caa a22 4500 477127258 CHVBK 20180405111653.0 cr unu---uuuuu 170330e19971001xx s 000 0 eng 10.1023/A:1012133008134 doi (NATIONALLICENCE)springer-10.1023/A:1012133008134 Glucuronidation of Entacapone, Nitecapone, Tolcapone, and Some Other Nitrocatechols by Rat Liver Microsomes [Elektronische Daten] [Pia Lautala, Maija Kivimaa, Hannele Salomies, Eivor Elovaara, Jyrki Taskinen] Purpose. Nitrocatechol COMT inhibitors are a new class of bioactive compounds, for which glucuronidation is the most important metabolic pathway. The objective was to characterize the enzyme kinetics of nitrocatechol glucuronidation to improve the understanding and predicting of the pharmacokinetic behavior of this class of compounds. Methods. The glucuronidation kinetics of seven nitrocatechols and 4-nitrophenol, the reference substrate for phenol UDP-glucuronosyltrans-ferase activity, was measured in liver microsomes from creosote-treated rats and determined by non-linear fitting of the experimental data to the Michaelis-Menten equation. A new method that combined densitometric and radioactivity measurement of the glucuronides separated by HPTLC was developed for the quantification. Results. Apparent Km values for the nitrocatechols varied greatly depending on substitution pattern being comparable with 4-nitrophenol (0.11 mM) only in the case of 4-nitrocatechol (0.19 mM). Simple nitrocatechols showed two-fold Vmax values compared with 4-nitrophenol (68.6 nmol min−1 mg−1), while all disubstituted catechols exhibited much lower glucuronidation rate. Vmax/Km values were about 10 times higher for monosubstituted catechols compared to disubstituted ones. The kinetic parameters for COMT inhibitors were in the following order: Km nitecapone >> entacapone > tolcapone; Vmax nitecapone > entacapone > tolcapone; Vmax/Km tolcapone > nitecapone > entacapone. Conclusions. Nitrocatechols can in principle be good substrates of UGTs. However, substituents may have a remarkable effect on the enzyme kinetic parameters. The different behaviour of nitecapone compared to the other COMT inhibitors may be due to its hydrophilic 5-substituent. The longer elimination half-life of tolcapone in vivo compared to entacapone could not be explained by glucuronidation kinetics in vitro. Plenum Publishing Corporation, 1997 nitrocatechols nationallicence glucuronidation nationallicence rat liver microsomes nationallicence Michaelis-Menten kinetics nationallicence HPTLC nationallicence Lautala Pia Department of Pharmacy, Division of Pharmaceutical Chemistry, University of Helsinki, Helsinki, Finland aut Kivimaa Maija Department of Pharmacy, Division of Pharmaceutical Chemistry, University of Helsinki, Helsinki, Finland aut Salomies Hannele Department of Pharmacy, Division of Pharmaceutical Chemistry, University of Helsinki, Helsinki, Finland aut Elovaara Eivor Finnish Institute of Occupational Health, Helsinki, Finland aut Taskinen Jyrki Department of Pharmacy, Division of Pharmaceutical Chemistry, University of Helsinki, Helsinki, Finland aut Pharmaceutical Research Kluwer Academic Publishers-Plenum Publishers 14/10(1997-10-01), 1444-1448 0724-8741 14:10<1444 1997 14 11095 https://doi.org/10.1023/A:1012133008134 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1023/A:1012133008134 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Lautala Pia Department of Pharmacy, Division of Pharmaceutical Chemistry, University of Helsinki, Helsinki, Finland aut NATIONALLICENCE 700 1- Kivimaa Maija Department of Pharmacy, Division of Pharmaceutical Chemistry, University of Helsinki, Helsinki, Finland aut NATIONALLICENCE 700 1- Salomies Hannele Department of Pharmacy, Division of Pharmaceutical Chemistry, University of Helsinki, Helsinki, Finland aut NATIONALLICENCE 700 1- Elovaara Eivor Finnish Institute of Occupational Health, Helsinki, Finland aut NATIONALLICENCE 700 1- Taskinen Jyrki Department of Pharmacy, Division of Pharmaceutical Chemistry, University of Helsinki, Helsinki, Finland aut NATIONALLICENCE 773 0- Pharmaceutical Research Kluwer Academic Publishers-Plenum Publishers 14/10(1997-10-01), 1444-1448 0724-8741 14:10<1444 1997 14 11095 Metadata rights reserved Springer special CC-BY-NC licence nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-springer