Biochemical properties of recombinant human beta-glucuronidase synthesized in baby hamster kidney cells

Tetramerization of GH2 β-Glucuronidases is Essential for Catalyzing the Hydrolysis of the Large Substrate Glycyrrhizin

In this study, structural analysis was employed to identify three hotspot residues that contribute most to the tetramer formation of a glycoside hydrolase family 2 (GH2) β-glucuronidase (GUS) from Aspergillus oryzae Li-3. Single-point mutation at these sites completely disrupted the tetramer structure and abolished the glycyrrhizin (GL)-hydrolyzing activity. Then, the W522A dimer was refactored into a tetramer by disulfide bonding, and partial GL activity was restored. Further saturated mutation showed a strong correlation between the GL activity of the mutants and their tetramer ratios. Molecular simulations were employed to illustrate the critical role of the tetramer interface in maintaining a functional active-site structure. The three highly conserved tetramer-forming residues were finally applied to two other GH2 GUSs for tetramer dissociation and demonstrated the significance of the homotetramerization for GL-hydrolyzing activity of GH2 GUSs. This study lays foundation for engineering GL-hydrolyzing GUSs at the quaternary structure level for function regulations.

Hyaluronidase inhibition accelerates functional recovery from stroke in the mouse brain

Perineuronal nets (PNNs) are presumed to limit plasticity in adult animals. Ischaemic stroke results in the massive breakdown of PNNs resulting in rejuvenating states of neuronal plasticity, but the mechanisms of this phenomenon are largely unknown. As hyaluronic acid (HA) is the structural backbone of PNNs, we hypothesized that these changes are a consequence of the altered expression of HA metabolism enzymes. Additionally, we investigated whether early hyaluronidase inhibition interferes with post-stroke PNN reduction and behavioural recovery. We investigated the mRNA/protein expression of these enzymes in the perilesional, remote and contralateral cortical regions in mice at different time points after photothrombosis, using quantitative real-time polymerase chain reaction and immunofluorescence. A skilled reaching test was employed to test hyaluronidase inhibitor L-ascorbic acid 6-hexadecanoate influence on post-stroke recovery. We found the simultaneous up-regulation of mRNA of HA synthesizing and degrading enzymes in the perilesional area early after stroke, suggesting an acceleration of HA turnover in ischaemic animals. Immunostaining revealed differential cellular localization of enzymes, with hyaluronidase 1 in astrocytes and hyaluronan synthase 2 in astrocytes and neurons, and post-stroke up-regulation of both of them in astrocytes. β-glucuronidase was observed in neurons but post-stroke up-regulation occurred in microglia. Inhibition of hyaluronidase activity early after stroke resulted in improved performance in skilled reaching test, without affecting the numbers of PNNs. These results suggest that after stroke, a substantial reorganization of polysaccharide content occurs, and interfering with this process at early time has a beneficial effect on recovery.

An improved purification method for the lysosomal storage disease protein β-glucuronidase produced in CHO cells

Human β-glucuronidase (GUS; EC 3.2.1.31) is a lysosomal enzyme that catalyzes the hydrolysis of β-d-glucuronic acid residues from the non-reducing termini of glycosaminoglycans. Impairment in GUS function leads to the metabolic disorder mucopolysaccharidosis type VII, also known as Sly syndrome. We produced GUS from a CHO cell line grown in suspension in a 15 L perfused bioreactor and developed a three step purification procedure that yields ∼99% pure enzyme with a recovery of more than 40%. The method can be completed in two days and has the potential to be integrated into a continuous manufacturing scheme.

Native Electrophoresis-Coupled Activity Assays Reveal Catalytically-Active Protein Aggregates of Escherichia coli β-Glucuronidase

β-glucuronidase is found as a functional homotetramer in a variety of organisms, including humans and other animals, as well as a number of bacteria. This enzyme is important in these organisms, catalyzing the hydrolytic removal of a glucuronide moiety from substrate molecules. This process serves to break down sugar conjugates in animals and provide sugars for metabolism in bacteria. While β-glucuronidase is primarily found as a homotetramer, previous studies have indicated that the human form of the protein is also catalytically active as a dimer. Here we present evidence for not only an active dimer of the E. coli form of the protein, but also for several larger active complexes, including an octomer and a 16-mer. Additionally, we propose a model for the structures of these large complexes, based on computationally-derived molecular modeling studies. These structures may have application in the study of human disease, as several diseases have been associated with the aggregation of proteins.

