LC-MS/MS-based enzyme assay for lysosomal acid lipase using dried blood spots.

Lysosomal acid lipase deficiency (LAL-D) (OMIM: 278000) is a lysosomal storage disorder with two distinct disease phenotypes such as Wolman disease and cholesteryl ester storage disorder (CESD), characterized by an accumulation of endocytosed cholesterol in the body. Due to the presence of multiple lipases in DBS, previous studies measured LAL enzyme activity in the presence of Lalistat-2, an established LAL-specific inhibitor (Hamilton J et al Chim Clin Acta (2012) 413:1207-1210). Alternatively, a novel substrate specific for LAL has been reported very recently (Masi S. et al Clin Chem (2018) 64:690-696). In this study, we examined the LAL enzyme activity of a Japanese population with the LAL-specific substrate using liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based enzyme assay whether an affected individual can be identified among this population. To achieve this, we first performed assay validation using LC-MS/MS. Under our experimental setting, typically we obtained LAL enzyme activity for QC High (100% enzyme activity) as 261.9 ± 3.2 μmol/h/L ( n  = 5) and for QC Low as (5% enzyme activity) as 14.7 ± 0.5 μmol/h/L ( n  = 5). The percentage of coefficient of variation for interday assay for QC High was 9.6% ( n  = 4) and for QC Low was 7.9% ( n  = 4), respectively. Based on these results, we further examined the LAL enzyme activity of control Japanese population and that of affected individuals with Wolman disease and CESD. The averaged enzyme activity for control newborns, Wolman, and CESD was 123.9 ± 53.9 μmol/h/L ( n  = 131), 6.6 ± 0.9 μmol/h/L ( n  = 3), and 4.8 ± 0.3 μmol/h/L ( n  = 3), respectively. These results suggest that an LAL-D-affected individual can be readily identified by enzyme activity using LC-MS/MS-based technique.