C. De Santi

Sulphation of resveratrol, a natural compound present in wine, and its inhibition by natural flavonoids

1. Resveratrol, a polyphenolic compound present in grape and wine, has beneficial effects against cancer and protective effects on the cardiovascular system. Resveratrol is sulphated, and the hepatic and duodenal sulphation might limit the bioavailability of this compound. The aim of this study was to see whether natural flavonoids present in wine, fruits and vegetables inhibit the sulphation of resveratrol in the human liver and duodenum. 2. In the liver, IC50 for the inhibition of resveratrol sulphation was 12 ± 2 pM (quercetin), 1.0 ± 0.04 μM (fisetin), 1.4 ± 0.1 μM (myricetin), 2.2 ± 0.1 μM (kaempferol) and 2.8 ± 0.2 μM (apigenin). Similarly, in the duodenum, IC50 was 15 ± 2 pM (quercetin), 1.3 ± 0.1 μM (apigenin), 1.3 ± 0.5 μM (fisetin), 2.3 ± 0.1 μM (kaempferol) and 2.5 ± 0.3 μM (myricetin). 3. The type of inhibition of quercetin on resveratrol sulphation was studied in three liver samples and was determined to be non-competitive and mixed in nature. Km (mean ± SD; μM) was 0.23 ± 0.07 (control), 0.40 ± 0.08 (5 pM quercetin) and 0.56 ± 0.09 (10 pM quercetin). Vmax (mean ± SD; pmol·min−1·mg−1) was 99 ± 11 (control), 73 ± 15 (5 pM quercetin) and 57 ± 10 (10 pM quercetin). K1 and K1es estimates (mean ± SD) were 3.7 ± 1.8 pM and 12.1 ± 1.7 pM respectively (p = 0.010). 4. Chrysin was a substrate for the sulphotransferase(s) and an assay was developed for measuring the chrysin sulphation rate in human liver. The enzyme followed Michaelis‐Menten kinetics and Km and Vmax (mean ± SD) measured in four livers were 0.29 ± 0.07 μM and 43.1 ± 1.9 pmol·min−1·mg−1 respectively. 5. Catechin was neither an inhibitor of resveratrol sulphation nor a substrate of sulphotransferase. 6. These results are consistent with the view that many, but not all, flavonoids inhibit the hepatic and duodenal sulphation of resveratrol, and such inhibition might improve the bioavailability of this compound.

Glucuronidation of resveratrol, a natural product present in grape and wine, in the human liver

1. Resveratrol, a polyphenolic compound present in grape and wine, has beneficial effects against cancer and protective effects on the cardiovascular system. It has been shown that the compound is sulphated in human liver and the aims of the present investigation were to study resveratrol glucuronidation in human liver microsomes and to determine whether flavonoids inhibit resveratrol glucuronidation. 2. A simple and reproducible radiometric assay for resveratrol glucuronidation was developed. The assay employed uridine-5′-diphosphoglucuronic acid-[14C] and unlabelled resveratrol. Resveratrol-glucuronide was isolated by TLC. The intra- and interassays variabilities were 1 and 1.5%, respectively. 3. The rate of resveratrol glucuronidation was measured in 10 liver samples. The mean ± SD and median of resveratrol glucuronidation rate were 0.69 ± 0.34 and 0.80 nmol/min/mg, respectively. Resveratrol glucuronosyl transferase followed Michaelis-Menten kinetics and the Km and Vmax (mean ± SD; n = 5) were 0.15 ± 0.09 mm and 1.3 ± 0.3 nmol/min/mg, respectively. The intrinsic clearance was 11 ± 4 × 10−3 ml/min.mg. 4. The flavonoid quercetin inhibited resveratrol glucuronidation and its IC50 (mean ± SD; n = 3) was 10 ± 1 μM. Myricetin, catechin, kaempferol, fisetin and apigenin (all at 20 μM) inhibited resveratrol glucuronidation and the percent of control ranged between 46% (catechin) to 72% (apigenin). 5. The present results show that resveratrol is glucuronated in the human liver. Glucuronidation may reduce the bioavailability of this compound however, flavonoids inhibit resveratrol glucuronidation and such an inhibition might improve the bioavailability of resveratrol.