Properties and structures of β-glucuronidases with different transformation types of glycyrrhizin

Substrate recognition mechanisms of three fungi β-glucuronidases with different types of GL hydrolysis were analyzed.

Genetic engineering of IgG-glucuronidase fusion proteins

beta-Glucuronidase (GUSB) is a lysosomal enzyme that could be developed as a brain therapy for Type VII Mucopolysaccharidosis. However, GUSB does not cross the blood-brain barrier (BBB). To enable BBB transport of the enzyme, human GUSB was re-engineered as a fusion protein with the chimeric monoclonal antibody (MAb) to the human insulin receptor (HIR). The HIRMAb crosses the BBB on the endogenous insulin receptor, and acts as a molecular Trojan horse to ferry into brain the GUSB. The 611 amino acid GUSB was fused to either the carboxyl or amino terminus of the heavy chain of the HIRMAb. This study illustrates the differential retention of functionality of IgG-enzyme fusion proteins depending on how the fusion protein is engineered.

Membrane-tethered proteins for basic research, imaging, and therapy

Secreted and intracellular proteins including antibodies, cytokines, major histocompatibility complex molecules, antigens, and enzymes can be redirected to and anchored on the surface of mammalian cells to reveal novel functions and properties such as reducing systemic toxicity, altering the in vivo distribution of drugs and extending the range of useful drugs, creating novel, specific signaling receptors and reshaping protein immunogenicity. The present review highlights progress in designing vectors to target and retain chimeric proteins on the surface of mammalian cells. Comparison of chimeric proteins indicates that selection of the proper cytoplasmic domain and introduction of oligiosaccharides near the cell surface can dramatically enhance surface expression, especially for single-chain antibodies. We also describe progress and limitations of employing surface-tethered proteins for preferential activation of prodrugs at cancer cells, imaging gene expression in living animals, performing high-throughput screening, selectively activating immune cells in tumors, producing new adhesion molecules, creating local immune privileged sites, limiting the distribution of soluble factors such as cytokines, and enhancing polypeptide immunogenicity. Surface-anchored chimeric proteins represent a rich source for developing new techniques and creating novel therapeutics.

Involvement of AP-2 binding sites in regulation of human beta-glucuronidase

The lysosomal hydrolase beta-glucuronidase (beta-gluc) can be used for the bioactivation of non-toxic glucuronide prodrugs of anticancer agents. The enzyme is present at high levels in many tumours and hence may lead to an enhanced drug targeting by tumour-selective release of the active anticancer drug. Individual expression and regulation of this enzyme is one factor modulating the bioactivation of glucuronide prodrugs. Nevertheless, in contrast to murine beta-gluc, which is inducible by androgens, the human enzyme has been regarded as an unregulated housekeeping gene due to a lacking TATA box and high G+C contents within the putative promotor sequence. Despite these facts, we were able to demonstrate downregulation of human beta-gluc expression by the calcium ionophore A23187 and the calcium ATPase inhibitor thapsigargin in the human hepatoma cell line HepG2. However, cis-acting elements responsible for this regulation have not yet been identified. We therefore characterised the 5'-untranslated region of the human beta-gluc gene using transient transfection assays with promotor-luciferase constructs in HepG2 cells and cloned fragments between 3,770 bp and 107 bp. A23187 reduced the beta-gluc promotor activity. This effect disappeared using fragments smaller than 356 bp. Using site-directed in vitro mutagenesis and gel-electrophoretic-mobility shift assays, we found evidence of an involvement of transcription factor activating protein-2 (AP-2) binding sites on the regulation of human beta-glucuronidase by A23187. Our studies provide a basis for the understanding of the transcriptional regulation of the human beta-glucuronidase gene and could be useful for the optimisation of glucuronide prodrug therapy.

