Human liver and plasma aspirin esterase

Do Aspirin and Flavonoids Prevent Cancer through a Common Mechanism Involving Hydroxybenzoic Acids?-The Metabolite Hypothesis

Despite decades of research to elucidate the cancer preventive mechanisms of aspirin and flavonoids, a consensus has not been reached on their specific modes of action. This inability to accurately pinpoint the mechanism involved is due to the failure to differentiate the primary targets from its associated downstream responses. This review is written in the context of the recent findings on the potential pathways involved in the prevention of colorectal cancers (CRC) by aspirin and flavonoids. Recent reports have demonstrated that the aspirin metabolites 2,3-dihydroxybenzoic acid (2,3-DHBA), 2,5-dihydroxybenzoic acid (2,5-DHBA) and the flavonoid metabolites 2,4,6-trihydroxybenzoic acid (2,4,6-THBA), 3,4-dihydroxybenzoic acid (3,4-DHBA) and 3,4,5-trihydroxybenzoic acid (3,4,5-THBA) were effective in inhibiting cancer cell growth in vitro. Limited in vivo studies also provide evidence that some of these hydroxybenzoic acids (HBAs) inhibit tumor growth in animal models. This raises the possibility that a common pathway involving HBAs may be responsible for the observed cancer preventive actions of aspirin and flavonoids. Since substantial amounts of aspirin and flavonoids are left unabsorbed in the intestinal lumen upon oral consumption, they may be subjected to degradation by the host and bacterial enzymes, generating simpler phenolic acids contributing to the prevention of CRC. Interestingly, these HBAs are also abundantly present in fruits and vegetables. Therefore, we suggest that the HBAs produced through microbial degradation of aspirin and flavonoids or those consumed through the diet may be common mediators of CRC prevention.

Metabolism of triflumuron in the human liver: Contribution of cytochrome P450 isoforms and esterases

Triflumuron (TFM) is a benzoylurea insecticide commonly used in Tunisian agriculture and around the world to control crop pests and flies as a promising alternative to conventional insecticides for its arthropod specificity and low toxicity. From the evidence available in animal models, it can be expected that the metabolism of TFM is catalyzed by cytochrome P450 (CYP) and esterases. However, no data are available on human metabolism of TFM with regards to phase I metabolism and CYP isoform specificity. Hence, this manuscript describes experimental investigations to underpin in vitro phase I TFM metabolism in human samples for the first time. TFM biotransformation by recombinant human CYPs was characterized, then human liver microsomes (HLM) and chemical specific inhibitors have been used to identify the relative contribution of CYPs and esterases. Our results showed that all CYP isoforms were able to metabolize TFM with different affinity and efficiency. The relative contribution based both on the kinetic parameters and the CYP hepatic content was 3A4 > >2C9 > 2C8 > 2A6 > 1A2 > 2B6 > 2D6 > 2C19 > 2C18 > 1A1 at low TFM concentration, whilst at high TFM concentration it was 1A2 > >2C9 = 3A4 = 2A6 > 2C19 > 2B6 = 2C8 > 2D6 > 1A1 > 2C18. Experiments with HLMs confirmed the involvement of the most relevant CYPs in the presence of specific chemical inhibitors with a catalytic efficiency (Cliapp) lower by an order of magnitude compared with recombinant enzymes. Esterases were also relevant to the overall TFM kinetics and metabolism, with catalytic efficiency higher than that of CYPs. It is foreseen that such isoform-specific information in humans will further support in silico models for the refinement of the human risk assessment of single pesticides or mixtures.

Effects of Panax Notoginseng Saponins on Esterases Responsible for Aspirin Hydrolysis In Vitro

