Metabolism of extracellular phospholipids in Tetrahymena pyriformis

We studied the metabolism of phospholopids exogenously added to cultures of the protozoan, Tetrahymena pyriformis. Tetrahymena cells were found to metabolize the extracelluar phospholipids and the fatty acyl chains of the latter were accumulated perdominantly as a form of triacylglyccrol in the cells. This metabolism was considered to be initated via endocytosis of phospholipid vesicles, as judged from the following facts: 1)Cytochalasin B, an inhibitor of endocytosis, suppressed the metabolism almost completely. 2) Phospholipid vesicles were incorporated into a phagosome-like structure in Tetrahymena cells, as observed under an electron microscope. When phospholipids were incubated with Tetrahymena cells, the glycerol moicty and fatty acyl chain at the sn-2-posotion of the exogenous phospholipids were incorporated into the celluar triacyglycerol fraction in a 1to 1 ratio. Monoacylglycerol acyltransferase activity was detected in the microsomal fraction of Tetrahymena cells. From these results, together with those of our previous study on lysosomal phospholipid hydrolysis in Tetrahymena(J. Biochem. 99, 125-133(1986)), it is suggested that the extracellular phospholipids which were taken up by the cells via endocytosis were hydrolyzed throught the action of lysosomal phospholipases acyl acceptor for the sunthesis of triacylglycerol via the microsomal "monoacylglycerol pathway".