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

Radiosensitization of calreticulin-overexpressing human glioma cell line by the polyphenolic acetate 7, 8-diacetoxy-4-methylcoumarin

BACKGROUND

Calreticulin (CRT), an endoplasmic reticulum-resident protein generally overexpressed in cancer cells, is associated with radiation resistance. CRT shows higher transacetylase activity, as shown by us earlier, in the presence of the polyphenolic acetates (like 7, 8-diacetoxy-4-methylcoumarin, DAMC) and modifies the activity of a number of proteins, thereby influencing cell signaling.

AIM

To investigate the relationship between CRT expression and radiation response in a human glioma cell line and to evaluate the radiomodifying effects of DAMC.

METHODS AND RESULTS

Studies were carried out in an established human glioma cell line (BMG-1) and its isogenic clone overexpressing CRT (CROE, CRT-overexpressing cells) by analyzing clonogenic survival, cell proliferation, micronuclei analysis, and protein levels by Western blotting as parameters of responses. CRT overexpression conferred resistance against radiation-induced cell death in CROE cells (D₃₇  = 7.35 Gy, D₁₀  = 12.6 Gy and D₀  = 7.25 Gy) as compared to BMG-1 cells (D₃₇  = 5.70 Gy, D₁₀  = 9.2 Gy and D₀  = 5.6 Gy). A lower level of radiation-induced micronuclei formation observed in CROE cells suggested that reduced induction and/or enhanced DNA repair partly contributed to the enhanced radioresistance. Consistent with this suggestion, we noted that CRT-mediated radioresistance was coupled with enhanced grp78 level and reduced P53 activation-mediated prodeath signaling, while no changes were noted in acetylation of histone H4. DAMC-enhanced radiation-induced delayed (secondary) apoptosis, which was higher in CROE cells.

CONCLUSION

CRT overexpression confers resistance against radiation-induced death of human glioma cells, which can be overcome by the polyphenolic acetate DAMC.

Evaluation of the Free Radical Scavenging Activities of Ellagic Acid and Ellagic Acid Peracetate by EPR Spectrometry

The purpose of this study was to examine the free radical scavenging and antioxidant activities of ellagic acid (EA) and ellagic acid peracetate (EAPA) by measuring their reactions with the radicals, 2,2-diphenyl-1-picrylhydrazyl and galvinoxyl using EPR spectroscopy. We have also evaluated the influence of EA and EAPA on the ROS production in L-6 myoblasts and rat liver microsomal lipid peroxidation catalyzed by NADPH. The results obtained clearly indicated that EA has tremendous ability to scavenge free radicals, even at concentration of 1 µM. Interestingly even in the absence of esterase, EAPA, the acetylated product of EA, was also found to be a good scavenger but at a relatively slower rate. Kinetic studies revealed that both EA and EAPA have ability to scavenge free radicals at the concentrations of 1 µM over extended periods of time. In cellular systems, EA and EAPA were found to have similar potentials for the inhibition of ROS production in L-6 myoblasts and NADPH-dependent catalyzed microsomal lipid peroxidation.

Developing polyphenolic acetates as radiation countermeasure agents: current status and future perspectives

Total-body exposure to ionizing radiation (TBI) results in life-threatening acute radiation syndrome (ARS), which encompasses hematopoietic and gastrointestinal (GI) injuries and results in dose-dependent morbidity and mortality. Management of ARS warrants the deployment of effective medical countermeasure agents (MCM) that protect against and/or mitigate lethal radiation injury. The polyphenolic acetate (PA) 7,8-diacetoxy-4-methylthiocoumarin (DAMTC) has been identified as a potential MCM against ARS by virtue of it mitigating the lethal effects of TBI in C57BL/6 mice. Herein, we describe current evidence, including mechanistic aspects, for the use of PAs as MCMs against ARS and provide perspectives for their further development as approved drugs for the mitigation of ARS.

