Effect of Aging on Glucuronidation of Valproic Acid in Human Liver Microsomes and the Role of UDP-Glucuronosyltransferase UGT1A4, UGT1A8, and UGT1A10

Abstract

ABSTRACT:Valproic acid (VPA) is a widely used anticonvulsant that is also approved for mood disorders, bipolar depression, and migraine. In vivo, valproate is metabolized oxidatively by cytochromes P450 and ␤-oxidation, as well as conjugatively via glucuronidation. The acyl glucuronide conjugate (valproate-glucuronide or VPAG) is the major urinary metabolite (30-50% of the dose). It has been hypothesized that glucuronidation of antiepileptic drugs is spared over age, despite a known decrease in liver mass. The formation rates of VPAG in a bank of elderly (65 years onward) human liver microsomes (HLMs) were measured by liquid chromatography/tandem mass spectrometry and compared with those in a younger (2-56 years) HLM bank. In vitro kinetic studies with recombinant UDPglucuronosyltransferases (UGTs) were completed. A 5-to 8-fold variation for the formation of VPAG was observed within the microsomal bank obtained from elderly and younger donors. VPAG formation ranged from 6.0 to 53.4 nmol/min/mg protein at 1 mM substrate concentration (n ‫؍‬ 36). The average velocities at 0.25, 0.5, and 1 mM VPA were 7.0, 13.4, and 25.4 nmol/min/mg protein, respectively, in the elderly HLM bank. Rates of VPAG formation were not significantly different in the HLM bank obtained from younger subjects. Intrinsic clearances (V max /K m ) for several cloned, expressed UGTs were determined. UGT1A4, UGT1A8, and UGT1A10 also were found to catalyze the formation of VPAG in vitro. This is the first reported activity of these UGTs toward VPA glucuronidation. UGT2B7 had the highest intrinsic clearance, whereas UGT1A1 demonstrated no activity. In conclusion, our investigation revealed no differences in VPAG formation in younger versus elderly HMLs and revealed three other UGTs that form VPAG in vitro.Glucuronidation is the conjugation of a glucuronic acid moiety to a range of functional groups-primary, secondary, tertiary, and aromatic amines, carboxylic acids, thiols, hydroxyls, and phenolic functional groups of xenobiotics or endogenous compounds. This conjugative reaction is carried out by UDP-glucuronosyltransferases (UGTs), enzymes expressed on the inner membrane of the endoplasmic reticulum (Dutton, 1966;Mackenzie et al., 1997;Remmel et al., 2008). Although predominantly expressed in the liver, UGTs are diverse in expression as well as function and are categorized into three subfamilies: UGT1A, UGT2A, and UGT2B (Mackenzie et al., 1997;Burchell et al., 1998;Tephly et al., 1998). Four UGT2B isoforms and nine UGT1A isoforms are expressed in humans and catalyze the glucuronidation of a variety of endogenous and exogenous substrates (Burchell et al., 1998;Mackenzie et al., 2000;Tukey and Strassburg, 2001). Glucuronidation is an important metabolic pathway for many drugs or oxidative metabolites. Several antiepileptic drugs (AEDs) including valproic acid (VPA) are excreted extensively as their glucuronides (Levy et al., 2002). VPA is directly glucuronidated to form its ester (acyl) glucuronide that is excreted in the urine, accounting for 30 -7...

Microsomal Quercetin Glucuronidation in Rat Small Intestine Depends on Age and Segment

“…Although human studies of intestinal glucuronidation in elderly individuals are lacking, in vitro results from liver bank studies indicate that advanced age makes little to no difference in the hepatic microsomal glucuronidation of valproate (Argikar andRemmel, 2009), S-oxazepam, trifluoperazine, serotonin, propofol, zidovudine (Court, 2010), and 4-methylumbelliferone (Parkinson et al, 2004). In rats, hepatic microsomal glucuronidation of acetaminophen was also unchanged with advanced age (Sweeny and Weiner, 1985;Woodhouse and Herd, 1993).…”

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“…31 CYP P450 enzyme system, though accounting only for about 10% of VPA metabolism, is mediated by polymorphic isoforms such as CYP4B, CYP2C9 and CYP2A6. 6 Other UGTs involved such as UGT2B7, UGT1A3 have also polymorphic forms 9, 32 e.g. UGT2B7 161C>T polymorphism has been reported to affect VPA concentrations in pediatric epilepsy patients.…”

“…1 The major metabolic pathways of VPA comprise glucuronidation, β-oxidation and ω-oxidation. 6,7 The former reaches up to 50% of the total metabolism of the initial dose. 8 A number of UGT isozymes including UGT1A3, UGT1A4, UGT1A6, UGT1A8, UGT1A9, UGT1A10, UGT2B7 and UGT2B15 were reported to be involved in VPA glucuronidation.…”

“…8 A number of UGT isozymes including UGT1A3, UGT1A4, UGT1A6, UGT1A8, UGT1A9, UGT1A10, UGT2B7 and UGT2B15 were reported to be involved in VPA glucuronidation. 6,9 Among these enzymes, UGT2B7, UGT1A6 and UGT1A9 account for the majority of VPA intrinsic hepatic clearance. 6,8 The gene encoding for UGT1A6 enzyme was demonstrated to be highly polymorphic having at least four alleles characterized by three single nucleotide polymorphisms (SNPs).…”

The efficacy and safety of valproic acid medications with controlled active ingredient release in adults in real clinical practice from the position of pharmacokinetic and pharmacogenetic approaches

“…Для ВК характерен печеночный путь биотрансформации (глюкуронизация, β-окисление в митохондриях, десатурация и гидроксилиро-вание при участии цитохрома Р450) [3]. Около 10-20% ВК метаболизируется через систему цитохрома Р450 [5] с обра-зованием 4-ene-VPA и гидрокси-метаболитов [6]. Хотя ци-тохром-катализируемый метаболизм ВК количественно не-значителен в сравнении с другими путями ее метаболизма, он, тем не менее, весьма интересен из-за развития явлений непереносимости и интоксикации вследствие образования ненасыщенных жирных кислот, являющихся промежуточ-ными продуктами метаболизма ВК (4-ene-VPA, 4-OH-VPA и 5-OH-VPA), поскольку в последние годы убедительно по-казан их токсический эффект на организм человека [7,8].…”