Glycerolipid metabolism in mammalian tissues

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Lee, Douglas P.
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The biological membrane surrounds all cells and delineates intracellular compartments. Lipids form building blocks of the biological membrane, and phosphatidylcholine is the principle lipid in the mammalian membrane. In this study, phosphatidylcholine metabolism and the direct acylation of glycerol for lipid biosynthesis were investigated. The inhibition of phosphatidylcholine biosynthesis by extracellular phosphocholine was studied in human umbilical vein endothelial cells. The activities of the enzymes in the CDP-choline pathway were not altered, but the intracellular phosphocholine pool was significantly reduced. The reduction was caused by competitive inhibition of choline uptake by phosphocholine. Phosphocholine also caused a limited stimulation of arachidonate release from phosphatidylcholine, and the release was potentiated by ATP. These studies clearly demonstrate that exogenous; phosphocholine has the potential to modulate phosphatidylcholine metabolism in mammalian tissues. The acylation of glycerol-'sn'-3-phosphate is regarded as the first committed step for glycerolipid biosynthesis. The direct acylation of glycerol in mammalian tissues has not been previously demonstrated. In this study, lipid biosynthesis in myoblast and hepatocyte cells was reassessed by conducting pulse-chase experiments with [1,3-3H] glycerol. The results suggested that a portion of labeled glycerol was directly acylated to form monoacylglycerol and subsequently diacylglycerol and triacylglycerol. This pathway became prominent when the glycerol-3-phosphate pathway was attenuated and when the exogenous glycerol concentration was elevated. The present study indicates the existence of a novel lipid biosynthetic pathway that may be important during hyperglycerolemia produced in diabetes or other pathological conditions. Glycerol: acyl-CoA acyltransferase, the enzyme which directly acylates glycerol is located in the microsomal fraction of tissue homogenate. It was identified as an 18 kDa protein after purification by FPLC gel filtration, photoaffinity labeling and gel electrophoresis. The purification was confirmed by immunoprecipitation studies, and sequence analysis of the protein identified the acyltransferase as similar to myoglobin. These studies suggest that myoglobin in the pig heart may be modified and subsequently translocated to the membrane where enzyme activity is then conferred.