Sphingomyelinase in cultured skin fibroblasts from normal and Niemann-Pick type C patients

The spingomyelinase activities of extracts of normal cultured skin fibroblasts were compared with those obtained from Niemann-Pick disease Type A and B patients, and from a number of patients with a provisional clinical diagnosis of Niemann-Pick disease Type C. Even though fibroblasts from Type A and B patients were shown to be clearly deficient (<5% residual activity) the activity in Type C fibroblasts varied from approximately 50% of the lowest control value to normal activity.
Isoelectric focussing of extracts of normal cultured skin fibroblasts on polyacrylamide gels revealed the presence of sphingomyelinase heterogeneity. However, no characteristic change in the behaviour of the enzyme in corresponding extracts from Niemann-Pick Type C cells was observed, suggesting that, if the defect in this condition is expressed in fibroblasts, it does not manifest biochemically in the appearance or disappearance of a specific sphingomyelinase isoenzyme.
Our data suggest that the heterogeneity observed in fibroblast extracts may simply reflect an interaction of the enzyme either with itself or with other hydrophobic components present in the cellular extracts.