Revisiting the Latency of Uridine Diphosphate-Glucuronosyltransferases (UGTs)-How Does the Endoplasmic Reticulum Membrane Influence Their Function?

Abstract

Uridine diphosphate-glucuronosyltransferases (UGTs) are phase 2 conjugation enzymes mainly located in the endoplasmic reticulum (ER) of the liver and many other tissues, and can be recovered in artificial ER membrane preparations (microsomes). They catalyze glucuronidation reactions in various aglycone substrates, contributing significantly to the body's chemical defense mechanism. There has been controversy over the last 50 years in the UGT field with respect to the explanation for the phenomenon of latency: full UGT activity revealed by chemical or physical disruption of the microsomal membrane. Because latency can lead to inaccurate measurements of UGT activity in vitro, and subsequent underprediction of drug clearance in vivo, it is important to understand the mechanisms behind this phenomenon. Three major hypotheses have been advanced to explain UGT latency: compartmentation, conformation, and adenine nucleotide inhibition. In this review, we discuss the evidence behind each hypothesis in depth, and suggest some additional studies that may reveal more information on this intriguing phenomenon.

Keywords: UDP-glucuronosyltransferase; glucuronidation; latency; microsomes; regulation.

Figure 1

The glucuronidation system in the endoplasmic reticulum. Substrates (X-OH) enter the lumen by diffusion (A), and UDPGA is transported via the UDPGA/UDP-GlcNAc antiporter (UGTrel7, B). Following the conjugation reaction, the glucuronide products (X-O-GA) are removed from the lumen by glucuronide transporter(s) (C). UDPGA: Uridine diphosphate-glucuronic acid; UDP-GLcNAc: UDP-N-acetylglucosamine.