Depression of the Ca2+-ATPase activity of the rat liver endoplasmic reticulum by the liver tumour promoters 1,1,1-trichloro-2,2-bis(p-chlorophenyl)- ethane and phenobarbital

Relative molecular similarity in selected chemical carcinogens and the nucleoside triphosphate chain

Several markers of cell toxicity are useful as screening tests for epigenetic carcinogens. The direct effects of chemicals on ATPase and GTPase function are pertinent to the early stages of carcinogenesis. Interference with triphosphate-diphosphate exchange mechanisms may result from the interaction of carcinogens with the substrate triphosphate chain. To investigate this hypothesis, a computational chemistry programme is used in this study to investigate molecular similarity in ATPase inhibitors, carcinogens and tumour promoters, in relation to the nucleoside triphosphate chain. The results show that atoms in the investigated molecular structures superimpose on sets of oxygen atoms in the triphosphate chain with interatomic distances < 0.3A. Relative molecular similarity to the substrate triphosphate chain is discussed in terms of the established inhibitory properties of carcinogens/tumour promoters on ATPase function, the carcinogen/ tumour promoting properties of ATPase inhibitors and the prediction of carcinogenic activity from chemical structure.

Alteration of rat liver endoplasmic reticulum Ca(2+)-ATPase thiol integrity by ciprofibrate, a peroxisome proliferator

Ciprofibrate (CP), a peroxisome proliferator, has been shown to reduce rat liver endoplasmic reticulum (ER) Ca(2+)-ATPase activity both in vitro and in vivo. The ER Ca(2+)-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 Ca(2+)-ATPase thiol groups as a possible mechanism of enzyme inhibition. Using a thiol immunoblot technique, free thiol groups specifically on the ER Ca(2+)-ATPase were localized. Exposure of freshly isolated rat liver microsomes to CP (500 microM) resulted in a loss of sulfhydryl reactivity on the ER Ca(2+)-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 Ca(2+)-ATPase were protected. Also, the reduction of ER Ca(2+)-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 Ca(2+)-ATPase activity through interactions with free thiol groups located on this enzyme.