Mitigation of radiation-induced gastro-intestinal injury by the polyphenolic acetate 7, 8-diacetoxy-4-methylthiocoumarin in mice

Radiation-induced intestinal injury (RIII) constitutes a crucial clinical element of acute radiation syndrome with life-threatening implications posing challenges in devising effective medical countermeasures. Herein, we report the potential of 7, 8-diacetoxy-4-methylthiocoumarin (DAMTC) to mitigate RIII following total-body irradiation (TBI) in C57BL/6 mice and underlying mechanisms. Administration of DAMTC 24 hours post TBI facilitated structural reconstitution and restoration of functional absorption linked to alleviation of radiation-induced apoptotic death of intestinal crypt progenitor/stem (ICPS) and villus stromal cells through induction of Bcl-2 family-mediated anti-apoptotic signalling. Reduction in TBI-induced DNA damage accumulation coupled with inhibition of cell cycle arrest through stimulation of anti-p53- and anti-p21-dependent synergistic signalling protected ICPS cells from radiation injury. Enhanced proliferation of crypt stem cells, induction of anti-oxidant defence, subjugation of TBI-induced lipid peroxidation and phenotypic polarization of intestinal macrophages to anti-inflammatory M2 class underlie amelioration of RIII. Stimulation of multiple mitigative signalling processes by DAMTC appeared to be associated with enhanced protein acetylation, an important regulator of cellular responses to radiation damage. Our findings establish the mitigative potential of DAMTC against RIII by hyper-acetylation-mediated epigenetic regulation, which triggers axes of anti-apoptotic and pro-survival pathways, enabling proliferation and maintenance of ICPS cells leading to epithelial regeneration.

Mitigation of radiation-induced hematopoietic injury by the polyphenolic acetate 7, 8-diacetoxy-4-methylthiocoumarin in mice

Protection of the hematopoietic system from radiation damage, and/or mitigation of hematopoietic injury are the two major strategies for developing medical countermeasure agents (MCM) to combat radiation-induced lethality. In the present study, we investigated the potential of 7, 8-diacetoxy-4-methylthiocoumarin (DAMTC) to ameliorate radiation-induced hematopoietic damage and the associated mortality following total body irradiation (TBI) in C57BL/6 mice. Administration of DAMTC 24 hours post TBI alleviated TBI-induced myelo-suppression and pancytopenia, by augmenting lymphocytes and WBCs in the peripheral blood of mice, while bone marrow (BM) cellularity was restored through enhanced proliferation of the stem cells. It stimulated multi-lineage expansion and differentiation of myeloid progenitors in the BM and induced proliferation of splenic progenitors thereby, facilitating hematopoietic re-population. DAMTC reduced the radiation-induced apoptotic and mitotic death in the hematopoietic compartment. Recruitment of pro-inflammatory M1 macrophages in spleen contributed to the immune-protection linked to the mitigation of hematopoietic injury. Recovery of the hematopoietic compartment correlated well with mitigation of mortality at a lethal dose of 9 Gy, leading to 80% animal survival. Present study establishes the potential of DAMTC to mitigate radiation-induced injury to the hematopoietic system by stimulating the re-population of stem cells from multiple lineages.

Biocatalytic Synthesis of Novel Partial Esters of a Bioactive Dihydroxy 4-Methylcoumarin by Rhizopus oryzae Lipase (ROL)

Highly regioselective acylation has been observed in 7,8-dihydroxy-4-methylcoumarin (DHMC) by the lipase from Rhizopus oryzae suspended in tetrahydrofuran (THF) at 45 °C using six different acid anhydrides as acylating agents. The acylation occurred regioselectively at one of the two hydroxy groups of the coumarin moiety resulting in the formation of 8-acyloxy-7-hydroxy-4-methylcoumarins, which are important bioactive molecules for studying biotansformations in animals, and are otherwise very difficult to obtain by only chemical steps. Six monoacylated, monohydroxy 4-methylcoumarins have been biocatalytically synthesised and identified on the basis of their spectral data and X-ray crystal analysis.

Downregulation of Rubisco Activity by Non-enzymatic Acetylation of RbcL

Atmospheric carbon dioxide (CO2) is assimilated by the most abundant but sluggish enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Here we show that acetylation of lysine residues of the Rubisco large subunit (RbcL), including Lys201 and Lys334 in the active sites, may be an important mechanism in the regulation of Rubisco activities. It is well known that Lys201 reacts with CO2 for carbamylation, a prerequisite for both carboxylase and oxygenase activities of Rubisco, and Lys334 contacts with ribulose-1,5-bisphosphate (RuBP). The acetylation level of RbcL in plants is lower during the day and higher at night, inversely correlating with the Rubisco carboxylation activity. A search of the chloroplast proteome database did not reveal a canonical acetyltransferase; instead, we found that a plant-derived metabolite, 7-acetoxy-4-methylcoumarin (AMC), can non-enzymatically acetylate both native Rubisco and synthesized RbcL peptides spanning Lys334 or Lys201. Furthermore, lysine residues were modified by synthesized 4-methylumbelliferone esters with different electro- and stereo-substitutes, resulting in varied Rubisco activities. 1-Chloroethyl 4-methylcoumarin-7-yl carbonate (ClMC) could transfer the chloroethyl carbamate group to lysine residues of RbcL and completely inactivate Rubisco, whereas bis(4-methylcoumarin-7-yl) carbonate (BMC) improved Rubisco activity through increasing the level of Lys201 carbamylation. Our findings indicate that RbcL acetylation negatively regulates Rubisco activity, and metabolic derivatives can be designed to dissect and improve CO2 fixation efficiency of plants through lysine modification.

