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 3.0. Highly thermostable. Enzyme activity is maintained up to 45 degrees Celsius. Active at 50 degrees Celsius but with reduced catalytic activity. Monomer. Belongs to the peptidase A1 family.