ReviewsFirst-Pass Metabolism via UDP-Glucuronosyltransferase: a Barrier to Oral Bioavailability of Phenolics
Section snippets
INTRODUCTION
Phenolics are a class of chemical compounds consisting of one or more hydroxyl group(s) (-OH) bonded directly to an aromatic ring. Phenolic compounds such as flavonoids (including flavones, isoflavones, flavonols, flavanones, chalcones, and catechins), stilbenes, coumarins, and quinones are widely distributed in the nature, especially in plant kingdom. For example, significant amounts of isoflavones (e.g., genistein and daidzein) are found in soybeans, catechins (e.g., epigallocatechin gallate
DISTRIBUTION OF UGTS IN GI TRACT AND LIVER
Human UGTs are classified into four families: UGT1, UGT2, UGT3, and UGT8, on the basis of amino acid sequence identity.17 The most important drug-conjugating UGTs belong to UGT1 and UGT2 families. The human UGT1A gene cluster, located on chromosome 2q37, spans approximately 200 kb. It contains 13 distinct individual promoters/first exons and shared exons 2–5. Each exon 1 spliced to the same exons 2–5 is regarded as a unique gene, which translates to the corresponding active UGT1A isoform
TOPOLOGY OF UGTS IN ER MEMBRANE
UDP-glucuronosyltransferase active site faces the lumen of endoplasmic reticulum (ER) where the conjugation occurs, different from CYP enzymes, which face the cytosolic side.26 Although the lipophilic compounds usually can passively permeate through ER membrane to access the active site of the enzyme, uridine diphosphoglucuronic acid (UDPGA) is transported into the ER lumen using nucleotide sugar transporters (NSTs) (Fig. 3a).27 NSTs act as antiporters requiring the counter transport of UDP-N
STRUCTURE AND CATALYTIC MECHANISMS
Human UGTs belong to family 1 of glycosyltransferases (GT1) according to the CAZY database (http://www.cazy.org). Enzymes in GT1 adopt GT-B fold (i.e., consist of two α/β/α or Rossmann fold domains) and an inverting catalytic mechanism (Fig. 4a). Approximately 50% of the GT1 family shares a highly conserved motif in the C-terminal domain, denoted as the UGT defining sequence or the UGT signature motif.34,35 Unlike mammalian UGTs, which are membrane-bound proteins, most plant or bacterial GT1
REGULATION OF UGTS BY PHOSPHORYLATION
UDP-glucuronosyltransferase gene expression is known to be regulated by a number of transcription factors, including hepatocyte HNF1 and HNF2, Ah receptor, and nuclear receptors (NRs).61,62 The contribution of those regulators to the large interindividual variation of hepatic UGT levels has been discussed.63 In recent years, it is becoming more evident that UGT enzymes are also regulated via phosphorylation mediated by protein kinase C (PKC) or Src tyrosine kinase (SrcTK).64,65 Phosphorylation
Flavones
Intestinal and/or hepatic disposition of flavones using apigenin (1,Fig. 6) as the model compound was systematically performed in Caco-2 and rat intestine perfusion models.71., 72., 73., 74. The experimental results are summarized in Table 2. In Caco-2,71,73 apigenin glucuronide was substantially excreted at high concentrations (≥25 μM), whereas the excretion of sulfate was significant at low concentrations (≤10 μM). Unlike the sulfate which was mainly effluxed apically, the glucuronide was
STILBENES
Resveratrol (20,Fig. 7), present in grape and wine, has beneficial effects against cancer and protective effects on the cardiovascular system. Bioavailability of resveratrol in human is very low, only trace amounts of unconjugated resveratrol (<5ng/mL) could be detected in plasma after oral administration of 25 mg resveratrol.123 On the contrary, a moderate bioavailability (38%) was reported in rats.124 The susceptibility of resveratrol to first-pass glucuronidation had been revealed in many
ANTHRAQUINONE
Emodin (23,Fig. 7) is a major active anthraquinone present in the rhubarb.135 Oral administration of emodin to rabbits resulted in a very low serum concentration.136 In an isolated rat small intestine model, emodin glucuronide (8.69%) and sulfate (1.84%) were detected at the vascular side, whereas the glucuronide (5.23%) and sulfate (1.08%) moieties were also found in the luminal perfusate.137 Liu et al.138 showed that rapid metabolism by UGTs is the major reason why emodin has poor
COUMARINS
Oral bioavailability of 4-methylumbelliferone (24,Fig. 7) in rats is less than 1.5%.139 The total body plasma clearance of 4-methylumbelliferone was accounted mostly by the hepatic conjugative metabolism.139 In the isolated perfused livers, cumulative biliary excretion of the glucuronide was extensive (= 25 μmol), after a 30 μmol dose of 4-methylumbelliferone.140 4-methylumbelliferone is known to be metabolized by multiple human UGT isoforms, especially by UGT1A6, 1A7, and 1A10.141
Daphnetin (25,
Acetaminophen
Acetaminophen (28,Fig. 8), a widely used over-the-counter analgesic and antipyretic, is subjected to extensive first-pass glucuronidation. Glucuronidation accounts for 40% to two-thirds of the metabolism in human.145 Acetaminophen glucuronide excreted to bile accounts for approximately 7% of the administered acetaminophen dose (100 mg/kg) in rats and approximately 10% in the isolated perfused rat liver at the equivalent acetaminophen dose.146 UGT1A1, 1A6, 1A9, and 2B15 contribute significantly
