Mechanistic evaluation of substrate inhibition kinetics observed from aldehyde oxidase-catalyzed reactions.

While most enzyme-catalyzed reactions are adequately described by Michaelis-Menten kinetics, Aldehyde Oxidase (AOX) metabolism might exhibit atypical kinetics due to possible substrate inhibition. Ignoring this phenomenon may lead to erroneous estimates of kinetic parameters and over simplification of the enzyme mechanism. In this study, in vitro metabolism data for 3 AOX substrates exhibiting varying degrees of substrate inhibition were analyzed with the following kinetic models: A) Michaelis-Menten (naïve) model; B) Substrate inhibition (empirical) model; and C) Twobinding site (mechanistic) model. The application of this mechanistic model is a novel interpretation for kinetic analysis of AOX metabolism whereby substrate can presumably bind to two enzymes' active site(s). Unlike the other models, this mechanistic model quantitatively captures the degree of substrate inhibition observed. Analysis by this model showed: A) All tested substrates have simultaneous access to the metabolic and inhibitory site of the enzyme with Ks (binding affinity for inhibitory site) greater (1.3- to 28-fold) than Km (binding affinity for metabolic site); B) Dissociation constants for binding of a second substrate in either the productive and nonproductive enzyme conformations decreased with factor α ranging from 2.58 to 15.6 between compounds; and C) In addition, a drastic decrease (from 64%-98%) in the metabolism rates between compounds was exhibited by factor ß (ranging from 0.02-0.36). Overall, the mechanistic two-binding site model best fitted the experimental data. Moreover, the observed differences between kinetic parameters generated by these models highlight the importance of appropriate model selection to adequately fit the substrate inhibition kinetics of AOX metabolism.

Intestinal lymphatic transport for drug delivery.

Intestinal lymphatic transport has been shown to be an absorptive pathway following oral administration of lipids and an increasing number of lipophilic drugs, which once absorbed, diffuse across the intestinal enterocyte and while in transit associate with secretable enterocyte lipoproteins. The chylomicron-associated drug is then secreted from the enterocyte into the lymphatic circulation, rather than the portal circulation, thus avoiding the metabolically-active liver, but still ultimately returning to the systemic circulation. Because of this parallel and potentially alternative absorptive pathway, first-pass metabolism can be reduced while increasing lymphatic drug exposure, which opens the potential for novel therapeutic modalities and allows the implementation of lipid-based drug delivery systems. This review discusses the physiological features of the lymphatics, enterocyte uptake and metabolism, links between drug transport and lipid digestion/re-acylation, experimental model (in vivo, in vitro, and in silico) of lymphatic transport, and the design of lipid- or prodrug-based drug delivery systems for enhancing lymphatic drug transport.

Breast cancer resistance protein (BCRP) and sulfotransferases contribute significantly to the disposition of genistein in mouse intestine.

The low bioavailability of genistein has impeded its development into a therapeutic agent. Our earlier studies indicate that glucuronidation is one of the major barriers to genistein oral bioavailability. This study will determine how sulfotransferases and efflux transporters affect its intestinal disposition. A rodent intestinal perfusion model and S9 fractions were used. Sulfate excretion rates were comparable to glucuronide excretion in mouse small intestine but significantly higher than glucuronide excretion in mouse colon, which is different from rat intestinal disposition but similar to disposition in Caco-2 cells. To define efflux transporter(s) involved in sulfate excretion, two organic anion inhibitors (estrone sulfate and dihydroepiandrosterone sulfate) or a multidrug resistance protein inhibitor (MK-571) were used but neither was able to decrease the excretion of genistein sulfates. In contrast, the excretion of genistein sulfate decreased substantially (>90%) in small intestine of breast cancer resistance protein (BCRP) knockout mice and became undetectable in colon of the knockout mice. The excretion rates of genistein glucuronide in the small intestine of BCRP knockout mice were also significant decreased (78%). This study shows clearly that BCRP facilitates the cellular genistein sulfate excretion by removing sulfates to prevent their backward hydrolysis and to limit substrate inhibition, indicating that BCRP plays a dominant role in genistein sulfate excretion and a significant role in genistein glucuronide excretion in the mouse intestine.

Disposition of naringenin via glucuronidation pathway is affected by compensating efflux transporters of hydrophilic glucuronides.

The purposes of this study were to investigate how efflux transporters and UDP-glucuronosyltransferases (UGT) affect the disposition of naringenin. A rat intestinal perfusion model with bile duct cannulation was used along with rat intestinal and liver microsomes. In the intestinal perfusion model, both absorption and subsequent excretion of naringenin metabolites were rapid and site-dependent (p < 0.05). Naringenin was absorbed the most in colon, and its glucuronides were excreted the most in duodenum. In metabolism studies, the intrinsic clearance value of naringenin glucuronidation was the highest in jejunum microsomes, followed by liver, ileal and colonic microsomes. The rapid metabolism in microsomes did not always translate into more efficient excretion in the rat perfusion model, however, because of presence of rate-limiting efflux transporters. When used separately, MK-571 (an inhibitor of multidrug resistance-related protein 2 or Mrp2) or dipyridamole (an inhibitor of breast cancer resistance protein or Bcrp1) did not affect excretion of naringenin glucuronides, but when used together, they significantly (p < 0.05) decreased intestinal and biliary excretion of naringenin glucuronides. In conclusion, efflux transporters Mrp2 and Bcrp1 are shown to compensate for each other and enable the intestinal excretion of flavonoid (i.e., naringenin) glucuronides.

