Hepatic conjugation/deconjugation cycling pathways. Computer simulations examining the effect of michaelis-menten parameters, enzyme distribution patterns, and a diffusional barrier on metabolite disposition

Abstract

Conjugation/deconjugation cycling plays an important role in the physiologic regulation of the concentration of endogenous compounds that form conjugated metabolites. Less is known concerning the deconjugation of xenobiotics. The model compound p-nitrophenol (pNP) is conjugated to sulfate and glucuronide metabolites which can also undergo hydrolysis, via separate enzyme systems, to regenerate pNP. In the present investigation, computer simulations were performed using literature values for the KM and Vmax for each of the four enzyme systems involved in net pNP conjugation. The apparent sulfation rate, apparent glucuronidation rate, and the extraction ratio (E) of pNP were each examined (i) as a function of pNP concentration, (ii) following alterations in the KM and Vmax values for the deconjugation enzymes, (iii) after modulating the enzyme distribution patterns along the liver flow path for both the conjugating and deconjugating enzymes, and (iv) in the presence of drug metabolite diffusional barriers for membrane transport. Results of these simulations demonstrated that changes in the KM or Vmax for deglucuronidation produced changes not only in net glucuronidation but also in net sulfation. Overall extraction (E) of the parent compound was only affected when glucuronidation was an important pathway, i.e., at higher pNP concentrations. Similar results were observed with changes in desulfation, with desulfation having the greatest effects at low pNP concentrations where sulfation represents the predominant metabolic pathway. Changes in the enzyme distribution patterns for the deconjugation pathways showed that the greatest influence on net conjugation rates occurred when hydrolase enzyme activity was distributed downstream from the respective forward reaction. In the presence of a diffusional barrier for metabolite transport (i.e., when the diffusional clearance was one tenth of blood flow), net metabolism of parent was diminished with E decreasing from 0.74, in the absence of a barrier, to 0.23, since the generated metabolite remained, to a great extent, within hepatocytes and underwent a more pronounced hydrolysis. In the presence of diffusional barriers for uptake of the conjugated metabolites, the lowest drug extraction and metabolite formation rates were observed when the distribution of the conjugation and deconjugation pathways across the liver were the same. Therefore, the effects of deconjugation on hepatic drug removal and metabolite formation are highly dependent on the enzymatic parameters of both the forward and reverse reactions, the parent drug concentration, the enzyme distribution patterns, and the presence of diffusional barriers for metabolite membrane transport. Since a change in the deconjugation of one metabolite can influence the net formation of not only itself but also other metabolites, and overall drug extraction, evaluation of conjugation/deconjugation cycling represents an important consideration in pharmacokinetic studies involving physiological-, pathological-, or pharmacological-i...

Carrier-mediated entry of 4-methylumbelliferyl sulfate: Characterization by the multiple-indicator dilution technique in perfused rat liver

“…8,9 Diffusion-limited entry was implied to exist for 4MUS in single-pass perfused liver studies, 4,10 and would reduce the net removal and extraction ratio of the compound. 9 In contrast, the biliary excretion clearances of 4MUS, as a preformed species or as a metabolite of 4MU, were found to be similar in perfused rat liver studies performed in our laboratory 8 ; the observation points toward the rapid equilibration of 4MUS across the sinusoidal membrane. Concentrationdependent hepatocellular entry of 4MUS was subsequently found in isolated rat hepatocytes.…”

“…8 The extent of futile cycling is expected to be modulated by the mode of hepatocellular entry of the precursor-product pair. 8,9 Diffusion-limited entry was implied to exist for 4MUS in single-pass perfused liver studies, 4,10 and would reduce the net removal and extraction ratio of the compound. 9 In contrast, the biliary excretion clearances of 4MUS, as a preformed species or as a metabolite of 4MU, were found to be similar in perfused rat liver studies performed in our laboratory 8 ; the observation points toward the rapid equilibration of 4MUS across the sinusoidal membrane.…”

Disposition of isosteviol in the rat isolated perfused liver

“…Extensive pharmacokinetic studies have shown that stevioside is absorbed as steviol (Fig. 1) in different animal species 3–9 . Steviol is the core structure of stevioside and it may be the active part that is responsible for the therapeutic effects of stevioside.…”

“…1) in different animal species. [3][4][5][6][7][8][9] Steviol is the core structure of stevioside and it may be the active part that is responsible for the therapeutic effects of stevioside. It has been proposed that steviol may be developed for therapeutic purposes.…”