GENDER-DEPENDENT GLUCURONIDATION
Liu et al.138 studied the disposition of emodin using rat intestine perfusion model, excretion rates of emodin-3-O-glucuronide were significantly different (p < 0.05) in four regions of the intestine and were higher in males than in females (p < 0.01). Similarly, we found duodenal excretion of glucuronide was significantly higher in male than female rats for the isoflavone daidzein (7, Fig. 6) (p < 0.05), which coincided with a higher absorption of the parent compound in duodenum (Fig. 10a
SPECIES-DEPENDENT GLUCURONIDATION
Species difference in glucuronidation activity was observed with human jejunum microsomes higher than rat intestinal microsomes for all hydroxylflavones, except for 3,7-dihydroxyflavone.201 Glucuronidation of the isoflavone prunetin (9, Fig. 6) (CLint) in human intestinal microsomes was threefold to sixfold higher than that in rat intestinal microsomes, but was similar in liver microsomes.182 In addition, rat Ugts might be able to produce an unusually prunetin C-glucuronide.182 Emodin
INTESTINAL VERSUS HEPATIC GLUCURONIDATION
In rats or mice, intestine probably plays a more significant role than liver in first-pass disposition of flavonoids via glucuronidation.72 Glucuronidation of flavonoids using rat intestinal microsomes generally shows higher catalytic efficiency than that using rat liver microsomes. In human, intestinal disposition may also be more important than hepatic disposition because human intestinal microsomes were more efficient than liver microsomes in glucuronidating phenolic compound such as
ISOFORM-SPECIFIC GLUCURONIDATION
The UGT isoforms that are responsible for glucuronidating phenolics are generally from UGT1A subfamily, especially, 1A1, 1A3, and 1A7–1A10.62,85 Substrate specificity of the six main contributors often exhibits significant overlaps.
For prunetin, UGT1A7, 1A8, and 1A9 were mainly responsible for the formation of 5-O-glucuronide, whereas UGT1A1, 1A8, and 1A10 were mainly responsible for the formation of 4′-O-glucuronide.182 UGT1A10 was also shown to be responsible for metabolizing 4′-OH of
STRUCTURE–GLUCURONIDATION RELATIONSHIP
The significances pertaining structure–glucuronidation relationship are emphasized by Wong et al.203 Briefly, the knowledge can usually be used to (but not limited to) (a) predict glucuronidation-mediated drug interactions that include both xenobiotics or endogenous compounds; (b) screen for compounds that are exclusively metabolized by a particular UGT isoform, which might be utilized as probe substrates for a particular UGT isoform; and (c) assist in the biosynthesis of flavonoid (or other
COMPENSATION BETWEEN UGT1AS AND 2BS
A recent study205 showed that flavonoids (i.e., apigenin and genistein) are efficiently metabolized by Ugt1a-deficient Gunn rats at a comparable or even higher level, in contrast to control Wistar rats. The equivalent or increased glucuronidation in Gunn rats was ascribed to the compensatory upregulation of intestinal Ugt2bs and hepatic anion efflux transporters.205 This was the first report to show that activities of other Ugt isoforms (2b isoforms) had changed (i.e., elevated) to compensate
DEGLUCURONIDATION BY β-GLUCURONIDASE
For the majority of dietary polyphenols, it remains unclear that their glucuronide still retain the biological functions in vivo. However, accumulating evidences suggest that β-glucuronidase -mediated deglucuronidation can occur in vivo, which converts the glucuronides back to free aglycones. This process was proposed to assist in the uptake, transport of the polar metabolite, and more importantly make the inactive metabolite active.206,207 Deglucuronidation had been frequently reported in the
POSSIBLE STRATEGIES TO IMPROVE BIOAVAILABILITY
First, inhibitors of UGT phosphorylation may be applied to reduce the magnitude of first-pass glucuronidation, thus improving the oral bioavailability. A study by Basu et al.164 represented the first work to show that targeted inhibition of glucuronidation could lead to enhanced drug (i.e., mycophenolic acid) uptake and efficacy. The authors successfully utilized curcumin (a PKC inhibitor) to downregulate UGT phosphorylation reversibly, thus, suppress glucuronidation; and demonstrated between a
CONCLUSIONS
First-pass glucuronidation in the intestine and liver is one of the major barriers limiting the oral bioavailability of phenolic compounds. UGTs (a large family of enzymes) catalyze the glucuronidation of phenolics in species-, gender-, organ-, and isoform-dependent manner. Furthermore, it was shown recently that Ugt1as deficiency in Gunn rats is compensated by the increases in Ugt2bs activities, which indeed complicates the attempts to bypass first-pass metabolism by suppressing the activity
Acknowledgements
This work was supported by grants from the National Institutes of Health [GM070737] to Ming Hu.
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