UBC Theses and Dissertations
Regulation of rat hepatic phosphatidylcholine biosynthesis Pelech, Steven
Several model systems were investigated to elucidate the mechanisms by which rat liver phosphatidylcholine synthesis is controlled. CTP: phosphocholine cytidylyltransferase was clearly the key regulatory enzyme for phosphatidylcholine formation from choline. This ambiquitous enzyme was detected in both the cytosolic and microsomal fractions of rat liver, although the majority of the cytidylyltransferase occurred in the soluble fraction. The distribution of cytidylyltransferase between these fractions was altered when the rate of phosphatidylcholine synthesis was perturbed. Translocation of cytidylyltransferase was observed in rat liver during early development, with starvation and during a diurnal rhythm. A redistribution of cytidylyltransferase was also detected in isolated hepatocytes which were treated with glucagon, cAMP analogues or fatty acids bound to albumin. The rate of phosphatidylcholine synthesis was found to reflect the amount of microsomal cytidylyltransferase activity. The inhibition of phosphatidylcholine synthesis by glucagon or cAMP analogues was likely due to phosphorylation and inhibition of the cytidylyltransferase. Several lines of evidence indicated that the cytidylyltransferase in fresh rat liver cytosol was probably phosphorylated and activated upon dephosphorylation by endogenous phosphoprotein phosphatases or alkaline phosphatase from hog intestine. Although the phosphorylation of cytidylyltransferase was apparently kinetically "silent", dephosphorylation resulted in an increased affinity of the enzyme for membranes. Fatty acids stimulated de novo phosphatidylcholine synthesis by acceleration of the cytidylyl-transferase-catalyzed reaction. Fatty acids and their CoA derivatives were shown to stimulate the cytosolic cytidylyltransferase activity. However, these compounds failed to activate partially purified cytidylyltransferase appreciably. Apparently, fatty acids, like dephosphorylation, enhanced the tenacity of cytidylyltransferase for membranes. Upon binding to membranes, cytidylyltransferase activity could be elevated up to 45-fold, and the affinity of the enzyme for the substrate, CTP, was increased 20-fold. The influence of glucagon, cAMP analogues and fatty acids on the synthesis of phosphatidylcholine by successive N-methylation was also examined in isolated rat hepatocytes. Glucagon and cAMP analogues inhibited the methylation pathway in these cells, but the activity of microsomal phosphatidylethanolamine methyltransferase was elevated. Fatty acids also reduced the formation of phosphatidylcholine from phosphatidylethanolamine. Fatty acids and their CoA derivatives directly inhibited the phosphatidylethanolamine methyltransferase in rat liver microsomes. The coordinate control of hepatic phosphatidylcholine synthesis by cAMP and fatty acids may be important during starvation when the intracellular levels of these compounds are increased.
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