Studies on sphingomyelinase activity in cultured cells and leucocytes.

Using both [3H]ceramide-sphingomyelin and [14C]methyl choline-sphingomyelin as substrate, enzyme activity was determined in skin fibroblasts, cultured amniotic fluid cells and leucocytes. Sphingomyelinase activities of fibroblasts from two patients with Niemann-Pick disease were about 1% of the mean control values (22.9 nmol/h/mg protein and 65.6 nmol/h/mg protein, with [3H]sphingomyelin and [14C]sphingomyelin respectively). Leucocyte sphingomyelinase activity is much lower than that of cultured cells. Optimum sphingomyelinase activity of cultured cells is at pH 5.0 and the apparent Km values are 58 mumol/1 and about 200 mumol/1 for [3H]sphingomyelin and [14C]sphingomyelin respectively. These properties are similar to those for residual sphingomyelinase activity in Niemann-Pick fibroblasts.