Differential inhibition of human liver and duodenum sulphotransferase activities by quercetin, a flavonoid present in vegetables, fruit and wine

1. Quercetin is a natural flavonoid present in vegetables, fruit and wine, and is known to inhibit sulphotransferase. Drugs are often taken orally and the intestinal mucosa is an early site of drug metabolism. The aims of this investigation were to study the inhibition of dopamine, (-)-salbutamol, minoxidil and paracetamol sulphation by quercetin in the duodenal mucosa and liver and to compare the IC 50 in these tissues. 2. The rates (pmol min -1 mg -1) of sulphation of 4-nitrophenol were 343 92 (liver) and 164 22 (duodenum; p = 0.031), of dopamine were 15 11 (liver) and 656 516 (duodenum; p = 0.049), of (-)-salbutamol were 153 31 (liver) and 654 277 (duodenum; p = 0.018), of minoxidil were 156 47 (liver) and 105 7 (duodenum; n.s.), and of paracetamol were 229 86 (liver) and 328 187 (duodenum; n.s.). 3. The IC 50 of quercetin for 4-nitrophenol was 48 11 nM (liver) and 56 1 nM (duodenum, n.s.), for dopamine was 5.7 0.7 µM (liver) and 170 12µM (duodenum, p < 0.0001), for (-)-salbutamol was 54 4 nM (liver) and 16 8µM (duodenum; p = 0.025), for minoxidil was 134 22 nM (liver) and 3 0.3 µM (duodenum, p = 0.013), and for paracetamol was 57 7 nM (liver) and 35 1 µM (duodenum; p = 0.0002). 4. Quercetin inhibited the sulphation of 4-nitrophenol, dopamine, (-)-salbutamol, minoxidil and paracetamol both in liver and duodenum. With dopamine, (-)-salbutamol, minoxidil and paracetamol as substrates, quercetin was a more potent inhibitor in the liver than the duodenum. Such a difference may reflect the different composition of sulphotransferase forms in the liver and duodenum.

Inhibition of human liver phenol sulfotransferase by nonsteroidal anti-inflammatory drugs

AbstractObjective: The aim of this investigation was to study the inhibition of 11 nonsteroidal anti-inflammatory drugs (NSAIDs) on the human liver phenol sulfotransferases (HL-PST) and catechol sulfotransferase (HL-CST). Methods: The activities of HL-PST and HL-CST were measured with 4 μM 4-nitrophenol and 60 μM dopamine (the sulfate acceptors) and 0.4 μM 3′-phosphoadenosine-5′-phosphosulfate [35S] (the sulfate donor). Samples of liver were obtained from five patients, aged 55–79 years, undergoing clinically indicated hepatectomy. The inhibition curves were constructed with at least five concentrations of the inhibitor. Results: With the exception of piroxicam, NSAIDs inhibited HL-PST, and the estimates of the inhibitory concentration for 50% of responses (IC50; μM) were: 0.02 (mefenamic acid), 3.7 (diflunisal), 5.4 (nimesulide), 9.5 (diclofenac), 30 (salicylic acid), 41 (ketoprofen), 74 (indomethacin), 159 (ibuprofen), 245 (ketoralac) and 473 (naproxen). With 4-nitrophenol as the variable substrate, the inhibition of salicylic acid on HL-PST was non-competitive and the Ki and Kies were 18 μM and 21 μM (n = 5; P = 0.548), respectively. HL-CST was less susceptible than HL-PST to inhibition by NSAIDs, with only five drugs inhibiting this enzyme. The IC50 estimates for these drugs (μM) were 76 (mefenamic acid), 79 (diflunisal), 103 (indomethacin), 609 (salicylic acid) and 753 (diclofenac). Conclusion: The comparison of the IC50 estimates of HL-PST with the therapeutic plasma concentrations of NSAIDs corrected for the plasma unbound fraction was consistent with the view that mefenamic acid and salicylic acid, when administered at therapeutic doses, should impair the hepatic sulfation of those compounds that are substrates of phenol sulfotransferase.

Catechol-O-methyltransferase: variation in enzyme activity and inhibition by entacapone and tolcapone.

AbstractObjective: The aim of this investigation was to study the variation of catechol-O-methyltransferase (COMT) activity in the human liver, duodenal mucosa and renal cortex, and to investigate the inhibition of COMT by entacapone and tolcapone. This study included 87 samples of human liver, 94 samples of the duodenum and 72 samples of the renal cortex. Results: The activity of COMT was measured with 3,4-dihydroxybenzoic acid (242 μmol · l−1), the methyl acceptor substrate, and adenosyl-l-methionine (44 μmol · l−1), the methyl donor substrate. The hepatic activity of COMT activity was significantly higher in men than in women, whereas it was not sex-dependent in the duodenum or renal cortex. The activity of COMT varied 4.4-fold in the liver of men, 2.6-fold in the duodenum and 5.3-fold in the renal cortex. The median estimates of COMT activity were 577, 499, 103 and 159 pmol · min−1 · mg−1 in the liver of men and women, in the duodenum and in the renal cortex, respectively. Conclusion: Entacapone and tolcapone were powerful inhibitors of COMT and their IC50 estimates were 151 and 773 nM (P = 0.008), respectively, in the liver; consistent results were obtained with the other tissues.

