Effect of typical inducers of microsomal drug-metabolizing enzymes on phospholipid metabolism in rat liver

Abstract

The effect of administration of phenobarbital (PB), polychlorinated biphenyls(PCBs) or 3-methyl-cholanthrene (3-MC) on the metabolism of phospholipids in rat liver was studied by the i.p. injection of [ 32P]orthophosphate or [Me- 14C]choline chloride. The inducers were given to animals for 2 successive days PB had no significant effect on the incorporation rate of 32Pi into liver microsomal phospholipid classes 48 hr after the first administration. The rate of incorporation of [ 14C]choline into phosphatidylcholine (PC) in both subcellular components of the liver and blood plasma decreased slightly at this experimental period. In addition, the ratio of [ 14C] sp. act. of phosphorylcholinc to that of microsomal phosphulipid was considerably higher on PB-trcated rats as compared with the ratio in control rats. These and previous findings strongly suggest that proliferation of liver endoplasmic reticulum (ER) membranes induced by PB would be accompanied by the stimulation of phospholipid synthesis at the early process of induction and subsequently followed by the decrease in turnover rate of microsomal phospholipids. The administration of PCBs, on the other hand, caused strong inhibition of both 32Pi and 14C incorporation into PC in liver subcellular fractions. The secretion of PC from liver cells to blood plasma was also strongly depressed. In addition, phospholipid catabolizing activity was found to be depressed in the liver. These results indicate that hypertrophy of ER membranes in the liver after PCBs administration could be due to a depression of both secretion of lipoprotein from liver cells to plasma and catabolic activity toward membranous phospholipids in liver cells. The decrease in 32Pi incorporation into liver microsomal PC was also observed in rats treated with 3-MC. There occurred a considerable accumulation of 14C activity in phosphorylcholine in the liver of rats treated with either PCBs or 3-MC, suggesting strongly that these drugs caused an inhibition of PC synthesis at the site of CDP-choline formation, namely the inhibition of the reaction catalyzed by cholinephosphate cytidylyltransferase.