Liver Microsomal β-Glucuronidase and UDP-GIucuronyltransferase

Abstract

Both UDP-glucuronyltransferase (GT) and β-glucuronidase (ßG) were assayed in untreated liver microsomes. Optimum assay conditions were established with rat liver microsomes using p-nitrophenol (pNP) and its glucuronide (pNPGA) at the pH optima of GT (7.5) and βG (4.5). The activities of the two enzymes were compared using microsomes from rats, mice, pigs, cattle and horses, with pNP, pNPGA, and phenolphthalein as substrate, in the presence of various cofactors and inhibitors at pH 7.5 and 4.5. These data disclose pronounced differences with respect to species, substrate and other experimental conditions, thereby precluding the establishment of general optimum conditions. The two enzymes were also assayed under strictly identical conditions using pNP and pNPGA and rat liver microsomes at pH 7.5 in the presence and absence of UDP-glucuronate disodium (UDPGA), activators (ATP; UDP-N-acetylglucosamine) and inhibitors. When provided with a functional level of UDPGA, both enzymes proved active under those conditions, and a conjugation-deconjugation interplay was indicated. The two processes could be selectively and totally inhibited by Zn^2+ and saccharolactone. The results suggest that conjugation-deconjugation-reconjugation cycles may be operative in the metabolism of drugs in vivo, taking place already at the level of the liver endoplasmic reticulum.

Interference of UDP-Glucuronyltransferase and β-Glucuronidase Activity in Rat Liver Microsomes at pH 7.5 with p-Nitrophenol and p-Nitrophenylgliicuronide as Substrates

“…In this experiment, UDPGA 14 C-pNP (0.25 mmol/1) was subwas omitted, stituted for pNP, and pNPGA was added at three different concentrations. Since GT does not have any activity in the absence of UDPGA (27), only the pNPGA deconjugating activity was therefore measured. Addition of 14C-pNP and the choice of pNPGA concentrations were done in an attempt to create conditions as similar as possible to those present in the experiment shown with microsomes in figure 1.…”

“…The figures can certainly not be extrapolated to the situation in vivo. In addition, they are limited to the model used in view of the considerable species, substrate, and other differences (27). However, the data shown in figure 3 clearly demonstrate that microsomal ß-G is active under cellular pH conditions and simul taneously with GT.…”

“…Even though each of the two enzymes has been extensively investigated, this has almost invariably been done by separate groups of workers and hence under different experimental conditions. Work dealing with the simultaneous actions of GT and ß-G is scanty (25,27). Activity of both GT and ß-G in liver microsomal preparations at a pH value close to the physiological value has been demonstrated.…”

The Differential Tissue Distribution of the Citrus Flavanone Naringenin Following Gastric Instillation

“…It is possible that naringenin glucuronides could be hydrolysed by tissue b-glucuronidase, resulting in the penetration of free naringenin into the tissues, followed by subsequent conjugation with glucuronic acid. This assumption is also plausible because of the presence of both UDP-glucuronosyltransferase and b-glucuronidase in close subcellular proximity within hepatocytes [32] and cells of other organs. [33,34] Our results have also shown a longer retention of the aglycone form compared to glucuronides over 18 h by most of the tissues, but mainly by the brain and lungs.…”

Ratio of Urinary 6β-Hydroxycortisol to 17-Hydroxycorticosteroids in Patients with Liver Disease

“…It would appear that struc tural damage should affect both the major reduction pathway and the 6/3-hydroxylation pathway for cortisol in a similar manner. The major cortisol metabolism route involves conjugation, which occurs in the rough endoplasmic reticulum (8). Perhaps there are conditions in which this latter process is af fected, reducing cortisol clearance by this pathway and resulting in a com pensatory increase in oxidative metabolites.…”