Acid protease active in intracellular protein breakdown. Plays a role in APP processing following cleavage and activation by ADAM30 which leads to APP degradation. Specificity similar to, but narrower than, that of pepsin A. Does not cleave the 4-Gln-|-His-5 bond in B chain of insulin. Occurs as a mixture of both a single chain form and two types of two chain (light and heavy) forms (PubMed:1883350). Interacts with ADAM30; this leads to activation of CTSD (By similarity). N- and O-glycosylated. Undergoes proteolytic cleavage and activation by ADAM30. Belongs to the peptidase A1 family.