4-Methylumbelliferyl palmitate serves as a potent fluorophore to assay acid lipase activity in human leukocytes, with its fluorescence significantly affected by acidity and solvent properties. In near-neutral aqueous solutions, it predominates in a neutral molecular form, emitting strong fluorescence at 445 nm, whereas in weakly alkaline solutions, it primarily exists in an anionic form, resulting in even stronger fluorescence at the same wavelength [1] [2].