Verapamil regulates activity and mRNA-expression of human beta-glucuronidase in HepG2 cells

A promising development in tumor therapy is the application of non-toxic prodrugs from which the active cytostatic is released by endogenous enzymes such as beta-glucuronidase (beta-gluc). Regulation of beta-gluc expression is one crucial factor modulating bioactivation of prodrugs. Recent experiments in rats indicate regulation of beta-gluc activity by the calcium channel blocker verapamil. To further explore this phenomenon, we investigated the effect of verapamil on beta-gluc enzyme activity, protein (western blot) and mRNA expression (RT-PCR) as well as the underlying mechanisms (effects of verapamil metabolites; promoter activity) in the human hepatoma cell line HepG2. Treatment of HepG2 cells with verapamil revealed down-regulation of beta-gluc activity, protein, and mRNA level down to 50% of the control with EC(50) values of 25 microM. Effects were similar for both enantiomers. Moreover, it was demonstrated that reduced promoter activity contributes to the observed effects. In summary, our data demonstrate regulation of human beta-glucuronidase expression by verapamil. Based on our findings we hypothesize that coadministration of verapamil may effect cleavage of glucuronides by beta-glucuronidase.

Chimeric TNT-3/human beta-glucuronidase fusion proteins for antibody-directed enzyme prodrug therapy (ADEPT)

ADEPT (antibody-directed enzyme prodrug therapy) is a novel therapeutic approach that targets an enzyme into tumors to convert a relatively nontoxic prodrug into an active cytotoxic agent. This method has a number of advantages, including the reduction of systemic toxicity, but to date it has not realized its full potential. A critical component of ADEPT is the choice of the monoclonal antibody (MAb) to target the enzyme into the tumor mass. Prior studies have utilized MAbs directed against tumor cell surface antigens which are oftentimes labile and heterogeneous in nature and do not provide an ideal site for the enzyme. As an alternative approach, we now describe the use of Tumor Necrosis Therapy (TNT) MAbs to deliver the enzyme to necrotic regions of tumors in order to enhance the effectiveness of ADEPT. Biodistribution and autoradiographic studies performed using TNT MAbs have shown that localization of these antibodies occurs in degenerating cells and necrotic regions of tumors and that binding is retained within the tumor mass for extended periods of time. Since necrotic regions are often located in the center of tumors, are universal in nature, and constitute between 30 and 80% of the tumor mass, TNT MAbs may be ideal targeting agents for ADEPT. To test this hypothesis, fusion proteins consisting of single chain Fv (scFv), Fab, or F(ab')2 fragments of chTNT-3 and the human beta-glucuronidase (betaG) enzyme were constructed for ADEPT. Each of these reagents was tested to assess specificity and avidity of antigen binding as compared to the parental antibody. In addition, studies were performed to demonstrate enzymatic function of the fusion proteins and retention of catalytic activity in circulating blood, specific tissues, and tumor after in vivo administration. Pharmacokinetic and biodistribution studies of radiolabeled fusion proteins were conducted over time to evaluate the characteristics of the fusion proteins. Finally, one of the constructs (chTNT-3 Fab/betaG) was used in a pilot treatment study with a glucuronide prodrug of doxorubicin to demonstrate the anti-tumor activity of ADEPT using the chemoresistant MAD109 murine lung carcinoma tumor model transplanted into BALB/c mice. The results of these experiments show that all three constructs retained their antigen binding capability and demonstrated active enzymatic function against substrate in vitro. Moreover, after in vivo administration, the betaG enzyme was shown to localize to tumor and remain active for up to 9 days demonstrating a key characteristic of TNT targeting. Pharmacokinetic and biodistribution studies confirmed specific localization of the fusion proteins and rapid clearance from blood and normal tissues over time. Finally, therapeutic studies using only two doses of fusion protein followed by prodrug administration demonstrated active cytotoxicity against established tumors without systemic toxicity. These preliminary studies show that the use of TNT MAbs to target the enzyme to the tumor may be a significant advance in ADEPT and that further studies are warranted to test this novel therapeutic approach in the treatment of solid tumors.

Expression of active human beta-glucuronidase in Sf9 cells infected with recombinant baculovirus

