Sphingomyelinases in Human Tissues. II. Absence of a Specific Enzyme from Liver and Brain of Niemann-Pick Disease, Type C

Extract: Sphingomyelinase was obtained in excellent yield from liver and brain by homogenization with 0.05 M citrate-phosphate buffer, pH 4.5, containing 0.25% Triton X-100 (v/v) followed by dialysis of the supernatant fluids against 1% glycine. Total recovery of enzyme was slightly less with tissue from Niemann-Pick disease compared with control tissue.Isoelectric focusing of liver and brain extracts was successfully used to resolve several species of sphingomyelinase. Three (I-III) of the five species were partially characterized. Enzyme I (pI 4.6) had a pH optimum of 4.8-5.0 in acetate buffer and a Km value of 0.026 mM. Both sphingomyelinases I and II were the major enzymes, whereas III, IV, and V were found at lower levels. Of the two major species in normal liver and brain (I and II), species I alone persisted in liver from the two cases of type C, while species III, IV, and V were present. In brain, only species II was decreased but the resolution of the brain enzymes was less satisfactory.Speculation: Sphingomyelinases exist in multiple molecular forms in human liver and brain. In Niemann-Pick disease, type C, a deficiency of a specific sphingomyelinase was noted in liver and brain from two patients. The lack of this enzyme probably reflects the genetic defect in this disease.