Characterization of Rat Intestinal Microsomal UDP-Glucuronosyltransferase Activity toward Mycophenolic Acid

Abstract

ABSTRACT:Mycophenolic acid (MPA) is the active immunosuppressive metabolite of the anti-organ rejection drug mycophenolate mofetil (MMF) and is implicated in the gastrointestinal toxicity associated with MMF therapy. Intestinal UDP-glucuronosyltransferases (UGT) have been proposed to provide intrinsic resistance against MMF-induced gastrointestinal toxicity by converting MPA to the inactive MPA 7-O-glucuronide. Using an optimized intestinal microsome preparation method that stabilized the intestinal MPA UGT activity, the MPA UGT activity of male Sprague-Dawley rat intestinal microsomes was characterized. A longitudinal gradient similar to that described for other phenolic compounds was observed, with the activity decreasing from the duodenum to the distal small intestine and colon. The catalytic efficiency of MPA glucuronidation decreased from the proximal to distal intestine as a result of decreasing V max and increasing K m . The finding that homozygous Gunn rats lack detectable intestinal MPA UGT activity indicates exclusive roles of UGT1A1, UGT1A6, and/or UGT1A7. Quantitative immunoblotting revealed a parallel between the MPA UGT activity and the content of UGT1A7-like immunoreactivity (18.7 and 7.3 g/mg for duodenum and colon, respectively). In contrast, the lesser MPA-metabolizing UGT, UGT1A1 and UGT1A6, were lower in abundance (1.6-2.1 and 1.7-2.9 g/mg, respectively), and their patterns of longitudinal distribution were distinct from the MPA UGT activity. These data suggest a dominant role of a UGT1A7-like enzyme, presumably UGT1A7 itself, in the catalysis of rat intestinal MPA glucuronidation. Studies are ongoing to investigate the relationship between intestinal UGT1A enzymes and susceptibility to MMF-induced gastrointestinal toxicity.

Disturbance of Hepatic and Intestinal UDP-glucuronosyltransferase in Rats with Trinitrobenzene Sulfonic Acid-induced Colitis

“…Enzymatic activities of UGTs were determined using selective substrates, ¢-estradiol, 4-MU, MPA, and naloxone for UGT1A1, 23) UGT1A6, 23) UGT1A7, 24) and UGT2B1, 25) respectively (Supplementary Table S1). As shown in Figure 3D, the hepatic enzymatic activities of all UGT isoforms were down-regulated in rats treated with TNBS.…”

Effect of Adrenalectomy on Expression and Induction of UDP-Glucuronosyltransferase 1A6 and 1A7 in Rats

“…These results agree with those of previous studies reporting that the expression of UGT1A6 protein is higher in the liver than in the small intestine. 28) UGT1A7 has been previously shown to be expressed abundantly in the small intestine, but was either not detected or was present at lower levels in the liver. 29) Thus, these findings indicate that UGT1A7 mRNA expression is regulated by another mechanism rather than by glucocorticoids.…”

Species Differences in UDP-Glucuronosyltransferase Activities in Mice and Rats

“…In the present study in mice, the CL int value in liver was higher than that in duodenum, but mouse Ugt catalyzing MPA O-glucuronidation has not been determined yet. Rat UGT1A7 is mainly involved in MPA O-glucuronidation (Miles et al, 2005(Miles et al, , 2006.…”