to readily impart green colour to the lipid peroxidation incubation mixture containing ADP and Fe3+, whereas DAMC formed green complex only upon incubation with liver microsomes, confirming our earlier observation that liver microsomal deacetylase hydrolyses DAMC to DHMC. Sensitive pH metric technique revealed the formation of ADP-Fe-DHMC ternary complex with highest stability, while Fe-DHMC and ADP-DHMC had negligible stabilities concluding that ADP-perferryl ion formation is prevented by DHMC resulting in the production of stable ternary mixed ligand complex (ADP-Fe-DHMC), thereby inhibiting the formation of O2-, and eventually other reactive oxygen species (ROS) responsible for membrane lipid peroxidation.

Mechanism of biochemical action of substituted 4-methylbenzopyran-2-ones. Part II: Mechanism-based inhibition of rat liver microsome-mediated aflatoxin B1-DNA binding by the candidate antimutagen 7,8-diacetoxy-4-methylcoumarin

7,8-Diacetoxy-4-methylcoumarin (DAMC), with no prerequisite for oxidative biotransformation has been reported to produce suicide inactivation of microsomal cytochrome P-450-catalysed formation of aflatoxin B1-8,9-oxide that binds to DNA. Parenteral administration of DAMC to rats caused significant inhibition of AFB1 binding to hepatic DNA in vivo as well as AFB1-induced micronuclei formation in bone marrow cells. These results highlight the antimutagenic potential of DAMC.

Some international law aspects of the Bhopal disaster

This article explores certain international law aspects of the Bhopal disaster, namely the principles and rules of international law establishing international accountability for environmental damage; the criteria for determining the liability of the Union Carbide Company (U. S. A.) for the Bhopal disaster; the criteria for determining compensation; and the international remedies available to the Indian government in the event that Bhopal victims fail to get justice within the Indian court system. The article discusses two applicable sets of proposed international standards--the U. N. Draft Code of Conduct on Transnational Corporations, and the U. N. International Law Commission's Draft on International Liability for Injurious Consequences Arising out of Acts Not Prohibited by International Law. The scattered 'hard' and 'soft' jurisprudence of international environmental law establishes liability and accountability for environmental hazards. It makes both state and non-state entities liable to pay compensation to the victims of environmental pollution. This jurisprudence, in addition to domestic law analogies, can influence Indian courts in determining the amount of damages payable to the victims of the Bhopal disaster. The authors conclude that the Bhopal disaster has demonstrated that enforceable international standards are clearly and urgently needed for hazardous industries, especially those operating in developing countries. Such standards would eliminate, or at least narrow, the gap between standards prevailing in the developed countries and those in the Third World. Even without enforcement, international standards could provide norms for measuring the performance of individual companies engaged in hazardous activities such as the manufacture of MIC at Bhopal.