Antibody directed enzyme prodrug therapy (ADEPT) using glucuronide prodrugs is an experimental approach to reduce systemic toxicity of anti-cancer agents. Bioactivation of such prodrugs is achieved by fusion proteins consisting of targeting moieties (e.g. ligands of tumor specific antigens) and human beta-glucuronidase. In order to test a large panel of possible beta-glucuronidase fusion proteins for their applicability in ADEPT, an easy, rapid and high-yield expression system like the baculovirus/insect cell expression system would be needed. A prerequisite for using such fusion proteins is functional and biochemical characterization of human beta-glucuronidase expressed in baculovirus-infected insect cells. Therefore, recombinant human beta-glucuronidase was expressed in Sf9 insect cells and characterized at the protein and functional level. As shown by Western blot analysis the recombinant enzyme consists of dimers with their monomers being linked via disulfide bonds. Posttranslational modifications of the monomers seem to be different as compared with mammalian cells or tissues. The enzyme is functionally active in cleaving the substrates 5-bromo-4-chloro-3-indolyl-beta-D-glucuronic acid, 4-methylumbelliferyl-beta-D-glucuronide and the glucuronide prodrug HMR 1826, respectively, with similar enzyme kinetic parameters as those found in human tissues. Our data demonstrate that beta-glucuronidase expressed in Sf9 cells displays the same enzymatic features as the protein expressed in mammalian cells. Therefore, we suggest that beta-glucuronidase fusion proteins produced in this cell line will be valuable tools for testing a large panel of various targeting moieties in human tumor xenograft models or may be used for ADEPT in man.

Expression and purification of Escherichia coli beta-glucuronidase

A strong and constitutive expression vector of Escherichia coli beta-glucuronidase with the isocitrate dehydrogenase promoter has been developed for producing a large amount of recombinant protein. More than 95% pure enzyme was obtained by a four step purification procedure-ammonium sulfate precipitation, DEAE ion-exchange chromatography, Superose 12 gel filtration, and hydroxyapatite steric ion-exchange chromatography. The overexpressed gene can produce 23 mg of pure enzyme from one liter of bacterial culture.

Regulation of human beta-glucuronidase by A23187 and thapsigargin in the hepatoma cell line HepG2

A novel approach to reducing organ toxicity of anticancer agents is the application of nontoxic glucuronide prodrugs from which the active drug is released by human beta-glucuronidase, an enzyme present at high levels in many tumors. In view of high interindividual variability in beta-glucuronidase expression, regulation of this enzyme is an essential factor modulating bioactivation of glucuronide prodrugs. However, data on regulation of human beta-glucuronidase expression are not available. Preliminary evidence from animal experiments points to a role of intracellular calcium in regulation of beta-glucuronidase activity. Therefore, we investigated regulation of beta-glucuronidase by the calcium ionophore A23187 and the calcium ATPase inhibitor thapsigargin in the human hepatoma cell line HepG2. The enzyme was characterized on activity, protein, and mRNA levels by cleavage of 4-methylumbelliferyl-beta-D-glucuronide, Western blotting, Northern blotting, and nuclear run-on transcription. Incubation of HepG2 cells with A23187 and thapsigargin, respectively, revealed a time and concentration dependent down-regulation of beta-glucuronidase activity to about 50% of the control level. This effect could also be demonstrated in several other cell lines (e.g., HL-60, ECV 304, 32M1, Caco-2/TC7). Effects on protein and mRNA levels paralleled those obtained on enzymatic activity. In line with these data, A23187 and thapsigargin decreased beta-glucuronidase transcriptional rate. Our data demonstrate regulation of human beta-glucuronidase by xenobiotics. Down-regulation of beta-glucuronidase by A23187 and thapsigargin is at least partly mediated by a transcriptional mechanism. Based on our findings, we speculate that beta-glucuronidase activity and hence bioactivation of glucuronide prodrugs in humans can be modulated by exogenous factors.

Towards a human repertoire of monocytic lysosomal proteins

The lysosomal compartment of human monocytic cells has never been investigated by a proteomic approach. By a combination of one-dimensional (1-D) and two-dimensional (2-D) gel electrophoresis, protein identification by N-terminal sequencing, matrix assisted laser desorption/ionization-mass spectrometry (MALDI-MS) peptide mass fingerprinting and tandem mass spectrometry (MS/MS) peptide sequence analysis, we initiated an exhaustive study of the human lyososomal proteome, which aims at establishing a 2-D reference map of human soluble lyososomal proteins. Human monocytic U937 cells were induced to secrete lysosomal soluble hydrolases by addition of NH4Cl in the culture medium. Since lysosomal soluble proteins are characterized by the presence of mannose-6-phosphate, they were purified on an affinity support bearing mannose-6-phosphate receptor. Analysis of the purified fraction led to the preliminary identification of fifteen proteins, among which twelve are well-known lysosomal hydrolases, one is assumed to be lysosomal on the basis of sequence homology to cysteine proteinases of the papain family, and two (leukocystatin and the human cellular repressor of E1A-stimulated genes) are described here for the first time as mannose-6-phosphate-containing proteins.