Herb–drug interactions strongly challenge the clinical combined application of herbs and drugs. Herbal products consist of complex pharmacological-active ingredients and perturb the activity of drug-metabolizing enzymes. Panax notoginseng saponins (PNS)-based drugs are often combined with aspirin in vascular disease treatment in China. PNS was found to exhibit inhibitory effects on aspirin hydrolysis using Caco-2 cell monolayers. In the present study, a total of 22 components of PNS were separated and identified by UPLC-MS/MS. Using highly selective probe substrate analysis, PNS exerted robust inhibitory potency on human carboxylesterase 2 (hCE2), while had a minor influence on hCE1, butyrylcholinesterase (BChE) and paraoxonase (PON). These effects were also verified through molecular docking analysis. PNS showed a concentration-dependent inhibitory effect on hydrolytic activity of aspirin in HepaRG cells. The protein level of hCE2 in HepaRG cells was suppressed after PNS treatment, while the level of BChE or PON1 in the extracellular matrix were elevated after PNS treatment. Insignificant effect was observed on the mRNA expression of the esterases. These findings are important to understand the underlying efficacy and safety of co-administration of PNS and aspirin in clinical practice.

Inhibitory Influence of Panax notoginseng Saponins on Aspirin Hydrolysis in Human Intestinal Caco-2 Cells

Herb-drug interactions are important safety concerns in clinical practice. The interactions occur firstly in the intestinal absorption for orally administered drugs. Aspirin and Panax notoginseng saponins (PNS)-based drugs are often combined in China to prevent larger-artery atherosclerosis. Here, we aimed to characterize the aspirin transport across Caco-2 cell monolayers, a model of the intestinal absorption, and further to evaluate the influence of PNS on aspirin hydrolysis and the relating mechanisms. Transcellular transport of aspirin and the influence of PNS were explored using Caco-2 cell monolayers. The protein expression of human carboxylesterase 1 (hCE1) and hCE2 in Caco-2 cells after PNS treatment was analyzed by ELISA, and the mRNA level were determined by qRT-PCR. In the study, Caco-2 cells showed high level of hydrolase activity, and most aspirin was hydrolyzed inside the cells during the transport process. Interestingly, PNS were demonstrated to inhibit the esterase activities responsible for aspirin hydrolysis in Caco-2 cells. PNS could also decrease the protein expression of hCE1 and hCE2, whereas exhibited minor effect on the mRNA expression. These results indicated that oral administration of PNS-based drugs might inhibit the hydrolysis of aspirin during intestinal absorption thus promoting its bioavailability.

Hansen solubility parameters for assay method optimization of simvastatin, ramipril, atenolol, hydrochlorothiazide and aspirin in human plasma using liquid chromatography with tandem mass spectrometry

A simple, specific, sensitive, validated method was developed using liquid chromatography with tandem mass spectrometry with electrospray ionization of human plasma for the simultaneous estimation of drugs (simvastatin, ramipril, atenolol, hydrochlorothiazide, and aspirin) of Polycap^TM capsule used in cardiovascular therapy. The interaction of these actives including internal standards between the stationary and mobile phase were investigated using Hansen solubility parameters. Chromatographic separation was performed on Phenomenex Synergi Polar-RP (30 × 2 mm, 4 μm) column with a gradient mobile phase composition of acetonitrile and 5 mM ammonium formate for positive mode and 0.1% formic acid in both water and acetonitrile for negative mode. The flow rate and runtime were 1.0 mL/min and 3.5 min, respectively. Sample extraction was done by protein precipitation using acetonitrile, enabling a fast analysis. The calibration ranges from 0.1 to 100, 0.1 to 100, and 1 to 1000 ng/mL for simvastatin, ramipril, and atenolol using internal standard carbamazepine in positive mode, respectively, whereas it was 0.3-300 and 2-2000 ng/mL for hydrochlorothiazide and aspirin using internal standard 7-hydroxy coumarin in negative mode, respectively. Hansen solubility parameters can be used as a high-throughput optimizing tool for column and mobile phase selection in bioanalysis. This validated bioanalytical method has the potential for future fixed dose combination based preclinical and clinical studies that can save analysis time.

