EVIDENCE FOR AN UDP-GLUCURONIC ACID PHENOL GLUCURONIDE ANTIPORT IN RAT-LIVER MICROSOMAL VESICLES

The transport of glucuronides synthesized in the luminal compartment o f the endoplasmic reticulum by UDP-glucuronosyl-transferase isoenzymes was studied in rat liver microsomal vesicles. Microsomal vesicles wer e loaded with p-nitrophenol glucuronide (5 mM), phenolphthalein glucur onide or UDP-glucuronic acid, by a freeze-thawing method. It was shown that: (i) the loading procedure resulted in millimolar intravesicular concentrations of the different loading compounds; (ii) addition of U DP-glucuronic acid (5 mM) to the vesicles released both intravesicular glucuronides within 1 min; (iii) glucuronides stimulated the release of UDP-glucuronic acid from UDP-glucuronic acid-loaded microsomal vesi cles; (iv) trans-stimulation of UDP-glucuronic acid entry by loading o f microsomal vesicles with p-nitrophenol glucuronide, phenolphthalein glucuronide, UDP-glucuronic acid and UDP-N-acetylglucosamine almost co mpletely abolished the latency of UDP-glucuronosyltransferase, althoug h mannose 6-phosphatase latency remained unaltered; (v) the loading co mpounds by themselves did not stimulate UDP-glucuronosyltransferase ac tivity. This study indicates that glucuronides synthesized in the lume n of endoplasmic reticulum can leave by an antiport, which concurrentl y transports UDP-glucuronic acid into the lumen of the endoplasmic ret iculum.