Enzyme replacement therapy in a feline model of MPS VI: modification of enzyme structure and dose frequency

Enzyme replacement therapy (ERT) in the MPS VI cat is effective at reducing or eliminating pathology in most connective tissues. One exception is that cartilage and chondrocytes remained distended with extensive lysosomal vacuolation after long-term, high-dose ERT. In this study, we demonstrate that recombinant human N-acetylgalactosamine-4-sulphatase (4S) is taken up by chondrocytes via a mannose-6-phosphate-dependent mechanism and is effective at removing MPS storage. In vitro, the penetration of 4S into articular cartilage is low (partitioning coefficient = 0.06) and i.v. administered enzyme does not distribute significantly into articular cartilage in vivo. To alter the tissue distribution of 4S, the enzyme was coupled to ethylene diamine or poly-L-lysine, increasing its overall charge and diffusion into cartilage, and the dosing frequency of unmodified 4S was increased. Modification resulted in active 4S that maintained its ability to correct MPS storage and increased the partitioning coefficient of 4S into cartilage by 77% and 50% for ethylene diamine and poly-L-lysine, respectively. However, in vivo ERT studies demonstrated that response to therapy was not significantly improved by either the enzyme modifications or change to the dosing regimen, when compared with ERT with unmodified enzyme. Distribution experiments indicated the majority of enzyme is taken up by the liver irrespective of modification. To optimize therapy and improve the amount of enzyme reaching cartilage and other tissues demonstrating poor uptake, it may be necessary to bypass the liver or prolong plasma half-life so that proportionately more enzyme is delivered to other tissues.

Active site residues of human beta-glucuronidase. Evidence for Glu(540) as the nucleophile and Glu(451) as the acid-base residue

Human beta-glucuronidase (hGUSB) is a member of family 2 glycosylhydrolases that cleaves beta-D-glucuronic acid residues from the nonreducing termini of glycosaminoglycans. Amino acid sequence and structural homology of hGUSB and Escherichia coli beta-galactosidase active sites led us to propose that residues Glu(451), Glu(540), and Tyr(504) in hGUSB are involved in catalysis, Glu(451) being the acid-base residue and Glu(540) the nucleophile. To test this hypothesis, we introduced mutations in these residues and determined their effects on enzymes expressed in COS cells and GUSB-deficient fibroblasts. The extremely low activity in cells expressing Glu(451), Glu(540), and Tyr(504) hGUSBs supported their roles in catalysis. For kinetic analysis, wild type and mutant enzymes were produced in baculovirus and purified to homogeneity by affinity chromatography. The k(cat)/K(m) values (mM(-1).s(-1)) of the E540A, E451A, and Y504A enzymes were 34,000-, 9100-, and 830-fold lower than that of wild type hGUSB, respectively. High concentrations of azide stimulated the activity of the E451A mutant enzyme, supporting the role of Glu(451) as the acid-base catalyst. We conclude that, like their homologues in E. coli beta-galactosidase, Glu(540) is the nucleophilic residue, Glu(451) the acid-base catalyst, and Tyr(504) is also important for catalysis, although its role is unclear. All three residues are located in the active site cavity previously determined by structural analysis of hGUSB.

Molecular farming of industrial proteins from transgenic maize

Recombinant egg white avidin and bacterial B-glucuronidase (GUS) from transgenic maize have been commercially produced. High levels of expression were obtained in seed by employing the ubiquitin promoter from maize. The recombinant proteins had activities that were indistinguishable from their native counterparts. We have illustrated that down-stream activities in the production of these recombinant proteins, such as stabilizing the germplasm and processing for purification, were accomplished without any major obstacles. Avidin (A8706) and GUS (G2035) are currently marketed by Sigma Chemical Co.

