Targeted Quantification of the Lysosomal Proteome in Complex Samples

The lysosome, as the main degradative organelle of eukaryotic cells, is involved in numerous cellular processes. A defect in one of its proteins often results in lysosomal storage diseases (LSDs). For the study of lysosomal proteins, mass spectrometry (MS) has emerged as the method of choice. Lysosomal proteins are, however, low-abundant, restricting the analysis of lysosomal proteins in unbiased approaches to the investigation of lysosome-enriched fractions. The use of targeted MS provides an attractive alternative to analyze lysosomal proteins in a complex background. In this study, the two targeted MS approaches data-independent acquisition (DIA) and parallel reaction monitoring (PRM) were compared with regard to their ability to analyse lysosomal proteins. These experiments were conducted with samples of different complexity: low-complex lysosome-enriched fractions, medium-complex mouse embryonic fibroblasts (MEF), and high-complex liver whole tissue lysate. While both MS approaches were able to identify and quantify lysosomal proteins, PRM outperformed DIA, especially in high-complex samples.