Acid protease active in intracellular protein breakdown. Specificity similar to, but narrower than, that of pepsin A. Does not cleave the 4-Gln-|-His-5 bond in B chain of insulin. Inhibited by pepstatin. Optimum pH is 2.5 with hemoglobin as substrate. Optimum temperature is 37 degrees Celsius. Monomer. Belongs to the peptidase A1 family.