A new fluorimetric enzyme assay for the diagnosis of Niemann–Pick A/B, with specificity of natural sphingomyelinase substrate
- Cite this article as:
- van Diggelen, O.P., Voznyi, Y.V., Keulemans, J.L.M. et al. J Inherit Metab Dis (2005) 28: 733. doi:10.1007/s10545-005-0105-y
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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” appears to be used with a very specific meaning. The Summary should be able to stand entirely alone from the text: should the use of this notation be expanded/explained more fully here in the Summary [or is the phrase “bearing the Q292K mutation” sufficient]?} 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.