Evidence from rat liver nuclear preparations that latency of microsomal UDP-glucuronosyltransferase is associated
Abstract:
1. Nuclear, nuclear-envelope and microsomal preparations were prepared from rat liver, and their purity and morphology monitored by electron microscopy. 2. UDP-glucuronosyltransferase activity in microsomal preparations, but not in standard nuclear or nuclear-envelope preparations, displays latency from the criterion of being enhanced ('activated') by a range of detergents or the endogenous activator UDP-N-acetyl-glucosamine. 3. Nuclear preparations resemble activated rather than native microsomal preparations in failing to transfer glucuronic acid from 4-nitrophenyl glucuronide to 2-aminophenol. 4. Electron microscopy indicates that membranes of nuclear preparations and of our standard nuclear-envelope preparations remain, as in vivo, in a cisternal arrangement, whereas those of microsomal preparations are vesiculated. 5. In nuclear-envelope preparations in which vesiculation has been encouraged, the transferase can be activated by detergents. 6. We suggest that latency of UDP-glucuronosyltransferase results from vesiculation of membranes during preparation and that the latency of the microsomal transferase is largely a preparative artefact.
Insights
Latency in UDP-glucuronosyltransferase activity, crucial for drug metabolism, is primarily caused by membrane vesiculation during sample preparation. This finding suggests microsomal enzyme latency is often a preparative artifact.
Area of Science:
- Biochemistry
- Cell Biology
- Pharmacology
Background:
- UDP-glucuronosyltransferase (UGT) is a key enzyme system involved in drug metabolism and detoxification.
- UGT activity in different cellular fractions (nuclear, nuclear-envelope, microsomal) has been studied, with latency observed in microsomal preparations.
Purpose of the Study:
- To investigate the cause of latency observed in UDP-glucuronosyltransferase (UGT) activity in rat liver preparations.
- To determine if membrane structure and preparation methods influence UGT latency.
Main Methods:
- Preparation of nuclear, nuclear-envelope, and microsomal fractions from rat liver.
- Morphological analysis of fractions using electron microscopy.
- Assay of UDP-glucuronosyltransferase activity using specific substrates and activators (detergents, UDP-N-acetyl-glucosamine).
Main Results:
- Microsomal preparations exhibited latency in UGT activity, which was enhanced by detergents or UDP-N-acetyl-glucosamine.
- Nuclear and nuclear-envelope preparations showed different activity patterns and maintained a cisternal membrane structure.
- Vesiculation of membranes in nuclear-envelope preparations, induced during processing, led to detergent-activatable UGT activity.
- UGT activity in nuclear preparations was similar to activated microsomal preparations, failing to transfer glucuronic acid to 2-aminophenol.
Conclusions:
- The latency of UDP-glucuronosyltransferase in microsomal preparations is largely a preparative artifact, resulting from membrane vesiculation during sample handling.
- Membrane integrity and vesiculation are critical factors influencing the observed latency of UGT activity.
- Understanding these preparation-dependent artifacts is essential for accurate assessment of enzyme activity in biochemical and pharmacological studies.
