Enzyme substrates for visualizing acidic organelles

The present invention relates to the visualization of acidic organelles based upon organelle enzyme activity. The organelle substrates of the invention are specific for enzyme activity of the organelle and label these organelles, such as lysosomes, rendering them visible and easily observed. Substrates of the present invention include substrates that produce a fluorescent signal. The fluorogenic acidic organelle enzyme substrates of this invention are designed to provide high fluorescence at low pH values and are derivatized to permit membrane permeation through both outer and organelle membranes of intact cells and can be used for staining cells at very low concentrations. They can be used for monitoring enzyme activity in cells at very low concentrations and are not toxic to living cells or tissues.