Leupeptin hemisulfate is a tripeptide aldehyde protease inhibitor, consisting of the amino acids acetyl-Leu-Leu-argininal. It is commonly supplied as a mixture of the acetate and sulfate salts. The compound is a reversible and competitive inhibitor that primarily targets serine proteases (such as trypsin, plasmin, and kallikrein) and cysteine proteases (such as papain, cathepsin B, and calpain). Its primary application is in life science research as a biochemical tool to inhibit proteolytic activity in cell lysates, tissue extracts, and during protein purification procedures. By preventing unwanted protein degradation, it helps maintain the integrity of proteins of interest. It is also used in studies of apoptosis, signal transduction, and various cellular processes where protease activity is involved. Furthermore, due to its specific inhibitory profile, it can serve as a process-related impurity standard or reference compound in the quality control of biopharmaceuticals, particularly those involving recombinant protein production. Leupeptin hemisulfate is typically used in aqueous buffer solutions at micromolar concentrations. It is cell-permeable and often used in combination with other protease inhibitors (e.g., pepstatin A, PMSF) for broad-spectrum protection. Appropriate safety precautions should be taken as it may be harmful if inhaled or absorbed through the skin.