Cathepsin A

Tumor cells express many hydrolytic enzymes, particularly peptidases, some of which are partially specific for certain tumor types, e.g., plasmin, plasminogen activator protease, and cathepsins. A number of prodrug strategies have been developed for the tumor-selective delivery of cytotoxic drugs [45-47], as illustrated below with a few representative examples. 

This enzyme [EC 3.4.16.5] (also known as serine-type carboxypeptidase I, cathepsin A, carboxypeptidase Y, and lysosomal protective protein) is a member of the peptidase family SIO and catalyzes the hydrolysis of the peptide bond, with broad specificity, located at the C-terminus of a polypeptide. The pH optimum ranges from 4.5 to 6.0. The enzyme is irreversibly inhibited by diisopropyl fluorophosphate and is sensitive to thiolblocking reagents. 

Lysosomal proteases are called cathepsins, a name derived from the Greek term meaning to digest. The interior of the lysosome is acidic and the cathepsins, like all lysosomal hydrolases, possess acidic pH optima and exhibit little enzymatic activity at neutral pH. This characteristic protects the cell from autolytic breakdown that might result from leakage of lysosomal contents into the neutral cytoplasm. 

Lactacystin and derivatives Lactacystin, / -Lactone Cathepsin A, TPPII... 

Sulphatides, glycolipids, glycosaminoglycans Galactosialidosis (protective protein/cathepsin A) protective protein/cathepsin A 20... 

Galactosialidosis Protective protein/ cathepsin A Sialyl-OS GM2, GM3, GMl, GDla... 

K. Worowski, Lactacystin, a specific inhibitor of the proteasome, inhibits human platelet lysosomal cathepsin A-like enzyme, Biochem. Biophys. Res. Commun. 1997, 234, 729-732. 

Cathepsins. Intracellular proteinascs obtained Irom animal tissue extracts, the richest sources being liver, kidney and spleen. Located primarily in the lysosomal fraction within the cell. Part of the genera] enzymic apparatus of animal cells in most cases they do not specialize in functions characteristic of individual tissues. Review of cathepsins A -C J. S, Fruton in The Enzymes vol, 4, P. D. Boyer et at., Eds. (Academic Press, New York, I960) pp 233-241 of cathepsins A-E M. J. Mycek, Methods JEnzymol. 19, 285-315 (1970) of cathepsins B, D and G several authors, Res. Monogr. Celt Tissue Physiol. 2, 57-89, 181 -248 (1977) of cathepsins B, D, G, H, L, N and S several authors, Ciba Found. Symp. 75, 1-68 (1980). 

D. Wang, S. Zaitsev, G. Taylor, A. d Azzo, and E. Bonten, Protective protein/ cathepsin A rescues N-glycosylation defects in neuraminidase-1, Biochim. Biophys. Acta, 1790 (2009) 275-282. 

A. van der Spoel, E. Bonten, and A. d Azzo, Transport of human lysosomal neuraminidase to mature lysosomes requires proteetive protein/cathepsin A, EMBOJ, 17 (1998) 1588-1597. 

E. J. Bonten and A. d Azzo, Lysosomal neuraminidase. Catalytic activation in insect cells is eontrolled by the protective protein/cathepsin A, J. Biol. Chem., 275 (2000) 37657-37663. 

The term cathepsin, as we use it, is valid for the proteolytic optimum at pH 3.8. We are aware that most probably this is a complex of cathepsin enzymes, included in the original broad concepts of Willstatter and Bamann, not in those of Bergmann and Fruton. Up to the present we did not examine the individual cathepsin enzymes, which are divided into at least 6 groups (viz., cathepsin A, B, C, leucinaminopeptidase, carboxypeptidase, tripeptidase), and for the determination of which at least 6 types of synthetic substrates are necessary. Up to the present, only cathepsin and leucinaminopeptidase activities have been compared, exhibiting quite different respective results (203). 

Birkus G, Wang R, Liu X, Kutty N, MacArthur H, Cihlar T, Gibbs C, Swaminathan S, Lee W, McDermott M (2007) Cathepsin A is the major hydrolase catalyzing the intracellular hydrolysis of the antiretroviral nucleotide phosphonoamidate prodrugs GS-7340 and GS-9131. Antimicrob Agents Chemother 51 543-550... 

Mammalian liver and muscle frucose bisphosphate aldolases are also very susceptible to limited proteolysis (5,53-55). Cathepsin B, cathepsin L, and papain catalyze the limited proteolysis of rabbit muscle and rat liver aldolases 50,51). In fact, decrease of aldolase activity in liver is observed during starvation 109) and after administration of lysosome-tropic agents 103). Leupeptin caused an increase in osmotic sensitivity of lysosomes and an increase in the activities of free lysosomal proteinases, such as cathepsin A and cathepsin D, and a moderate increase of cathepsin B and L, and resulted in a decrease in aldolase activity. The molecular properties of aldolase isolated from the livers of control rats and leupeptin-treated rats indicated that the decrease of aldolase activity is attributable to hydrolysis of a peptide linkage(s) near the carboxyterminal of the enzyme. However, care is necessary in determining whether proteolytic modification of enzymes... 

Inhibitor peptides low molecular mass oligopeptide-fatty acid compounds of microbial origin which irreversibly inactivate plant and animal proteases. The inhibition is stoichiometric, i.e. 1 molecule I.p. inhibits 1 molecule enzyme. Examples are Leupeptin [acetyl-(or propionyl-)L-Leu-L-Leu-arginal the L-leu-cine can also be replaced by L-isovaline or L-valine], from Streptomyces species, inhibits cathepsin B, papain, trypsin, plasmin and cathepsin D, the effectiveness of the inhibition decreasing in that order. Pepsta-tin (isovaleryl-L-Val-L-Val-P-hydroxy-Y-NH2- -CH3-heptanoyl-L-Ala-P-hydroxy-Y-NHj-e-heptanoic acid), from actinomycetes, inhibits pepsin and cathepsin D. Chymostatin inhibits all known chymotrypsin types, cathepsin A, B, and D and papain. Antipain inhibits papain trypsin and plasmin. 

The Discovery of Cathepsin A Inhibitors A Project-Adapted Fragment Approach Based on HTS Results... 

Unlike the well-described cysteine proteases cathepsin S (CatS) and cathepsin L (CatL), the serine protease cathepsin A (CatA) is less well known in the scientific community as a potential drug target. We became aware of pharmacological studies conducted during the 1990s with the natural product ebelactone B [1]. These studies postulated therapeutic benefits in cardiovascular diseases after CatA inhibition. Based on these reports, we started our own research program for small-molecule inhibitors of CatA and we published some of oiu" results in two research articles [2,3]. In this chapter, we will provide the reader with an in-depth discussion of our lead discovery strategy and include a siunmary of our results. 

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