Relationship between translocation of long-chain acyl-CoA hydrolase, phosphatidate phosphohydrolase and CTP:Phosphocholine cytidylyltransferase and the synthesis of triglycerides and phosphatidylcholine in rat liver

Abstract

Translocation of long-chain acyl-coenzyme A hydrolase from the microsomal fraction to the cytosolic fraction was promoted in cell-free extracts of rat liver by palmitic acid, oleic acid, tetradecylthioacetic acid, and tetradecylthiopropionic acid, and by their CoA esters. The CoA esters were more effective than the non-esterified acids in the translocation of the enzyme. Treatment of normolipidemic rats with sulfur-substituted non-β-oxidizable fatty acid analogues resulted in a transitory increase in hepatic concentration of long-chain acyl-CoA. Longer feeding times almost normalized the hepatic long-chain acyl-CoA content. Microsomal long-chain acyl-CoA hydrolase activity was inhibited, whereas the activity of the cytosolic form was stimulated. The rise in enzyme activity coincided with a reduction in liver content of triglyceride and an increase in hepatic phospholipid content. The results suggest that the activity of long-chain acyl-CoA hydrolase in the cytosol may control the amount of acyl-CoA thioesters in the liver. Esterified and non-esterified fatty acids causedin vitro translocation of phosphatidate phosphohydrolase and cytidine 5′-triphosphate (CTP):phosphocholine cytidylyltransferase from the cytosolic fraction to the microsomal fraction. However, the translocation of these two enzyme systems was not obtainedin vivo. The activity of phosphatidate phosphohydrolase decreased in microsomal and cytosolic fractions while the activity of cytidylyltransferase in these fractions increased. The activities of soluble phosphatidate phosphohydrolase and long-chain acyl-CoA hydrolase appeared to be inversely correlated. The results imply that in cytoplasm, long-chain acyl-CoA hydrolase may compete with the biosynthetic enzymes for the acyl-CoA substrate, thus influencing the rate of lipid synthesis. The reduced hepatic triglyceride content observed in tetradecylthioacetic acid-treated rats is probably due to reduced triglyceride synthesis, which is mediated by an inhibition of phosphatidate phosphohydrolase accompanied with translocation and stimulation of long-chain acyl-CoA hydrolase. Development of fatty liver as an effect of tetradecylthiopropionic acid is probably due to accelerated triglyceride biosynthesis, which is mediated by a stimulation of phosphatidate phosphohydrolase and a decrease in cytosolic palmitoyl-CoA hydrolase activity.

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