Disposition of flavonoids via enteric recycling: UDP-glucuronosyltransferase (UGT) 1As deficiency in Gunn rats is compensated by increases in UGT2Bs activities.

Flavonoids have poor bioavailabilities largely because of metabolism via UDP-glucuronosyltransferases (UGTs). This study aims to further understand the functions of UGT in metabolizing genistein and apigenin, two compounds metabolized more extensively in the gut than in the liver. Because Gunn rats are deficient in UGT1As, we determined whether this deficiency would result in less flavonoid glucuronidation, using rat intestinal perfusion model and microsomes prepared from rat liver and intestine. In yeast-expressed rat UGT isoforms, rat UGT1A isoforms (especially UGT1A7) were mainly responsible for flavonoid metabolism. In perfusion studies, the two flavonoids were rapidly absorbed at comparable rates, but the intestinal excretions of glucuronides in Gunn rats compared with Wistar rats were not only comparable for genistein but also were higher (p < 0.05) for apigenin, suggesting up-regulation of UGT isoforms in Gunn rats. To determine the possible compensatory UGT isoforms, we first verified that UGT1A activities were significantly lower (p < 0.05) in Gunn rats by using UGT1A-specific probes 7-ethyl-10-hydroxycamptothecin (SN-38) and prunetin. We then demonstrated using UGT2B probes testosterone, ezetimibe, and indomethacin that UGT2B activities were usually significantly higher in Gunn rats. In addition, testosterone was metabolized much faster in liver microsomes than in intestinal microsomes, and in microsomes prepared from Gunn rats compared with Wistar rats. In conclusion, flavonoids are efficiently metabolized by UGT1A-deficient Gunn rats because of compensatory up-regulation of intestinal UGT2Bs and hepatic anion efflux transporters, which increases their disposition and limits their oral bioavailabilities.

Variable isoflavone content of red clover products affects intestinal disposition of biochanin A, formononetin, genistein, and daidzein.

BACKGROUND: Marketed red clover (Trifoleum pratense) products use a wide variety of labels, and the isoflavone content from the label is ambiguous. MATERIALS AND METHODS: In the present study, we analyzed the content of various isoflavone products, and determined (1) the content and (2) how the sample matrix of red clover products affects the intestinal disposition of main isoflavones within it, using the human intestinal Caco-2 cell model. RESULTS: Analysis using high- and ultraperformance liquid chromatography indicates that the isoflavone content varied significantly (p < 0.05) between the chosen products. Consequently, rates of isoflavone absorption across the Caco-2 cell monolayers varied (p < 0.05) greatly. Unexpectedly, permeabilities of biochanin A and formononetin (two key biomarkers) were found to be significantly affected (p < 0.05) by the product matrix. As expected, biochanin A was the only isoflavone with noticeable metabolite peaks in both the apical and basolateral sides. Interestingly, rates of metabolism and the polarity of the glucuronidated biochanin A excretion were also significantly altered (p < 0.05) by the product matrix. Studies using the breast cancer resistance protein (BCRP) inhibitor, dipyridamole, showed that both the apical and basolateral excretion of biochanin A glucuronides were significantly (p < 0.05) reduced (7.5- and 9.4-fold, respectively) when dipyridamole is present. This provides evidence that BCRP is the main transporter responsible for the apical efflux of isoflavone glucuronides. CONCLUSIONS: The isoflavone content of the marketed red clover products is highly variable, and the product matrix significantly affected the intestinal disposition of red clover isoflavones by altering their absorption rates, permeabilities, biochanin A glucuronide excretion rates, and the polarity of biochanin A glucuronide excretion. This research provides scientific evidence to support the standardization effort, so that consumers can make intelligent product choices.

Disposition of flavonoids via enteric recycling: enzyme stability affects characterization of prunetin glucuronidation across species, organs, and UGT isoforms.

