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. Consists of a light chain and a heavy chain. Interacts with ADAM30; this leads to activation of CTSD. Interacts with GRN; stabilizes CTSD; increases its proteolytic activity (PubMed:28453791). N- and O-glycosylated. Undergoes proteolytic cleavage and activation by ADAM30. Belongs to the peptidase A1 family.