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The biosynthesis of sphingomyelin and the role of phosphatidylcholine as its precursor in baby hamster kidney-21F cells Ruff, Blair A.


The last step in sphingomyelin's de novo biosyn-thetic pathway was investigated in Baby Hamster Kidney (BHK-21F) cells in tissue culture. Three types of pulse-chase experiments were done to try to identify the precursor for sphingomyelin's phosphocholine moiety. First, [Me- ³H]-choline was used to monitor the movement of the choline moiety in all the possible phosphocholine donors: ie. phosphocholine, CDP-choline, phosphatidylcholine, lysophospha-tidylcholine, and glycerophosphocholine. Radioactivity was observed in phosphocholine before appearing in phosphatidylcholine, glycerophosphocholine, and sphingomyelin. Specific radioactivities of phosphatidylcholine and sphingomyelin revealed a peculiar pattern, if representative of a precursor-product relationship between these two phospholipids. Their specific radioactivities became equal at 22 hours of chase and remained quite similar for the next 24 hours. The other two types of pulse-chase experiments both utilized prelabeled BHK phosphatidyl[Me- ³H]choline in phospholipid vesicles as their 'pulse' source. Phospholipid Exchange Protein(PLEP)-mediated exchange and polyethylene glycol/phytohemagglutinin (PEG/PHA)-mediated fusion between phospholipid vesicles and BHK cells were used to introduce the labeled phosphatidylcholine. In addition, in the 'fusion' experiments, other labeled compounds (glycerophosphocholine and sphingomyelin) were substituted for labeled phosphatidylcholine. PLEP- mediated exchange of labeled phosphatidylcholine did not result in enough transfer of radioactivity into the cell to adequately monitor individual cell phospholipids or any transfer of label - ie. from phosphatidylcholine to sphingomyelin. However, PEG/PHA-mediated fusion of vesicles and cells did result in enough radioactivity showing up in the cells. When labeled phosphatidylcholine was used, the radioactivity ratio between it and sphingomyelin averaged around 16, depending on the length of the chase (2-52 hours)m The use of either labeled glycerophosphocholine or sphingomyelin resulted in a ratio of about 1.8. One 'cold-trap1 experiment was done by including a large amount of unlabeled glycerophosphocholine with labeled phosphatidylcholine in the vesicle preparation. The resultant radioactivity in sphingomyelin was 50% less than previously, but phosphatidylcholine's had remained the same. The evidence seems to indicate a reversible precursor-product relationship between phosphatidylcholine and sphingomyelin, but does not clearly show whether or not any other intermediate (such as glycerophosphocholine) is also involved.

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