Sphingomyelinases in Human Tissues. III. Expression of Niemann-Pick Disease in Cultured Skin Fibroblasts

Abstract

Extract: Sphingomyelinase was effectively extracted from cultured fibroblasts homogenized in 1% glycine. The average specific activity was 24.6 nmol substrate hydrolyzed per hr per mg protein. Cultured cells from two cases of Niemann-Pick disease type A and one case of type B had markedly reduced enzyme activity, whereas, in type E cells, total activity was twice normal. Sphingomyelinase was resolved by isoelectric focusing into three peaks of activity (I-III), where I and II were the major forms. Species I had a pI of 4.6, II a pI of 4.8, and III a pI of 5.0. All three peaks, although markedly reduced, were present in cell extracts from both types A and B. All species of enzyme in B cells were found at higher levels than in A cells. Cultured cells from a case of type E contained sphingomyelinase I at a level several fold higher than normal, species II was absent, whereas III was found at near normal levels. Speculation: The data suggest that multiple forms of sphingomyelinase exist in cultured cells. Each species may contain polypeptide sequences in common since all forms are affected in fibroblasts in both types A and B. It is also likely that each sphingomyelinase species possesses some unique structure since a specific deficiency of species II was found in a case of type E. Type C and type E Niemann-Pick disease appear to be related since both involve a specific deficiency of sphingomyelinase II.