ALTERATION OF RAT-LIVER ENDOPLASMIC-RETICULUM CA2+-ATPASE THIOL INTEGRITY BY CIPROFIBRATE, A PEROXISOME PROLIFERATOR

Ciprofibrate (CP), a peroxisome proliferator, has been shown to reduce rat liver endoplasmic reticulum (ER) Ca2+-ATPase activity both in vitro and in vivo. The ER Ca2+-ATPase is highly susceptible to thiol reactivity, and maintenance of maximal enzyme activity is critically dependent upon the integrity of these thiol groups. We therefore investigated whether CP alters ER Ca2+-ATPase thiol groups as a possible mechanism of enzyme inhibition. Using a thiol immunoblot technique, free thiol groups specifically on the ER Ca2+-ATPase were localized. Exposure of freshly isolated rat liver microsomes to CP (500 muM) resulted in a loss of sulfhydryl reactivity on the ER Ca2+-ATPase protein at 107 kDa, as identified using the thiol immunoblot assay. However, when rat liver microsomes were exposed to CP in the presence of reduced glutathione (GSH), thiol groups on the ER Ca2+-ATPase were protected. Also, the reduction of ER Ca2+-ATPase activity by CP could be ameliorated by co-incubation of rat liver microsomes with GSH. These observations indicate that CP reduces rat liver ER Ca2+-ATPase activity through interactions with free thiol groups located on this enzyme.