The enzymatic formation of sphingomyelin from ceramide and lecithin in mouse liver.

The conversion of W-labeled fatty acylsphingosine and phosphatidyl[14C]choline to sphingomyelin could be demonstrated in lyophilized microsomes from mouse liver. Radioactive CDP-choline did not act as a choline donor. No cofactors were needed, although CDP-choline and P-choline exerted some stimulatory action. The unnatural ceramide, acetyl threo-sphingosine, could be converted to the corresponding sphingomyelin but this reaction required Mn2f and CDP-choline. Evidence from isotopic trapping experiments was interpreted to indicate that the P-choline transfer from lecithin to ceramide did not go through free CDPcholine, choline, or P-choline. The transferase was found in kidney, lung, liver, spleen, and heart (in decreasing order of activity) but not in brain. The enzyme was inhibited by diglyceride and lysolecithin.