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. Interacts with ADAM30; this leads to activation of CTSD (By similarity). Interacts with GRN; stabilizes CTSD; increases its proteolytic activity (By similarity). N- and O-glycosylated. Undergoes proteolytic cleavage and activation by ADAM30. Defects in CTSD are a cause of congenital ovine neuronal ceroid lipofuscinosis (CONCL). CONCL is an autosomal recessive disorder. Newborn lambs are weak, trembling, and unable to rise and support their bodies. However, they are able to vocalize, support their heads, and to suckle if bottle-fed. At autopsy, the brains of affected lambs are strikingly small. The deep layers of the cerebral cortex show pronounced neuronal loss, reactive astrocytosis, and infiltration of macrophages. There is severe degeneration of hippocampal pyramidal neurons. Belongs to the peptidase A1 family.