Construction and Characterization of a Fusion Protein of Single-Chain Anti-CD20 Antibody and Human β-Glucuronidase for Antibody-Directed Enzyme Prodrug Therapy

The CD20 antigen is an attractive target for specific treatment of B-cell lymphoma. Antibody-directed enzyme prodrug therapy (ADEPT) aims at the specific activation of a nontoxic prodrug at the tumor site by an enzyme targeted by a tumor-specific antibody such as anti-CD20. We constructed a fusion protein of the single-chain Fv anti-CD20 mouse monoclonal antibody (MoAb) 1H4 and human β-glucuronidase for the activation of the nontoxic prodrug N-[4-doxorubicin-N-carbonyl(-oxymethyl) phenyl] O-β-glucuronyl carbamate to doxorubicin at the tumor site. The cDNAs encoding the light- and heavy-chain variable regions of 1H4 were cloned, joined by a synthetic sequence encoding a 15-amino acid linker and fused to human β-glucuronidase by a synthetic sequence encoding a 6-amino acid linker. An antibody-enzyme fusion protein-producing cell line was established by transfection of the construct into human embryonic kidney 293/EBNA cells. The yield of active fusion protein was 100 ng/mL transfectoma supernatant. Antibody affinity, antibody specificity, and enzyme activity were fully retained by the fusion protein. Immunoprecipitation and analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) showed that the fusion protein has a relative molecular weight (Mw) of 100 kD under denaturing conditions. Gel filtration analysis indicated that the enzymatically active form of the fusion protein is a tetramer with an Mw of approximately 400 kD. The nontoxic prodrug N-[4-doxorubicin-N-carbonyl(-oxymethyl) phenyl] O-β-glucuronyl carbamate was hydrolyzed by the fusion protein at a hydrolysis rate similar to that of human β-glucuronidase. When the fusion protein was specifically bound to Daudi lymphoma cells, the prodrug induced similar antiproliferative effects as doxorubicin. Thus, it is feasible to construct a eukaryotic fusion protein consisting of a single-chain anti-CD20 antibody and human β-glucuronidase for future use in the activation of anticancer prodrugs in B-cell lymphoma.

The role of beta-glucuronidase in drug disposition and drug targeting in humans

Glucuronides of drugs often accumulate during long term therapy. The hydrolysis of glucuronides can be catalysed by beta-glucuronidase, an enzyme expressed in many tissues and body fluids in humans. The possible contribution of beta-glucuronidase to drug disposition in humans has not been assessed in a systematic manner, but this enzyme may be able to release, locally or systemically, the active or inactive parent compound from drug glucuronides, thereby modifying the disposition and action of these drugs. Based on the information available on the localisation, expression and variability of beta-glucuronidase, the concept of beta-glucuronidase-mediated drug metabolism is outlined in this article using examples from the literature. Since some issues surrounding the beta-glucuronidase-mediated deconjugation of drug glucuronides still need to be clarified in humans, additional data from animal models supporting this concept have been included. Moreover, as beta-glucuronidase has already been proven to be useful in tumour specific bioactivation of glucuronide prodrugs of anticancer agents, we also focus on anticancer prodrug approaches utilising beta-glucuronidase. This review summarises the role of beta-glucuronidase in drug disposition and drug targeting in humans.

High-performance liquid chromatographic quantification of 4-methylumbelliferyl-beta-D-glucuronide as a probe for human beta-glucuronidase activity in tissue homogenates

An internally standardized HPLC method to determine the concentration of 4-methylumbelliferone liberated from 4-methylumbelliferyl-beta-D-glucuronide by human beta-glucuronidase was developed. The assay allows the precise and rapid measurement of specific enzyme activity in human tissue homogenates. Without prior extraction the incubation mixture can be separated using a C8 column followed by fluorescence detection. The assay showed good accuracy and precision with a detection limit of 20 nM and a limit of quantification of 167 nM. The suitability of the method was shown in enzyme kinetic experiments with human liver homogenates.

Structure of human beta-glucuronidase reveals candidate lysosomal targeting and active-site motifs

The X-ray structure of the homotetrameric lysosomal acid hydrolase, human beta-glucuronidase (332,000 Mr), has been determined at 2.6 A resolution. The tetramer has approximate dihedral symmetry and each promoter consists of three structural domains with topologies similar to a jelly roll barrel, an immunoglobulin constant domain and a TIM barrel respectively. Residues 179-204 form a beta-hairpin motif similar to the putative lysosomal targeting motif of cathepsin D, supporting the view that lysosomal targeting has a structural basis. The active site of the enzyme is formed from a large cleft at the interface of two monomers. Residues Glu 451 and Glu 540 are proposed to be important for catalysis. The structure establishes a framework for understanding mutations that lead to the human genetic disease mucopolysaccharidosis VII, and for using the enzyme in anti-cancer therapy.