INTRALUMINAL CALCIUM OF THE LIVER ENDOPLASMIC-RETICULUM STIMULATES THE GLUCURONIDATION OF P-NITROPHENOL

The relationship between the intraluminal Ca2+ content of endoplasmic reticulum and the rate of the glucuronidation of p-nitrophenol was inv estigated in isolated rat hepatocytes. Different agents which decrease the Ca2+ level in the endoplasmic reticulum [calcium ionophores (A231 87, ionomycin) or Ca2+-ATPase inhibitors (thapsigargin, 2,5-di-(t-buty l)-1,4-benzohydroquinone)] inhibited the conjugation of p-nitrophenol. Depletion of intracellular Ca2+ stores by preincubation of hepatocyte s in the absence of free Ca2+ (in the presence of excess EGTA) also de creased the rate of glucuronidation; Ca2+ re-admission to EGTA-treated hepatocytes restored glucuronidation. In intact liver microsomes the p-nitrophenol UDP-glucuronosyl-transferase activity was not modified b y varying the external free Ca2+ concentrations within a cytosol-like range. Emptying of the Ca2+ from the lumen of microsomal vesicles by A 23187, after MgATP-stimulated Ca2+ sequestration, decreased the glucur onidation of p-nitrophenol. A similar effect was observed in filipin-p ermeabilized hepatocytes. In native and in detergent-treated microsome s, Ca2+ (1-10 mM) increased the p-nitrophenol UDP-glucuronosyltransfer ase activity. It is suggested that the physiological concentration of Ca2+ in the lumen of the endoplasmic reticulum is necessary for the op timal activity of p-nitrophenol UDP-glucuronosyltransferase; the deple tion of Ca2+ decreases the activity of the enzyme.