Aspirin as a COX inhibitor and anti-inflammatory drug in human skeletal muscle

Although aspirin is one of the most common anti-inflammatory drugs in the world, the effect of aspirin on human skeletal muscle inflammation is almost completely unknown. This study examined the potential effects and related time course of an orally consumed aspirin dose on the inflammatory prostaglandin E₂ (PGE₂)/cyclooxygenase (COX) pathway in human skeletal muscle. Skeletal muscle biopsies were taken from the vastus lateralis of 10 healthy adults (5 male and 5 female, 25 ± 2 yr old) before (Pre) and 2, 4, and 24 h after (Post) a standard dose (975mg) of aspirin and partitioned for analysis of 1) in vivo PGE₂ levels in resting skeletal muscle and 2) ex vivo skeletal muscle PGE₂ production when stimulated with the COX substrate arachidonic acid (5 μM). PGE₂ levels in vivo and PGE₂ production ex vivo were generally unchanged at each time point after aspirin consumption. However, most individuals clearly showed suppression of PGE₂, but at varying time points after aspirin consumption. When the maximum suppression after aspirin consumption was examined for each individual, independent of time, PGE₂ levels in vivo (184 ± 17 and 104 ± 23pg/g wet wt at Pre and Post, respectively) and PGE₂ production ex vivo (2.74 ± 0.17 and 2.09 ± 0.11pg·mg wet wt^-1·min^-1 at Pre and Post, respectively) were reduced ( P < 0.05) by 44% and 24%, respectively. These results provide evidence that orally consumed aspirin can inhibit the COX pathway and reduce the inflammatory mediator PGE₂ in human skeletal muscle. Findings from this study highlight the need to expand our knowledge regarding the potential role for aspirin regulation of the deleterious influence of inflammation on skeletal muscle health in aging and exercising individuals. NEW & NOTEWORTHY This study demonstrated that orally consumed aspirin can target the prostaglandin/cyclooxygenase pathway in human skeletal muscle. This pathway has been shown to regulate skeletal muscle metabolism and inflammation in aging and exercising individuals. Given the prevalence of aspirin consumption, these findings may have implications for skeletal muscle health in a large segment of the population.

A brief review on high on-aspirin residual platelet reactivity

Although aspirin is effective in secondary prevention in coronary heart disease, new thromboembolic events in patients on aspirin are frequently seen. In trials on aspirin-treated patients, platelet function tests have revealed large variability in platelet aggregation. This phenomenon has been named aspirin resistance, aspirin non-responsiveness or high-on-aspirin residual platelet reactivity. The mechanism of aspirin antiplatelet effect is due to the inhibition of cyclooxygenase-1 enzyme in platelets. In some trials, almost all patients on aspirin have a very low level of serum thromboxane B2, indicating that the measured platelet reactivity in aspirin-treated patients might be due to platelet activation via other pathways, such as ADP or thrombin. The prevalence of real aspirin resistance seems to be very low, and probably the term "high-on-aspirin residual platelet reactivity" should be preferred to describe this phenomenon.

Role of novel terpenes in transcutaneous permeation of valsartan: effectiveness and mechanism of action

CONTEXT

The greatest obstacle for transdermal delivery is the barrier property of the stratum corneum. Many approaches have been employed to breach the skin barrier; the most widely used one is that of chemical penetration enhancers. Of the penetration enhancers, terpenes are arguably the most highly advanced and proven category.

OBJECTIVE

The aim of this investigation was to study effectiveness and mechanism of seven novel terpenes, namely iso-eucalyptol, β-citronellene, valencene, rose oxide, safranal, lavandulol acetate, and prenol, as potential penetration enhancers for improved skin permeation of valsartan through rat skin and human cadaver skin (HCS) with reference to established terpene eucalyptol.

METHODS

Skin permeation studies were carried out using Automated Transdermal Diffusion Cell Sampling System (SFDC 6, LOGAN Instruments Corp., NJ) on rat skin and HCS. The mechanism of skin permeation enhancement of valsartan by terpenes treatment was evaluated by Fourier transform infrared spectroscopy (FT-IR) analysis, differential scanning calorimetry (DSC) thermogram, and histopathological examination.

RESULTS AND DISCUSSION

Among all study enhancers, iso-eucalyptol produced the maximum enhancement via rat skin [enhancement ratio (ER) = 7.4] and HCS (ER = 3.60) over control. FT-IR spectra and DSC thermogram of skin treated with aforesaid terpenes indicated that permeation occurred due to the disruption of lipid bilayers. No apparent skin irritation (erythema, edema) was observed on treatment with terpenes except β-citronellene, safranal, lavandulol acetate, and prenol, which caused mild irritation.

