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 investigated in isolated rat hepatocytes. Different agents which decrease the Ca2+ level in the endoplasmic reticulum [calcium ionophores (A23187, ionomycin) or Ca(2+)-ATPase inhibitors(thapsigargin,2,5-di-(t-butyl)-1,4-benzohydroquinone+ ++)] inhibited the conjugation of p-nitrophenol. Depletion of intracellular Ca2+ stores by preincubation of hepatocytes in the absence of free Ca2+ (in the presence of excess EGTA) also decreased 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 by varying the external free Ca2+ concentrations within a cytosol-like range. Emptying of the Ca2+ from the lumen of microsomal vesicles by A23187, after MgATP-stimulated Ca2+ sequestration, decreased the glucuronidation of p-nitrophenol. A similar effect was observed in filipin-permeabilized hepatocytes. In native and in detergent-treated microsomes, Ca2+ (1-10 mM) increased the p-nitrophenol UDP-glucuronosyltransferase activity. It is suggested that the physiological concentration of Ca2+ in the lumen of the endoplasmic reticulum is necessary for the optimal activity of p-nitrophenol UDP-glucuronosyltransferase; the depletion of Ca2+ decreases the activity of the enzyme.
Bánhegyi
G
G
1st Institute of Biochemistry, Semmelweis Medical University, Budapest, Hungary.
Bellomo
G
G
Fulceri
R
R
Mandl
J
J
Benedetti
A
A
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D014453:Glucuronosyltransferase / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism*; D009596:Nitrophenols / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley
D009596:Nitrophenols; D014453:Glucuronosyltransferase; D008274:Magnesium; D002118:Calcium; C024836:4-nitrophenol