Calreticulin transacetylase mediated upregulation of thioredoxin by 7,8-diacetoxy-4-methylcoumarin enhances the antioxidant potential and the expression of vascular endothelial growth factor in peripheral blood mononuclear cells

Extensive research carried out in our group on polyphenolic acetates (PAs) substantiated the potential role of PAs in causing diverse biological and pharmacological actions. Our earlier investigations firmly established the calreticulin transacetylase (CRTAase) catalyzed activation of nitric oxide synthase (NOS) by PAs. In this report, we have studied the effect of 7,8-diacetoxy-4-methylcoumarin (DAMC, a model PA) and other acetoxy coumarins on the thioredoxin and VEGF expression in human peripheral blood mononuclear cells (PBMCs), with a view to substantiate our earlier observation that DAMC was a superb inducer of angiogenesis. Real time RT-PCR analysis revealed the enhanced expression of thioredoxin reductase (TRXR) and diminished expression of thioredoxin interacting protein (TRXIP) leading to the increased expression and activity of thioredoxin (TRX) in PBMCs due to the the action of DAMC. The fact that TRX activity of PBMCs was enhanced by various acetoxy coumarins in tune with their affinity to CRTAase as substrate, suggested the possible activation of TRX due to acetylation. The overexpression of thioredoxin was found to correlate with that of VEGF as proved by real time RT-PCR and VEGF -ELISA results, apart from the DAMC-caused enhanced production of NO acting as an inducer of VEGF. Moreover, the intracellular ROS levels were also found to be reduced drastically, by DAMC thus reducing the oxidative stress in cells. These observations strongly evidenced the crucial role of TRX in DAMC-induced tissue angiogenesis with the involvement of VEGF.

Calreticulin Transacetylase mediated activation of human platelet nitric oxide synthase by acetyl group donor compounds

Polyphenols have attracted immense interest because of their diverse biological and pharmacological activities. Surprisingly, not much is documented about the biological activities of acetoxy derivatives of polyphenol called polyphenolic acetates (PA). In our previous reports, we have conclusively established the Calreticulin Transacetylase (CRTAase) catalyzed activation of neuronal nitric oxide synthase (nNOS) and tumor necrosis factor-α (TNF-α) induced nitric oxide synthase (iNOS) by PA. In the present work, specificity of CRTAase to various classes of PA was characterized in human platelet. The effect of PA, on platelet NOS and intracellular cyclic guanosine monophosphate (cGMP), and adenosine diphosphate (ADP)-induced platelet aggregation were studied in an elaborated manner. Platelet CRTAase exhibited differential specificities to polyphenolic acetates upon incubation with l-arginine leading to activation of NOS. The intraplatelet generation of NO was studied by flowcytometry using DCFH-DA. The differential specificities of CRTAase to PA were found to positively correlate with increased production of NO upon incubation of PRP with PA and l-arginine. Further, the inhibitory effect of l-NAME on PA induced NO formation in platelets substantiated the CRTAase catalyzed activation of NOS. The real-time RT-PCR profile of NOS isoforms confirmed the preponderance of eNOS over iNOS in human platelets on treatment with PA. Western blot analysis also reiterated the differential pattern of acetylation of eNOS by PA. PA were also found effective in increasing the intraplatelet cGMP levels and inhibiting ADP-induced platelet aggregation. It is worth mentioning that the effects of PA were found to be in tune with the specificities of platelet CRTAase to PA as the substrates.

Calreticulin transacetylase: a novel enzyme-mediated protein acetylation by acetoxy derivatives of 3-alkyl-4-methylcoumarins

Our earlier investigations culminated in the discovery of a unique membrane-bound enzyme Calreticulin transacetylase (CRTAase) in mammalian cells catalyzing the transfer of acetyl group from polyphenolic acetates (PAs) to certain functional proteins viz. Glutathione S-transferase (GST), NADPH Cytochrome c reductase and Nitric oxide synthase (NOS) resulting in the modulation of their biological activities. In order to develop SAR study, herein, we studied the influence of alkyl group at C-3 position of acetoxy coumarins on the CRTAase activity. The alkylated acetoxy coumarins lead to inhibition of catalytic activity of GST, and ADP induced platelet aggregation by the way of activation of platelet Nitric oxide synthase (NOS). Furthermore, the increase in size of the coumarin C-3 alkyl group was found to decrease the CRTAase activity.

