Magnesium-dependent sphingomyelinase of infantile brain. Effect of detergents and a heat-stable factor

The properties of the Mg²⁺-dependent sphingomyelinase, whose pH optimum is between 7 and 8, were investigated using post-mortem infantile brain. The enzyme could be extracted with 0.2% Triton X-100 and remained soluble when centrifuged at 170 000 × g. Subsequent removal of the detergent with SM2-Biobeads resulted in resedimentation of the enzyme at 80 000 × g. A detergent was needed for assaying enzymatic activity; either Triton X-100 or bile salts could be used. With increasing concentrations of detergent, the rates of hydrolysis of sphingomyelin increased, reached an optimum and then decreased, suggesting inhibition of the enzyme. The concentrations of detergent which resulted in optimal reaction rates were directly related to the protein concentration of the enzymatic preparation. A heat-stable factor which counteracts inhibition by the above detergents is present in brain as well as several other tissues. A lipid extract of the enzymatic preparation, or several purified lipids could not mimic the effect of the heat-stable factor. The interrelationship between enzyme, detergent and the heat-stable factor was investigated.