A specific hydroxysteroid UGT is responsible for the conjugation of aliphatic alcohols in rats: an estimation of the importance of glucuronidation versus oxidation

Endogenous formation of 1-propanol and methanol after consumption of alcoholic beverages

INTRODUCTION

In cases of drunk-driving, allegations that alcohol has been consumed after the incident, are proved by analyzing congener alcohols in the blood sample. 1-Propanol, one of the main congener compounds, was tested, whether it is also endogenously formed when a person has consumed alcoholic beverages.

METHODS

Eleven male and 13 female volunteers consumed congener-free vodka (37.5 vol% ethanol, individual doses: 0.15-0.32 l) within one hour. Blood samples were taken up to 10 h and analyzed for ethanol and congener alcohols by headspace gas chromatography-mass spectrometry.

RESULTS

Ethanol concentrations reached in blood a maximum of 0.65-1.23 g/l and decreased by 0.18 g/l/h (median values). Of the congener alcohols analyzed, only methanol and 1-propanol were detected in the plasma samples of all subjects. The endogenous methanol concentration increased from 0.66 mg/l by 0.22 mg/l/h to 2.19 mg/l (medians). 1-Propanol was not detected prior to alcohol consumption. Maximum concentrations of 0.10-0.32 mg/L were measured after 1.0-4.5 h. A plateau of the 1-propanol concentration was observed in the plasma samples of the 18 subjects lasting for 0.5-4.0 h and this alcohol was completely eliminated at ethanol concentrations of 0.17 g/l (median, range 0.03-0.55 g/l).

CONCLUSION

The results of the study confirm the formation of 1-propanol after consumption of 1-propanol-free beverages, which should be taken into account when evaluating its concentration.

Pharmacokinetic Interaction between Metformin and Verapamil in Rats: Inhibition of the OCT2-Mediated Renal Excretion of Metformin by Verapamil

The incidence of hypertension in diabetic patients has been increasing and contributing to the high mortality of diabetic patients. Recently, verapamil use was found to lower fasting blood glucose levels in diabetic patients, which led to a new indication of verapamil as combination treatment with anti-diabetic agents such as metformin. As pharmacokinetic (PK) interaction can affect drug efficacy and safety in drug combination, their PK-based interaction is recommended to be evaluated in preclinical levels as well as clinical levels. In case of metformin and verapamil, organic cation transporter (OCT) 1 and 2 primarily mediate metformin distribution to the liver and its elimination into urine, whereas cytochrome P450 is responsible for the hepatic metabolism of verapamil. Verapamil is also known as a potential OCT2 inhibitor. Thus, PK interaction between metformin (30 mg/kg) and verapamil (20 mg/kg) were investigated after their simultaneous administration to rats. In our results, verapamil inhibited the OCT2-mediated renal excretion of metformin, subsequently leading to increase of the systemic exposure of metformin. In contrast, metformin did not influence the pharmacokinetic pattern of verapamil. Although the further clinical investigation is required, our finding suggests a possibility of OCT2-mediated interaction of metformin and verapamil.

In vitro glucuronidation of 2,2-bis(bromomethyl)-1,3-propanediol by microsomes and hepatocytes from rats and humans

2,2-Bis(bromomethyl)-1,3-propanediol (BMP) is a brominated flame retardant used in unsaturated polyester resins. In a 2-year bioassay BMP was shown to be a multisite carcinogen in rats and mice. Because glucuronidation is the key metabolic transformation of BMP by rats, in this study the in vitro hepatic glucuronidation of BMP was compared across several species. In addition, the glucuronidation activities of human intestinal microsomes and specific human hepatic UDP-glucuronosyltransferase (UGT) enzymes for BMP were determined. To explore other possible routes of metabolism for BMP, studies were conducted with rat and human hepatocytes. Incubation of hepatic microsomes with BMP in the presence of UDP-glucuronic acid resulted in the formation of a BMP monoglucuronide. The order of hepatic microsomal glucuronidation activity of BMP was rats, mice >> hamsters > monkeys >>> humans. The rate of glucuronidation by rat hepatic microsomes was 90-fold greater than that of human hepatic microsomes. Human intestinal microsomes converted BMP to BMP glucuronide at a rate even lower than that of human hepatic microsomes. Among the human UGT enzymes tested, only UGT2B7 had detectable glucuronidation activity for BMP. BMP monoglucuronide was the only metabolite formed when BMP was incubated with suspensions of freshly isolated hepatocytes from male F-344 rats or with cryopreserved human hepatocytes. Glucuronidation of BMP in human hepatocytes was extremely low. Overall, the results support in vivo studies in rats in which BMP glucuronide was the only metabolite found. The poor glucuronidation capacity of humans for BMP suggests that the pharmacokinetic profile of BMP in humans will be dramatically different from that of rodents.

