HEPATIC CONJUGATION DECONJUGATION CYCLING PATHWAYS - COMPUTER-SIMULATIONS EXAMINING THE EFFECT OF MICHAELIS-MENTEN PARAMETERS, ENZYME DISTRIBUTION PATTERNS, AND A DIFFUSIONAL BARRIER ON METABOLITE DISPOSITION/

Conjugation/deconjugation cycling plays an important role in the physi ologic regulation of the concentration of endogenous compounds that fo rm conjugated metabolites. Less is known concerning the deconjugation of xenobiotics. The model compound p-nitrophenol (pNP) is conjugated t o sulfate and glucuronide metabolites which can also undergo hydrolysi s, via separate enzyme systems, to regenerate pNP. In the present inve stigation, computer simulations were performed using literature values for the K-M and V-max for each of the four enzyme systems involved in net pNP conjugation. The apparent sulfation rate, apparent glucuronid ation rate, and the extraction ratio (E) of pNP were each examined (i) as a function of pNP concentration, (ii) following alterations in the K-M and V-max values for the deconjugation enzymes, (iii) after modul ating the enzyme distribution patterns along the liver flow path for b oth the conjugating and deconjugating enzymes, and (iv) in the presenc e of drug metabolite diffusional barriers for membrane transport. Resu lts of these simulations demonstrated that changes in the K-M or V-max for deglucuronidation produced changes not only in net glucuronidatio n but also in net sulfation. Overall extraction (E) of the parent comp ound 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 m etabolic pathway. Changes in the enzyme distribution patterns for the deconjugation pathways showed that the greatest influence on net conju gation rates occurred when hydrolase enzyme activity was distributed d ownstream from the respective forward reaction. In the presence of a d iffusional barrier for metabolite transport (ie., when the diffusional clearance was one tenth of bloodflow), net metabolism of parent was d iminished with E decreasing from 0.74, in the absence of a barrier, to 0.23, since the generated metabolite remained, to a great extent, wit hin hepatocytes and underwent a more pronounced hydrolysis. In the pre sence of diffusional barriers for uptake of the conjugated metabolites , the lowest drug extraction and metabolite formation rates were obser ved when the distribution of the conjugation and deconjugation pathway s across the liver were the same. Therefore, the effects of deconjugat ion on hepatic drug removal and metabolite formation pre highly depend ent on the enzymatic parameters of both the forward and reverse reacti ons, the parent drug concentration, the enzyme distribution patterns, and the presence of diffusional barriers for metabolite membrane trans port. Since a change in the deconjugation of one metabolite can influe nce the net formation of not only itself but also other metabolites, a nd overall drug extraction, evaluation of conjugation/deconjugation cy cling represents an important consideration in pharmacokinetic studies involving physiological-, pathological-, or pharmacological-induced a lterations in conjugate formation.