Leupeptin hemisulfate (Ac-Leu-Leu-argininal) is a reversible inhibitor of serine and cysteine endopeptidases. Leupeptin hemisulfate is capable of inhibiting plasmin, trypsin, papain, kallikrein, calpain and cathepsin B. No inhibition has been found with pepsin, cathepsins A and D, elastase, renin or chymotrypsin. Leupeptin is also a lysosomal inhibitor, which promotes axon elongation due to the overexpression of FGFR1 in neurons. Through the inhibition of calpain, it prevents apoptosis of motoneurons and increases muscle function after nerve injury. Leupeptin is capable of reacting

with various opioid receptors, reversibly inhibiting opioid receptor-ligand binding. Leupeptin is usually used in a concentration range of 10 to 100μM. Leupeptin is widely used in many protease inhibitor cocktails due to its potency, low toxicity, and range of inhibition. Leupeptin is also used in a quantitative assay to measure macroautophagic flux in mammals and is being investigated for its use in treating malaria.