Pathways of sphingomyelin metabolism in cultured fibroblasts from normal and sphingomyelin lipidosis subjects

The metabolism of endogenous sphingomyelin labeled with ³²P or [methyl-³H]choline and of exogenous [choline-methyl-³H], [³²P]-, or [N-acyl-1-¹⁴C]sphingomyelin was studied in normal and Niemann-Pick Type A (NP-A) cultured fibroblasts. Despite a >96% decrease in lysosomal sphingomyelinase activity in the NP-A cells, they were able to degrade endogenously produced [³²P]- or [methyl-³H]sphingomyelin at normal or near normal rates. Exogenous [methyl-³H]-, [methyl-³H, ³²P]-, and [methyl-³H, N-acyl-1-¹⁴C] sphingomyelin was taken up intact by normal and NP-A cells, with NP-A cells accumulating 4-8 times more lipid. By 20 hr, 50% of the control cell-associated ³H and ³²P was recovered in lecithin, and the ratio of activities (³H/³²P) indicated most of the phosphorylcholine derived from sphingomyelin had been transferred intact. By comparison in NP-A cells, after a 40-hr incubation only 20% of the labeled phosphorylcholine derived from sphingomyelin was recovered in lecithin. With both cell lines, 20 to 50 times more sphingomyelin was hydrolyzed than was taken up by the cells; the reaction products in the medium were ceramide and a mixture of water-soluble compounds such as phosphorylcholine and choline. These results indicate that there are at least two metabolic pathways for sphingomyelin modification in cultured fibroblasts in addition to degradation by the lysosomal acid sphingomyelinase. One route is hydrolysis by a cellular sphingomyelinase. The second is the hydrolysis and/or transfer of phosphorylcholine from sphingomyelin and results in the synthesis of lecithin.