Cyclic ketone inhibitors of the cysteine protease cathepsin K.

Cathepsin K (EC 3.4.22.38), a cysteine protease of the papain superfamily, is predominantly expressed in osteoclasts and has been postulated as a target for the treatment of osteoporosis. Crystallographic and structure--activity studies on a series of acyclic ketone-based inhibitors of cathepsin K have led to the design and identification of two series of cyclic ketone inhibitors. The mode of binding for four of these cyclic and acyclic inhibitors to cathepsin K is discussed and compared. All of the structures are consistent with addition of the active site thiol to the ketone of the inhibitors with the formation of a hemithioketal. Cocrystallization of the C-3 diastereomeric 3-amidotetrahydrofuran-4-one analogue 16 with cathepsin K showed the inhibitor to occupy the unprimed side of the active site with the 3S diastereomer preferred. This C-3 stereochemical preference is in contrast to the X-ray cocrystal structures of the 3-amidopyrrolidin-4-one inhibitors 29 and 33 which show these inhibitors to prefer binding of the 3R diastereomer. The 3-amidopyrrolidin-4-one inhibitors were bound in the active site of the enzyme in two alternate directions. Epimerization issues associated with the labile alpha-amino ketone diastereomeric center contained within these inhibitor classes has proven to limit their utility despite promising pharmacokinetics displayed in both series of compounds.

Azepanone-based inhibitors of human and rat cathepsin K.

The synthesis, in vitro activities, and pharmacokinetics of a series of azepanone-based inhibitors of the cysteine protease cathepsin K (EC 3.4.22.38) are described. These compounds show improved configurational stability of the C-4 diastereomeric center relative to the previously published five- and six-membered ring ketone-based inhibitor series. Studies in this series have led to the identification of 20, a potent, selective inhibitor of human cathepsin K (K(i) = 0.16 nM) as well as 24, a potent inhibitor of both human (K(i) = 0.0048 nM) and rat (K(i,app) = 4.8 nM) cathepsin K. Small-molecule X-ray crystallographic analysis of 20 established the C-4 S stereochemistry as being critical for potent inhibition and that unbound 20 adopted the expected equatorial conformation for the C-4 substituent. Molecular modeling studies predicted the higher energy axial orientation at C-4 of 20 when bound within the active site of cathepsin K, a feature subsequently confirmed by X-ray crystallography. Pharmacokinetic studies in the rat show 20 to be 42% orally bioavailable. Comparison of the transport of the cyclic and acyclic analogues through CaCo-2 cells suggests that oral bioavailability of the acyclic derivatives is limited by a P-glycoprotein-mediated efflux mechanism. It is concluded that the introduction of a conformational constraint has served the dual purpose of increasing inhibitor potency by locking in a bioactive conformation as well as locking out available conformations which may serve as substrates for enzyme systems that limit oral bioavailability.

Effect of simulated intraoperative heating and shaping on mechanical properties of a bioabsorbable fracture plate material.

Heating bioabsorbable fracture fixation plates to above their glass transition temperature renders them temporarily malleable, thus facilitating their adaptation to the underlying bone geometry, although the consequence of heating is not well understood. Poly (L-lactide-co-glycolide) copolymer specimens were heated under various conditions, and the effects on specimen mechanics were assessed. Heating temporarily increased toughness while slight reducing flexural modulus. No lasting effects on in vitro material degradation were seen.

A comparative study of the asparagine-linked oligosaccharides on siglec-5, siglec-7 and siglec-8, expressed in a CHO cell line, and their contribution to ligand recognition.

The siglecs (sialic acid-binding immunoglobulin-like lectins) mediate sialic acid-dependent cellular interactions and may in some cases signal through SH2-binding domains. In addition to the previously characterized siglecs, sialoadhesin, CD22, CD33 and myelin-associated glycoprotein, several new ones, siglec-5, siglec-7 and siglec-8, have recently been cloned. Although these novel receptors have generated considerable interest as therapeutic targets because of their expression pattern on immune cells, very little is known about how their lectin activity is regulated. Previous studies with sialoadhesin, CD22 and CD33 have shown that siglec glycosylation has significant effects on binding. To determine any differences in the glycan composition of siglec-5, siglec-7 and siglec-8 that may modify their function, we released and characterized the N-linked oligosaccharide distribution in these three glycoproteins. The glycan pools from siglec-5 and siglec-7 contained a larger proportion of sialylated and core-fucosylated biantennary, triantennary and tetra-antennary oligosaccharides, whereas the carbohydrate mixture released from siglec-8 is noticeably less sialylated and is more abundant in 'high-mannose'-type glycans. In addition, we show that, in contrast with CD22 and CD33, mutating the conserved potentially N-linked glycosylation site in the first domain has no effect on binding mediated by siglec-5 or siglec-7.

