6-Hexadecanoylamino-4-methylumbelliferylphosphorylcholine (HMUPC) was shown to be a specific substrate for the determination of acid (lysosomal) sphingomyelinase (ASM; gene SMPD1). Fibroblasts (n = 27) and leukocytes (n = 8) from both the A and B types of Niemann-Pick disease showed <6% and <10% of mean normal ASM activity, respectively. Niemann-Pick A or B patients bearing the Q292K mutation had apparently normal ASM activity with our new artificial substrate. These patients with false-normal sphingomyelinase activity, however, could readily be detected by determining the extent of inhibition of enzymatic hydrolysis of the artificial substrate HMU-PC by an unlabelled natural substrate, in particular lysosphingomyelin. This approach is generally applicable. Our novel assay for ASM combines the ease of a rapid and robust enzyme assay using a fluorogenic substrate with the specificity of an ASM assay using a natural substrate. Such assays are obviously more convenient to the diagnostic laboratory, since radiolabelled substrates are not required.

doi.org/10.1007/s10545-005-0105-y, hdl.handle.net/1765/55956
Journal of Inherited Metabolic Disease
Department of Clinical Genetics

van Diggelen, O., Voznyi, Y. V., Keulemans, J. L. M., Schoonderwoerd, K. G. C., Ledvinová, J., Mengel, E., … Harzer, K. (2005). A new fluorimetric enzyme assay for the diagnosis of Niemann-Pick A/B, with specificity of natural sphingomyelinase substrate. Journal of Inherited Metabolic Disease, 28(5), 733–741. doi:10.1007/s10545-005-0105-y