Sphingomyelinases in human tissues. II. Absence of a specific enzyme from liver and brain of Niemann-Pick disease, type C.

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.