CONCLUSION

It is concluded that the iso-eucalyptol can be successfully used as safe and potential penetration enhancer for enhancement of skin permeation of lipophilic drug such as valsartan.

Serum aspirin esterase activity is lower in end-stage renal disease patients than in healthy control subjects and increases after haemodialysis

Background

Studies regarding aspirin metabolism can be important in patients with renal failure who have an increased risk of cardiovascular diseases. We undertook this study to assess the aspirin esterase (AE) status in end-stage renal disease (ESRD) patients.

Methods

A total of 42 patients on long-term haemodialysis (HD) with a mean dialysis course of 6.1 y were recruited.

Results

Serum AE levels were 44% lower and cholinesterase (ChE) levels were 22% lower in ESRD patients before dialysis as compared with control subjects ( P = 0.0001). A very strong correlation was found between AE and ChE levels. AE levels increased on average 28% after dialysis with adjustments for age, gender, total cholesterol, triglyceride and high-density lipoprotein cholesterol ( P = 0.002). In addition, ChE levels were significantly increased (48%) after dialysis ( P = 0.0001). Changes in AE activity were significantly and positively correlated with those of ChE ( r = 0.427, P = 0.005). When we adjusted for several confounders, we found that the changes in AE activity operated by dialysis are significant independently of age, gender, aspirin (ASA) intake, cholesterol, triglycerides, high-density lipoprotein cholesterol and ChE.

Conclusions

We report that serum AE activity is significantly lower in ESRD and that treatment by HD results in an increase of activity. We confirm that AE is associated with lipid parameters and ChE. Our results show variations in ASA catabolism between the dialysis sessions, suggesting an oscillating pattern in ASA disposal in these patients. The mechanisms for reduced AE activity in uraemia and the effects of HD need further investigation.

Paraoxonase 1 (PON1) modulates the toxicity of mixed organophosphorus compounds

A transgenic mouse model of the human hPON1(Q192R) polymorphism was used to address the role of paraoxonase (PON1) in modulating toxicity associated with exposure to mixtures of organophosphorus (OP) compounds. Chlorpyrifos oxon (CPO), diazoxon (DZO), and paraoxon (PO) are potent inhibitors of carboxylesterases (CaE). We hypothesized that a prior exposure to these OPs would increase sensitivity to malaoxon (MO), a CaE substrate, and the degree of the effect would vary among PON1 genotypes if the OP was a physiologically significant PON1 substrate in vivo. CPO and DZO are detoxified by PON1. For CPO hydrolysis, hPON1(R192) has a higher catalytic efficiency than hPON1(Q192). For DZO hydrolysis, the two alloforms have nearly equal catalytic efficiencies. For PO hydrolysis, the catalytic efficiency of PON1 is too low to be physiologically relevant. When wild-type mice were exposed dermally to CPO, DZO, or PO followed 4-h later by increasing doses of MO, toxicity was increased compared to mice receiving MO alone, presumably due to CaE inhibition. Potentiation of MO toxicity by CPO and DZO was greater in PON1(-/-) mice, which have greatly reduced capacity to detoxify CPO or DZO. Potentiation by CPO was more pronounced in hPON1(Q192) mice than in hPON1(R192) mice due to the decreased efficiency of hPON1(Q192) for detoxifying CPO. Potentiation by DZO was similar in hPON1(Q192) and hPON1(R192) mice, which are equally efficient at hydrolyzing DZO. Potentiation by PO was equivalent among all four genotypes. These results indicate that PON1 status can have a major influence on CaE-mediated detoxication of OP compounds.

