The relationships among sphingomyelin synthesis, cytosoic free Ca+ + level and cytotoxi-city in the presence of 27-hydroxycholesterol were investigated by measurement of [methyl-3H] choline incorporation into sphingomyelin, phosphatidylcholine, phosphocholine and cyti-dine choline 5’-diphosphate, 45Ca+ + uptake, cytosolic free Ca+ + levels and viable cell num-bers. The results indicated that 27-hydroxycholesterol at higher than normal level induced in-creased [3H] choline incorporation in sphingotnyelin accompanied by decreased radioactivity inphosphatidylcholine but did not alter [3H] choline content in phosphocholine and cytidinecholine 5’-diphosphate. The increased radioactivity in sphingomyelin by 27- hydroxycholesterolwas detected first, followed by detection of the enhanced Ca+ + uptake and cytosolic freeCa++. Finally, a decrease in viable cell number occurred. We suggest that the increasedtransfer from phosphatidylcholine to sphingomyelin by 27-hydroxycholesterol may berelatedto enhanced cytosolic free
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