Development of an optimized procedure for the preparation of rat intestinal microsomes: comparison of hepatic and intestinal microsomal cytochrome P450 enzyme activities in two rat strains

The objective of this study was to characterize cytochrome P450 (CYP) activities in both intestinal and hepatic microsomes from Wistar and Sprague-Dawley rats. Specific probes for measuring CYP activities were selected using rat recombinant CYP. The intestinal microsome preparation was optimized getting a more relevant and reproducible abundance of CYPs to measure CYP activities. Testosterone, propranolol, diclofenac, and midazolam were determined as specific substrates of rat CYP2C11, CYP2D2, CYP2C6, and CYP3A, respectively. Ethoxyresorufin and pentoxyresorufin were not specific substrates of CYP1A2 and CYP2B1, respectively. Hepatic and intestinal microsomes expressed active CYP1A1, CYP1A2, CYP2B1, and CYP3A2. Only liver expressed active CYP2C6, CYP2C11, and CYP2D2. Wistar liver expressed more active CYP1A and CYP3A2, but less active CYP2B1 than Wistar intestine. Sprague-Dawley liver expressed more active CYP2B1 and CYP3A2, but less active CYP1A than Sprague-Dawley intestine. In conclusion, CYP activities were qualitatively equivalent but not quantitatively in both strains.

Forensic relevance of glucuronidation in phase-II-metabolism of alcohols and drugs

Forensic toxicology means detecting toxic or pharmacologically active substances in body fluids and organs and the evaluation and judgement of the respective results. In the legal judgement, not only the taken in active drugs, but also their metabolites are to be included. Regarding metabolism one distinguishes phase-I- and phase-II-metabolism. In the phase-I-metabolism, active substances are converted by oxidation, reduction or hydrolysis, but influencing the polarity of more lipophilic substances often not decisively. The pharmacological activity is often preserved or even increased. In phase-II-metabolism a highly hydrophilic substance--mostly glucuronic acid--is coupled to the active substances or the respective phase-I-metabolites. This reaction step decisively increases hydrophilicity of lipophilic substances, thus enhancing renal elimination and often also abolishing pharmacologically and/or toxicologically effects. Nevertheless the interaction of different drugs and alcohols in glucuronidation and the glucuronides of phase-II-metabolism still do not play a substantial role in the forensic-toxicological analysis and interpretation of results so far. However, in vitro investigations since 1999 in our lab show that such interactions are not unlikely. For valid interpretation of complex cases in the future it may become necessary not only to quantify drugs and the phase-I-metabolites but also the phase-II-metabolites and discuss possible interactions in the metabolism.

Glucuronidation of aliphatic alcohols in human liver microsomes in vitro

The glucuronidation of several short-chained aliphatic alcohols in vitro, with the use of human liver microsomes (HLM) as catalyst, was performed, and the kinetics were studied. The concentrations of the glucuronides were determined by gas chromatography-mass spectrometry after derivatization to the trimethylsilyl compounds. Alcohols from ethanol to pentanols were found to couple with activated glucuronic acid in HLM to a widely varying amount. For analytic reasons the glucuronide of methanol, which is formed after methanol consumption in human beings in vivo, could not be determined. The length of the alkyl chain played the decisive role in the maximum turnover rate of the glucuronidation. The affinity of the alcohols to UDP-glucuronosyltransferase (UDPGT), which catalyzes the glucuronidation reaction, was shown to depend strongly on their structure. Alcohols with a very short alkyl chain and secondary alcohols were glucuronidated much more slowly in comparison with findings for the longer chain primary alcohols and showed less affinity to UDPGT. The alcohols mutually inhibited glucuronidation. However, ethanol inhibited the glucuronidation of isopentanol or n-pentanol only in high concentrations, whereas the two pentanols inhibited each other to a high degree. The glucuronidation of aliphatic alcohols is probably catalyzed by only one of several very similar enzymes of the UDPGT. This finding was indicated by the fact that the Michaelis-Menten constants of the alcohols--with the use of different lots of the HLM from different liver donors--had nearly the same values.