Nafamostat is hydrolysed by human liver cytosolic long-chain acyl-CoA hydrolase

Although the authors recently reported that nafamostat, a clinically used serine protease inhibitor, was mainly hydrolysed by carboxylesterase in human liver microsomes, the involvement of human liver cytosol has not been elucidated. The current study examined the in vitro metabolism of nafamostat with human liver cytosols. Kinetic analysis indicated that the Vmax and Km values in the liver cytosols were 9.82 nmolmin(-1) mg(-1) protein and 197 microM for a liver sample HL-1, and 15.1 nmolmin(-1) mg(-1) protein and 157 microM for HL-2, respectively. The Vmax/Km values in both cytosols were at least threefold higher than those in the corresponding microsomes. The liver cytosolic activity for nafamostat hydrolysis was inhibited by phenylmethylsulfonyl fluoride (PMSF) (43% inhibition at 100 microM), whereas diisopropyl fluorophosphate (DFP) and bis(p-nitrophenyl)phosphate (BNPP) failed to inhibit the activity. Furthermore, the hydrolytic activity was also reduced by palmitoyl-CoA (67% inhibition at 100 microM) but not by acetyl-CoA. Effects of PMSF, DFP and BNPP on cytosolic palmitoyl-CoA hydrolytic activity were comparable with those of the cytosolic nafamostat hydrolytic activity. In addition, the palmitoyl-CoA hydrolytic activity was competitively inhibited by nafamostat with the apparent Ki value of 164 microM for the liver cytosol from HL-2. These results suggest that an isoform of long-chain acyl-CoA hydrolase may be responsible for the nafamostat hydrolysis in human liver cytosol.

Plasma esterases and inflammation in ageing and frailty

OBJECTIVES

Esterases are enzymes of drug metabolism known to be reduced in frail older people and during acute illness. The mechanism for this is unknown. The aim of this study was to examine esterase activity and inflammation in ageing and frailty.

METHODS

Thirty frail patients (mean age 84.9 years) dependent on continuing inpatient care, 40 patients of intermediate frailty attending Day Hospital (84.2 years), 40 fit older controls (82.7 years) and 30 young controls (23.3 years) were studied. Frailty indicators, plasma esterase activities and markers of inflammation were measured.

RESULTS

With increasing patient frailty, C-reactive protein (CRP), interleukin-6 (IL-6) and tumour necrosis factor-alpha (TNF-alpha) increased significantly and esterase activity, with the exception of aspirin esterase, fell significantly (p < 0.005). There were significant negative correlations between log-transformed IL-6 and acetylcholinesterase (r = -0.354, p < 0.01), butyrylcholinesterase (r = -0.392, p < 0.01) and benzoylcholinesterase activity (r = -0.241, p < 0.05) and significant negative correlations between TNF-alpha and acetylcholinesterase (r = -0.223, p < 0.01), butyrylcholinesterase (r = -0.279, p < 0.01) and benzoylcholinesterase activity (r = -0.253, p < 0.01). Aspirin esterase activity did not correlate with IL-6 or TNF- alpha.

CONCLUSION

Frailty was associated with higher inflammatory markers and lower esterase activity. There was a weak but significant negative correlation between both IL-6 and TNF-alpha and the activity of three of four esterases. The negative correlation between esterase activity and inflammatory markers may have a causal basis, comparable to the inflammatory suppression of cytochrome P-450 enzymes.

Nonsteroidal anti-inflammatory drugs in cats: a review

OBJECTIVE

To review the evidence regarding the use of nonsteroidal anti-inflammatory drugs (NSAIDs) in cats.

DATABASES USED

PubMed, CAB abstracts.

CONCLUSIONS

Nonsteroidal anti-inflammatory drugs should be used with caution in cats because of their low capacity for hepatic glucuronidation, which is the major mechanism of metabolism and excretion for this category of drugs. However, the evidence presented supports the short-term use of carprofen, flunixin, ketoprofen, meloxicam and tolfenamic acid as analgesics in cats. There were no data to support the safe chronic use of NSAIDs in cats.

Malathion detoxification by human hepatic carboxylesterases and its inhibition by isomalathion and other pesticides

