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We have previously shown that heparin can increase the activity of the proenzyme form of Alzheimer's β-site amyloid precursor protein cleaving enzyme 1 (BACE1). Cathepsin D (CD) is a member of the aspartic protease family and has sequence similarity to BACE1. Therefore, we examined whether heparin and other glycosaminoglycans (GAGs) can influence the activity of CD. Heparin and other GAGs were found to stimulate the activity of recombinant proCD. Desulfation of heparin almost abolished the stimulation, indicating that sulfate groups were important for the stimulatory effect. In addition, the stimulation was dependent on the length of the GAG chain, as larger GAGs were more potent in their ability to stimulate proCD than shorter fragments. In the presence of heparin, limited autocatalytic proteolysis of the proenzyme was increased, suggesting that heparin increases the activity of proCD by accelerating the conversion of proCD, which has little activity, to pseudoCD, an active form lacking residues 1–26 of the prodomain. Furthermore, the activity of spleen-derived mature CD, which lacks the entire 44 amino acid residue prodomain, was also increased by heparin, indicating that the catalytic domain of CD contains at least one region to which GAGs bind and stimulate enzyme activity. Because heparin also stimulated the activity of pseudoCD, proenzyme activation was probably accelerated by the interaction of heparin with the catalytic domain of pseudoCD. However, it is possible that heparin may also activate the proenzyme directly. On the basis of this study, we propose that GAGs may regulate CD activity in vivo.