Expression in Escherichia coli, refolding, and purification of human procathepsin K, an osteoclast-specific protease.

We have constructed and optimized a high yielding Escherichia coli expression system to produce glycosylation-free human procathepsin K and have developed conditions for refolding this enzyme. Recombinant human procathepsin K (EC 3.4.22.38) was expressed in E. coli, refolded from inclusion bodies, and further purified by Superdex 75 size-exclusion chromatography. Purified procathepsin K had a [MH]+ of 35,063 Da which is in agreement with the predicted mass of the construct. Amino-terminal sequence analysis matched the predicted sequence with no secondary sequence detected. Purified procathepsin K activated under autocatalytic conditions to a final specific activity of 23 micromol 7-amido-4-methylcoumarin liberated/min/mg of enzyme using the fluorescent peptide substrate benzyloxycarbonyl-phenylalanine-arginine-7-amido-4-methylcoumarin. This expression and refolding procedure yielded 50 mg of purified, glycosylation-free human procathepsin K from 1 liter of E. coli cell culture and enabled the determination of the structure of human procathepsin K at 2.6 A resolution.

Type I IL-1 receptor: ligand specific confirmation differences and the role of glycosylation in ligand binding.

The extracellular domain of the type I Interleukin-1 receptor (sIL-1R) was expressed in Drosophila S2 cells as a secreted 43 kDa glycoprotein, as evidenced by its binding to Concanavalin A and enzymatic deglycosylation. sIL-1R bound IL-1 beta with a K(D) of 2 nM as determined by competition ELISA. N-Glycanase treated sIL-1R had a C. 100 fold lower affinity than glycosylated sIL-1R for IL-1 beta, suggesting that glycosylation is a key component of the IL-1 beta/IL-1 receptor interaction. Crosslinking of sIL-1R to (125)I-IL-1 beta could be competed with unlabelled IL-1 alpha, IL-1 beta, IL-1 receptor antagonist (IL-1ra), and a mutant of IL-1 (Th9Gly) which has reduced bioactivity but wild type receptor binding affinity. Limited proteolysis of sIL-1R in the presence of IL-1 alpha, IL-1 beta, IL-1ra, and Thr9Gly IL-1 beta with several different proteases followed by analysis of sIL-1R by Western blot was used to assess the effect of binding on sIL-1R conformation. While some proteases showed no differences in cleavage patterns or sensitivity between free and bound sIL-1R, others showed differences in either cleavage sites or sensitivity with different ligands. This implies that upon ligand binding there is a conformational change in the receptor which is sensitive to the particular ligand bound, and hence has implications for the ability of different ligands to trigger responses after binding to receptor.

A comparative biomechanical analysis of resorbable rigid fixation versus titanium rigid fixation of metacarpal fractures.

Linear (two-dimensional) and three-dimensional (3D) plating systems (Poly-Medics) composed of the resorbable copolymer of polyglycolic acid (PGA) and poly-l-lactic acid (PLLA) (Lactosorb) were studied in vitro. The plates were applied to osteotomized fresh frozen human cadaveric metacarpal bones that were then tested for torsional rigidity and three-point bending strength and rigidity. The results were compared to those from another study of two low-profile titanium plating systems (Leibinger and Synthes). Analysis of variance revealed that the linear-flat Lactosorb plate and screws had apex dorsal rigidity and force-to-displacement measurements equal to all but two of the titanium plates (3D). The 3D-flat Lactosorb plate had the highest torsional rigidity of the resorbable system, but it was only moderately rigid compared to the titanium plating systems. This in vitro biomechanical study of the copolymer PGA-PLLA plating system indicates that, in clinical applications, it may be better suited for metacarpal fractures rather than proximal phalangeal fractures due to the lower demands of torsional loading compared to apex bending.

The crystal structure of human procathepsin K.

Cathepsin K is a cysteine protease present in human osteoclasts that plays an important role in bone resorption. Cathepsin K is synthesized as an inactive proenzyme and activated under conditions of low pH. Autoproteolytic processing of the N-terminal 99 amino acid propeptide produces the active, mature form of cathepsin K. It is presumed that the activation of procathepsin K in vivo occurs in the bone resorption pit, which has a low-pH environment. We have determined the structure of human procathepsin K at 2.8 A resolution. The structure of the mature enzyme domain within procathepsin K is virtually identical to that of mature cathepsin K. The fold of the propeptide of procathepsin K is similar to that observed in procathepsins B and L despite differences in length and sequence. A portion of the propeptide occupies the active site cleft of cathepsin K. Hydrophobic interactions, salt bridges, and hydrogen-bonding interactions are observed in the structure of the propeptide and between the propeptide and the mature enzyme of procathepsin K. These interactions suggest an explanation for the stability of the proenzyme. The structure of procathepsin K contributes to an understanding of the molecular basis of inhibition by the propeptide portion of the molecule and activation of this important member of the cysteine protease family.