The organophosphorothioate (OPT) pesticide malathion (MAL) in mammals is readily hydrolyzed by mammalian carboxylesterases (CE). The reaction competes with the CYP-catalyzed formation of malaoxon (MOX), the toxic metabolite. Alterations or individual variations in CE activity may result in increased MOX formation, enhancing MAL toxicity. We have characterized the human hepatic CE activity in a panel of 18 human liver microsomes as well as the inhibitory effect of IsoMAL, a major impurity of MAL commercial formulations, parathion (PAR), chlorpyrifos (CPF), and chlorpyrifos-oxon (CPFO). CE activity showed a low level of variation among individuals (4-fold). The reaction consists of two different phases, differing in their affinity for mal (k(m1)=0.25-0.69 microm; K(m2)=10.3-26.8 microM). The relatively low K(m1) values confirmed that human CE efficiently detoxify MAL. IsoMAL was shown to be a potent noncompetitive inhibitor of MAL detoxification (K(i)=0.6 microM), with a higher inhibitory potency than CPF and PAR (K(i)=7.5 microM and 50 microM, respectively). These two latter compounds very likely act as mixed inhibitors. CPFO showed the highest inhibitory potency toward CE-mediated detoxification, being characterized by a K(i)=22 nM. The present results provide useful information for a better understanding of possible interactions between different OPTs and for assessing the potential cumulative risk for exposure to OPT mixtures.

Stereoselective disposition of flurbiprofen from a mutual prodrug with a histamine H2-antagonist to reduce gastrointestinal lesions in the rat

The in vitro and in vivo stereoselective hydrolysis characteristics of the mutual prodrug FP-PPA, which is a conjugate of flurbiprofen (FP) with the histamine H2-antagonist PPA, to reduce gastrointestinal lesions induced by FP were investigated and compared with those of FP methyl ester (rac-FP-Me) and FP ethyleneglycol ester (rac-FP-EG). The rac-FP derivatives were hydrolyzed preferentially to the (+)-S-isomer in plasma and to the (-)-R-isomer in liver and small intestinal mucosa. Interestingly, in the gastric mucosa, the stereoselectivity of hydrolysis of (-)-R-FP-PPA was opposite from that of rac-FP-Me and rac-FP-EG, which suggested that the stereoselective hydrolysis of FP-PPA was helpful in reducing gastric damage induced by (+)-S-FP. However, hydrolysis of all rac-FP derivatives was found to be catalyzed by carboxylesterases in the gastric mucosa. The stereoselective disposition of FP enantiomers early after intravenous administration of rac-FP-PPA could be explained by the stereoselective formation of (-)-R-FP from rac-FP-PPA in the liver. (-)-R-FP-PPA was completely hydrolyzed to form (-)-R-FP in vivo, while 78% of (+)-S-FP-PPA was hydrolyzed to (+)-S-FP, with a corresponding decrease in the area under the curve. Twenty-five percent of (+)-S-FP-PPA might be eliminated as the intact prodrug or its metabolites other than FP. The most important bioconversion of FP-PPA occurred in plasma, and additional hydrolysis of the R-enantiomer in liver resulted in the stereoselectivity observed following both i.v. and p.o. administration.

Pharmacokinetics and pharmacodynamics in copper deficiency. I. Antiinflammatory activity of aspirin

The effect of nutritional copper (Cu) deficiency on the antiinflammatory activity and pharmacokinetics of aspirin (ASA) was investigated in rats. Male, weanling Sprague-Dawley rats were fed either a Cu-deficient (CuD) or Cu-sufficient (CuS) diet for 49-50 d. The antiinflammatory activity of ASA was studied using the carrageenan-induced paw edema (CPE) test. ANOVA analyses of edema volumes at 2, 3, 4, 5, and 21 h postcarrageenan indicated significant differences between groups. The percent inhibition of edema due to ASA treatment in CuS was lower than that in CuD rats at 5 h, AUC5h, and AUC21h. ASA was found to be significantly more effective in inhibiting the CPE in CuD rats when compared to the CuS rats. Thus, we hypothesized that the increase in ASA's antiinflammatory activity in CuD rats was a result of a decrement in its elimination during nutritional Cu deficiency. The elimination of ASA in CuD and CuS rats was studied using an iv dose of 200 mg/kg. Concentrations of ASA and salicylic acid (SA) were determined in blood; whereas the concentrations of SA, salicylic phenol-glucuronide (SPG), and salicyluric acid (SUA) were determined in urine by HPLC. The results of the pharmacokinetic analyses from blood and urinary data indicated no significant differences in the disposition of ASA between CuD and CuS rats. For instance, the total body clearance for ASA (mean +/- SD, mL/min/kg) was 37.9 +/- 9.4 and 38.5 +/- 13.9 (p > 0.05); and the volume of distribution (Vd) for ASA (mean +/- SD, mL/kg) was 385.5 +/- 110.3 and 397.1.1 +/- 137.9 (p > 0.05) for CuD and CuS groups, respectively. Thus, contrary to our hypothesis, the enhanced antiinflammatory activity of ASA in CuD rats does not appear to be mediated via a decrement in the elimination of the drug. In addition, plasma ASA-esterase activity was found to be independent of Cu nutritional status.