Characterization of protein acyltransferase function of recombinant purified GlnA1 from Mycobacterium tuberculosis: a moon lighting property

The protein acetyltransferase (MTAase) function of glutamine synthetase of Mycobacterium smegmatis was established earlier. In this paper, studies were undertaken to examine MTAase function of recombinant glutamine synthetase (rGlnA1) of Mycobacterium tuberculosis, which showed >80% similarity with M. smegmatis GlnA. The specificity of MTAase to several acyl derivative of coumarins was examined. The results clearly indicated that MTAase exhibited differential specificities to several acyloxycoumarins. Further, MTAase was also found capable of transferring propionyl and butyryl groups from propoxy and butoxy derivatives of 4-methylcoumarin. These observations characterized MTAase in general as a protein acyltransferase. MTAase catalyzed acetylation of GST by 7,8-diacetoxy-4-methylcoumarin (DAMC), a model acetoxy coumarin was confirmed by MALDI-TOF-MS as well as western blot analysis using acetylated lysine polyclonal antibody. In order to validate the active site of rGlnA1 for TAase activity, effect of DAMC and L-methionine-S-sulfoximine (MSO) on GS and TAase activity of rGlnA1 were studied. The results indicated that the active sites of GS and TAase were found different. Acetyl CoA, a universal biological acetyl group donor, was also found to be a substrate for MTAase. These results appropriately characterize glutamine synthetase of Mtb exhibiting transacylase action as a moonlighting protein.

Prevention of benzene-induced genotoxicity in bone marrow and lung cells: superiority of polyphenolic acetates to polyphenols

Previous investigations carried out in our laboratory have highlighted that 7,8-diacetoxy-4-methylcoumarin demonstrates a mechanism-based inhibition of cytochrome P450 (Cyt-P450) activities such as microsome-mediated aflatoxin B1 (AFB1) epoxidation, dealkylation of alkylated resorufin, and toxicokinetics of benzene. 7,8-Diacetoxy-4-methylcoumarin, quercetin pentaacetate, and ellagic acid peracetate were also found to be effective in giving the protection of AFB1-induced genotoxicity in rat's bone marrow and lung cells possibly due to acetylation of Cyt-P450 apoprotein mediated by acetoxy drug: protein transacetylase. Later, this transacetylase was identified as calreticulin, and the acetyltransferase function of calreticulin was appropriately termed calreticulin transacetylase. In this communication, we have focused on the superiority of several classes of polyphenolic acetates to polyphenols in the modification of Cyt-P450-linked mixed function oxidases (MFOs) such as 7-ethoxyresorufin O-deethylase (EROD) and pentoxyresorufin O-dealkylase (PROD). Special attention has also been focused on benzene-induced genotoxicity in bone marrow and lung cells. Results clearly indicated that polyphenolic acetates demonstrated time-dependent inhibition of Cyt-P450-linked MFOs, while parent polyphenols failed to demonstrate the same. Polyphenolic acetates were found to be more superior to polyphenols in preventing benzene-induced micronuclei formation. The pattern of inhibition of Cyt-P450-dependent MFOs and benzene-induced micronuclei formation by polyphenolic acetates was found in tune with their specificities to calreticulin transacetylase. These results further substantiated that inhibition of Cyt-P450-linked MFOs and benzene-induced genotoxicity in bone marrow and lung cells by polyphenolic acetates are mediated by the action of calreticulin transacetylase that catalyzes the acetylation of concerned proteins.

7, 8-diacetoxy-4-methylcoumarin induced cell death in human tumor cells is influenced by calreticulin

Calreticulin (CRT), an endoplasmic reticulum resident protein demonstrates transacetylase activity in presence of 7, 8 diacetoxy-4-methyl coumarin (DAMC) in vitro. To investigate the possible role of CRT and DAMC mediated protein acetylation in cells, we investigated the effects of DAMC in tumor cells with different levels of CRT. DAMC was more toxic (clonogenicity, metabolic viability and proliferation) to human glioma cells (BMG-1) expressing low endogenous CRT level as compared to head and neck carcinoma cells (KB) with a high CRT level. The cytotoxicity was accompanied by loss of mitochondrial membrane potential in both the cells, which correlated with corresponding changes in the levels of pro-apoptotic (Bax) and anti-apoptotic (NFkB) regulators. Manipulation of CRT protein level in KB cells by application of small RNA interference enhanced the sensitivity by four folds while over expression of CRT in BMG-1 cells reduced their sensitivity to DAMC by ~20% strongly suggesting the influence of CRT on DAMC induced cytotoxicity. The partial rescue of CROE cells from DAMC induced toxicity was accompanied by changes in NFkB levels and over all protein acetylation status, besides increase in the NADPH-cytochrome c reductase activity related to its well known antioxidant property. Since CRT is over-expressed in cancer cells, which are generally resistant to radio- and chemotherapy; targeting CRT transacetylase system, may be an attractive approach for increasing the efficacy of anticancer therapies.