We characterized the in vitro glucuronidation of prunetin, a prodrug of genistein that is a highly active cancer prevention agent. Metabolism studies were conducted using expressed human UGT isoforms and microsomes/S9 fractions prepared from intestine and liver of rodents and humans. The results indicated that human intestinal microsomes were more efficient than liver microsomes in glucuronidating prunetin, but rates of metabolism were dependent on time of incubation at 37 degrees C. Human liver and intestinal microsomes mainly produced metabolite 1 (prunetin-5- O-glucuronide) and metabolite 2 (prunetin-4'- O-glucuronide), respectively. Using 12 human UGT isoforms, we showed that UGT1A7, UGT1A8, and UGT1A9 were mainly responsible for the formation of metabolite 1, whereas UGT1A1, UGT1A8, and UGT1A10 were mainly responsible for the formation of metabolite 2. This isoform-specific metabolism was consistent with earlier results obtained using human liver and intestinal microsomes, as the former (liver) is UGT1A9-rich whereas the latter is UGT1A10-rich. Surprisingly, we found that the thermostability of the microsomes was isoform- and organ-dependent. For example, human liver microsomal UGT activities were much more heat-stable (37 degrees C) than intestinal microsomal UGT activities, consistent with the finding that human UGT1A9 is much more thermostable than human UGT1A10 and UGT1A8. The organ-specific thermostability profiles were also evident in rat microsomes and mouse S9 fractions, even though human intestinal glucuronidation of prunetin differs significantly from rodent intestinal glucuronidation. In conclusion, prunetin glucuronidation is species-, organ-, and UGT-isoform-dependent, all of which may be impacted by the thermostability of specific UGT isoforms involved in the metabolism.

Disposition of flavonoids via enteric recycling: structural effects and lack of correlations between in vitro and in situ metabolic properties.

The purpose of this study is to determine the importance of coupling of efflux transporters and metabolic enzymes in the intestinal disposition of six isoflavones (genistein, daidzein, formononetin, glycitein, biochanin A, and prunetin), and to determine how isoflavone structural differences affect the intestinal disposition. A rat intestinal perfusion model was used, together with rat intestinal and liver microsomes. In the intestinal perfusion model, significant absorption and excretion differences were found between isoflavones and their respective glucuronides (p <0.05), with prunetin being the most rapidly absorbed and formononetin glucuronides being the most excreted in the small intestine. In contrast, glucuronides were excreted very little in the colon. In an attempt to account for the differences, we measured the glucuronidation rates of six isoflavones in microsomes prepared from rat intestine and liver. Using multiple regression analysis, intrinsic clearance (CL(int)) and other enzyme kinetic parameters (V(max) and K(m)) were determined using appropriate kinetic models based on Akaike's information criterion. The kinetic parameters were dependent on the isoflavone used and the types of microsomes. To determine how metabolite excretion rates are controlled, we plotted excretion rates versus calculated microsomal rates (at 10 microM), CL(int) values, K(m) values, or V(max) values, and the results indicated that excretion rates were not controlled by any of the kinetic parameters. In conclusion, coupling of intestinal metabolic enzymes and efflux transporters affects the intestinal disposition of isoflavones, and structural differences of isoflavones, such as having methoxyl groups, significantly influenced their intestinal disposition.

Stereospecific high-performance liquid chromatographic analysis of ibuprofen in rat serum.

A simple, rapid and sensitive high-performance liquid chromatographic method was developed for determination of ibuprofen, (+/-)-(R, S)-2-(4-isobutylphenyl)-propionic acid, enantiomers in rat serum. Serum (0.1 ml) was extracted with 2,2,4-trimethylpentane/isopropanol (95:5, v/v) after addition of the internal standard, (S)-naproxen, and acidification with H(2)SO(4). Enantiomeric resolution of ibuprofen was achieved on ChiralPak AD-RH column with ultraviolet (UV) detection at 220 nm without interference from endogenous co-extracted solutes. The calibration curve demonstrated excellent linearity between 0.1 and 50 microg/ml for each enantiomer. The mean extraction efficiency was >92%. Precision of the assay was within 11% (relative standard deviation (R.S.D.)) and bias of the assay was lower than 15% at the limit of quantitation (0.1 microg/ml). The assay was applied successfully to an oral pharmacokinetic study of ibuprofen in rats.

Stereospecific high-performance liquid chromatographic analysis of flurbiprofen: application to pharmacokinetic studies.

A method of analysis of flurbiprofen (+/- 2-(2-fluoro-4-biphenyl)-propionic acid) in biological fluids is necessary to study the kinetics of in vitro and in vivo metabolism and tissue distribution. A simple high-performance liquid chromatographic method was developed for simultaneous determination of flurbiprofen enantiomers in rat serum. Serum (0.1 ml) was extracted with 2,2,4-trimethylpentane-isopropanol (95:5, v/v) after addition of the internal standard (IS), S-naproxen and acidification with H(2)SO(4). Separation was achieved on a Chiralpak AD-RH column with UV detection at 247 nm. The calibration curve was linear ranging from 0.05 to 50 microg/ml for each enantiomer. The mean extraction efficiency was >95%. Precision of the assay was <11% (CV), and was within 12.6% at the limit of quantitation (LOQ) (0.05 microg/ml). Bias of the assay was lower than 13.1%, and was within 12.8% at the LOQ. The assay was applied successfully to the in vivo kinetic study of flurbiprofen in rats.