Human xenobiotic metabolizing esterases in liver and blood

Esterases in human liver microsomes hydrolysed fluazifop-butyl (Vmax 9.8 +/- 1.6 mumol/min/g tissue), paraoxon (Vmax 47.4 +/- 7.5 nmol/min/g tissue) and phenylacetate (Vmax 57 +/- 8 mumol/min/g tissue), whereas esterases found in the human liver cytosol hydrolysed fluazifop-butyl (Vmax 10.0 +/- 0.5 mumol/min/g tissue) and phenylacetate (Vmax 37 +/- 2.9 mumol/min/g tissue) but not paraoxon. Human plasma esterase hydrolysed fluazifop-butyl (Vmax 0.09 +/- 0.006 mumol/min/mL), paraoxon (Vmax 210 +/- 14 nmol/min/mL) and phenylacetate (Vmax 250 +/- 17 mumol/min/mL). Inhibitory studies using paraoxon, bis-nitrophenol phosphate and mercuric chloride indicated fluazifop-butyl hydrolysis involved carboxylesterase in liver microsomes and cytosol, and cholinesterase and carboxylesterase in plasma. Phenylacetate hydrolysis involved arylesterase in plasma, both arylesterase and carboxylesterase in liver microsomes and carboxylesterase in liver cytosol. Plasma hydrolysis is less important and overall esterase activity is lower in humans than in the rat which is therefore a poor model.

Postoperative decline in plasma aspirin-esterase and cholinesterase activity in surgical patients

We studied preoperative and postoperative plasma hydrolysis of aspirin and plasma cholinesterase activity in surgical patients. Postoperative aspirin esterase and cholinesterase activities fell sharply (119 +/- 32 micrograms ml h-1 and 3746 +/- 1068 U l-1 respectively; P < 0.01) in comparison to basal preoperative values (157 +/- 33 micrograms ml h-1 and 4620 +/- 940 U l-1 respectively, P < 0.01). We suggest that alterations in plasma esterase during the postoperative period may be related to the metabolic response to anesthesia- and surgery-induced stress.

Esterase activity in rat hepatocytes

Hydrolysis of acetylsalicylate, benorylate, phenetsal, fluazifop butyl and paraoxon has been studied with freshly isolated rat hepatocytes maintained as a monolayer. Acetylsalicylate and paraoxon were the poorest substrates for hydrolysis whereas benorylate was hydrolysed one hundred times faster. Phenetsal and fluazifop butyl were both hydrolysed at one-tenth of the rate of benorylate. Inhibitor studies with paraoxon, BNPP and physostigmine indicated the involvement of different carboxylesterase isozymes. Studies with acetylsalicylate indicated that uptake of the substrate into the hepatocyte may influence the rate of formation of the hydrolysis product. Studies of hydrolysis in hepatocytes more closely reflect in vivo hepatic hydrolysis than subcellular fractions as cytosolic and microsomal esterases can act in parallel.

Benorylate hydrolysis by human plasma and human liver

1. Benorylate (4-acetamido phenyl-O-acetylsalicylate) hydrolysis in vitro by human plasma and by human liver microsomes and cytosol has been investigated. 2. Benorylate was hydrolysed by a route involving initial hydrolysis of the acetyl group to yield phenetsal followed by hydrolysis to paracetamol and salicylate. Hydrolysis via acetylsalicylate was minor. 3. Benorylate was more actively hydrolysed by liver cytosol than microsomes and about 10 times faster than plasma. 4. Following a single oral dose benorylate (4 g) to volunteers only salicylate and paracetamol were detected in the plasma. 5. The therapeutic effects of benorylate appear to be mediated by salicylate and paracetamol.