Interindividual variability in the rate of salbutamol sulphation in the human lung

The β2-adrenergic agonist salbutamol is administered by inhalation to treat lung-obstructive disease. Salbutamol is metabolized by conjugation with sulphate, and the sulphation of salbutamol was investigated in human lung. Specimens of lung were obtained at lobectomy from 11 non-smokers, 39 smokers and 46 ex-smokers, the latter refraining from smoking at least 6 months before surgery. Neither sex nor ageing influenced the activity of sulphotransferase. The rate of salbutamol sulphation (pmol·min-1·mg-1) was greater in non-smokers (27.7) than in smokers (21.3), whereas it was similar in smoker and ex-smokers (22.8). The rate of salbutamol sulphation ranged up to six fold and its distribution did not deviate from normality. As the rate of formation of the inactive salbutamol sulphate varied in the lung, the availability of salbutamol and, in turn, the evoked pharmacological effect should vary in parallel. The activities of salbutamol and dopamine sulphotransferase correlated, suggesting that catechol sulphotransferase takes part in the sulphation of salbutamol. The sulphation of salbutamol is stereoselective in the human lung, the kM estimate for (+)-salbutamol (1198 μM) being greater than those for either (-)-salbutamol (190 μM) and racemic salbutamol (142 μM). These results are consistent with the view that (-)-salbutamol is a better substrate than (+)-salbutamol for sulphotransferase.

Fenamates and the potent inhibition of human liver phenol sulphotransferase

1. The inhibition of the human liver phenol sulphotransferase (HL-PST) and catechol sulphotransferase (HL-CST) by five fenamates has been studied and the activities of HLPST and HL-CST were measured with 4-nitrophenol and dopamine as substrates, respectively. 2. The IC50 for inhibition of HL-PST were 0.02 μM (mefenamic acid); 0.12 μM (tolfenamic acid); 0.28 μM (niflumic acid); 0.87 μM (meclofenamic acid) and 1.50 μM (flufenamic acid). 3. HL-CST was less susceptible than HL-PST to the inhibition by fenamates and the IC50 for HL-CST were 36 μM (tolfenamic acid); 70 μM (flufenamic acid); 76 μM (mefenamic acid); 180 μM (niflumic acid) and 185 μM (meclofenamic acid). 4. The ratios of the IC50 for HL-CST:HL-PST were drug-dependent and ranged from 47 (flufenamic acid) to 3800 (mefenamic acid). Mefenamic acid is a relatively potent and selective inhibitor of HL-PST. 5. The IC50 for HL-PST obtained with mefenamic acid was three orders of magnitude lower than the peak plasma concentration of this drug after an oral dose of 0.5 g. Accordingly, mefenamic acid should impair sulphation in vivo.

Interethnic and interindividual variabilities of platelet sulfotransferases activity in Italians and Finns

AbstractObjective: The aim of this investigation was to see whether there was interethnic variability in the platelet activities of catechol- and phenol sulfotransferases in Italians and Finns. Methods: The activities of catechol- and phenol sulfotransferases were measured in platelets obtained from 103 Italian and 74 Finnish individuals. Blood donors were obtained from healthy volunteers free from drugs and without apparent disease. The activities of catechol- and phenol sulfotransferases were measured with 60 μM dopamine and 4 μM 4-nitrophenol as substrates, respectively Results: The activity of catechol sulfotransferase was not gender dependent and the median estimates (pmol/min/mg) were 9.10 in Italians and 6.37 in Finns (P = 0.0018). The activity of phenol sulfotransferase activity was gender dependent in Finns but not in Italians. The median estimates (pmol/min/mg) were 3.81 in Finnish men and 1.18 in Finnish women (P = 0.0007). In Italian men and women, the median estimates (pmol/min/mg) of phenol sulfotransferase activity were 1.25 and 1.24, respectively (NS). Conclusion: This study shows that platelet catechol sulfotransferase activity is greater in Italians than Finns and that the activity of phenol sulfotransferase is gender regulated in Finns but not in Italians. Thus, interethnic differences exist in platelet sulfotransferases between Italians and Finns.

Interindividual variability of histamine N-methyltransferase in the human liver and kidney

1. The aim was to investigate the possibility of interindividual variability of histamine N-methyltransferase (HNMT) in the human liver and renal cortex. The activity of HNMT was measured in 99 specimens of the human liver and in 75 specimens of the renal cortex. 2. In the liver the activity of HNMT was positively skewed. It ranged 2.9-fold with a median of 1.72 pmol/min/mg. In the renal cortex the activity of HNMT was normally distributed and ranged 2.6-fold with a mean and coefficient of variation of 1.35 pmol/min/mg and 21%, respectively. 3. The activities of catechol methyltransferase and thiopurine methyltransferase were measured in the renal cortex and any correlations with HNMT activity were assessed. There was a weak but significant correlation (r = 0.294, p = 0.010) between HNMT and catechol methyltransferase activities whereas HNMT activity was not correlated with thiopurine methyltransferase activity. 4. These results are consistent with the view that HNMT is well expressed in the human liver and renal cortex and that it varies among subjects.