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.

Antioxidant activity of 4-methylcoumarins

Polyphenolic coumarins are known to act as antioxidants in biological systems, but it is difficult to distinguish their antioxidant activity from the many other effects they produce in cells. We have determined the radical scavenging capacity of 22 structurally related natural and synthetic 4-methylcoumarins, by measuring their reaction with radicals, galvinoxyl and 2,2-diphenyl-1-picrylhydrazyl, using electron paramagnetic resonance spectroscopy. Efficient antioxidant activity of 4-methylcoumarins in cells was verified using the DCF fluorescent probe assay for determination of intracellular reactive oxygen species levels. As expected, the o-dihydroxysubstituted coumarins were found to be excellent radical scavengers and better than the m-dihydroxysubstituted or monohydroxysubstituted analogues, but surprisingly the corresponding o-diacetoxy derivatives also turned out to be good scavengers, even in the absence of an esterase. Another unexpected result was that the antioxidant efficiency of 4-methylcoumarins could be modulated by introducing an ethoxycarbonylethyl substituent at the C-3 position; this effect cannot be explained by simple electron donating/withdrawing properties. Coumarin concentrations of 10 microM or less were used in all experiments, corresponding to the levels relevant for therapeutic purposes. Considering that 4-methylcoumarins, in contrast to many other coumarins, are not metabolized to toxic epoxide intermediates, these results indicate promising new strategies for the design of non-toxic antioxidant coumarin-based drugs.

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.

Mechanism of biochemical action of substituted 4-methylcoumarins. Part 11: Comparison of the specificities of acetoxy derivatives of 4-methylcoumarin and 4-phenylcoumarin to acetoxycoumarins: protein transacetylase

Our earlier observations led to the identification of a microsomal enzyme termed as acetoxy drug: protein transacetylase (TAase) catalyzing the transfer of acetyl groups from acetylated polyphenols to the receptor proteins. TAase was conveniently assayed by the irreversible inhibition of cytosolic glutathione S-transferase (GST) by the model acetoxycoumarin, 7,8-diacetoxy-4-methylcoumarin (1). The specificities of the acetoxy group on the benzenoid ring and position of the pyran carbonyl group of the coumarin with respect to oxygen heteroatom for the catalytic activity of TAase were also reported earlier. In this communication, we have demonstrated that the acetoxy coumarins and acetoxy dihydrocoumarins having a methyl group instead of a phenyl ring at the C-4, when used as the substrates, resulted in enhancement of TAase activity, while the saturation of double bond at C-3 and C-4 position had no effect on TAase activity. A comparison of the optimized structures of 1 and 7,8-diacetoxy-4-phenylcoumarin (2) suggested that the observed influence may be due to out of plane configuration of the phenyl ring at C-4. Further, the TAase-catalyzed activation of NADPH cytochrome c reductase and inhibition of aflatoxin B1 (AFB1)-DNA binding by acetoxy 4-phenylcoumarins and dihydrocoumarins were significantly lower as compared to those caused by acetoxy 4-methylcoumarins.

Non-enzymatic platelet-activating factor formation by acetylated proteins

Substantial amounts of platelet-activating factor (PAF 1-O-alkyl-2-acyl-sn-glycero-3-phosphocholine), the potent phospholipid mediator of allergic and inflammatory reactions, are formed upon incubation of acetylated low-density lipoprotein, acetylated bovine serum albumin (BSA) and acetylated apolipoprotein A-I with 1-0-hexadecyl-sn-glycero-3-phosphocholine (lyso-PAF). Acetylated BSA produced 0.3 nmol PAF/mg of protein after a 6 h incubation period with 40 microM lyso-PAF. The transfer of acetate bound to acetylated proteins to lyso-PAF was non-enzymatic. Chemical PAF formation by acetylated proteins, involved in lipid metabolism and transport, could lead to complication of inflammatory and allergic events.

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.