A simple, accurate method for the measurement of lysosomal activity

Lysosomes are responsible for the degradation of intra- and extracellular components and are thus essential for the quality control of proteins and organelles. Lysosomal dysfunction leads to lysosomal storage diseases, and it is therefore important to identify which types of stress cause functional abnormalities. Lysosomal function is generally evaluated by measuring the enzyme activity of lysosomes with fluorescent dyes. However, fluorescence microscopy can lead to different outcomes due to variations in the field of view, the analysis software used, and the parameter settings. We therefore developed a method that uses only a microplate reader and DQ Green BSA, a dye that emits fluorescence upon lysosomal degradation, to ascertain lysosomal activity. HEK293 cells were treated with DQ Green BSA with or without bafilomycin A1 and lysates extracted using cell lysis buffer. Fluorescence intensities and protein concentrations in the cell lysates were then measured using a microplate reader and the bicinchoninic acid method, respectively, and the fluorescence intensity divided by the protein concentration. Results indicated a significant lysosome inhibitor-induced dose-dependent decrease in the lysosomal activity. The Z'-factor of 0.77 obtained using the proposed method is a significant improvement over the - 0.06 obtained using the conventional method. The versatility of the method was evaluated with different cell types, cell lysis buffers, inhibitors, and protease substrates. These results suggest that the method works regardless of the cells or reagents used, and indicates the relative simplicity and accuracy of the proposed method compared to the currently utilized method.