Autocatalytic activation of human cathepsin K.

The in vitro activation of the recombinant purified human cathepsin K (EC 3.4.22.38) was examined by mutagenesis. Cathepsin K was expressed as a secreted proenzyme using baculovirus-infected Sf21 insect cells. Spontaneous in vitro activation of procathepsin K occurred at pH 4 and was catalyzed by exogenous mature cathepsin K. Three intermediates were identified as resulting from cleavages after Glu19, Ser98, and Glu110. The mature enzyme was composed of mixture of enzymes with N termini of Gly113, Arg114, and Ala115 with varying ratios depending on the preparation. Molecular weight determinations were consistent with the absence of carbohydrate in the mature protein, while electrospray mass spectroscopy indicated that six of the eight cysteine residues were in disulfide linkage, and that the protein had Met329 as the C-terminal residue. A mutant was constructed in which the active site Cys139 was changed to Ser. [Ser139,Ala163]Procathepsin K (containing mutation C139S,S163A) failed to spontaneously process and was only partially processed in the presence of 1% exogenous wild-type mature cathepsin K forming intermediates, which were identical to those observed in the activation of wild-type. [Ser139,Ala163]Procathepsin K could be fully processed to mature enzyme by including one equivalent of wild-type procathepsin K in the activation mixture. These results indicated that in vitro activation of the procathepsin K was an autocatalytic process.

Identification of SAF-2, a novel siglec expressed on eosinophils, mast cells, and basophils.

BACKGROUND: Eosinophils, basophils, and mast cells are believed to be the central tenet cells in allergic conditions including allergic rhinitis, asthma, and eczema. The molecular mechanisms underlying the recruitment of these cells to sites of allergic inflammation are poorly understood. OBJECTIVES: Our aim was to identify a common adhesion molecule that could potentially be responsible for mediating the recruitment of the allergic cell types to the lungs and other sites of allergy. METHODS: We have cloned a sialoadhesin molecule from a human eosinophil library with the use of expressed sequence tag technology and characterized its expression on allergic cells by the use of flow cytometry and specific mAbs. RESULTS: With the use of expressed sequence tag sequencing, we have identified a novel siglec molecule, SAF-2. SAF-2 has homology with other sialoadhesin family members (CD33 and siglec-5) and belongs to a subgroup of the Ig superfamily. SAF-2 is a 431-amino acid protein composed of 3 Ig domains with a 358-amino acid extracellular domain and a 47-amino acid tail. SAF-2 is highly restricted to eosinophils, basophils, and mast cells. Antibodies to SAF-2 do not modulate Ca(++) mobilization or chemotaxis of human eosinophils induced by eotaxin. CONCLUSION: SAF-2 is a highly restricted sialoadhesin molecule, which may be useful in the detection and/or modulation of allergic cells.

Potent dipeptidylketone inhibitors of the cysteine protease cathepsin K.

Cathepsin K (EC 3.4.22.38) is a cysteine protease of the papain superfamily which is selectively expressed within the osteoclast. Several lines of evidence have pointed to the fact that this protease may play an important role in the degradation of the bone matrix. Potent and selective inhibitors of cathepsin K could be important therapeutic agents for the control of excessive bone resorption. Recently a series of peptide aldehydes have been shown to be potent inhibitors of cathepsin K. In an effort to design more selective and metabolically stable inhibitors of cathepsin K, a series of electronically attenuated alkoxymethylketones and thiomethylketones inhibitors have been synthesized. The X-ray co-crystal structure of one of these analogues in complex with cathepsin K shows the inhibitor binding in the primed side of the enzyme active site with a covalent interaction between the active site cysteine 25 and the carbonyl carbon of the inhibitor.

Design of potent and selective human cathepsin K inhibitors that span the active site.

Potent and selective active-site-spanning inhibitors have been designed for cathepsin K, a cysteine protease unique to osteoclasts. They act by mechanisms that involve tight binding intermediates, potentially on a hydrolytic pathway. X-ray crystallographic, MS, NMR spectroscopic, and kinetic studies of the mechanisms of inhibition indicate that different intermediates or transition states are being represented that are dependent on the conditions of measurement and the specific groups flanking the carbonyl in the inhibitor. The species observed crystallographically are most consistent with tetrahedral intermediates that may be close approximations of those that occur during substrate hydrolysis. Initial kinetic studies suggest the possibility of irreversible and reversible active-site modification. Representative inhibitors have demonstrated antiresorptive activity both in vitro and in vivo and therefore are promising leads for therapeutic agents for the treatment of osteoporosis. Expansion of these inhibitor concepts can be envisioned for the many other cysteine proteases implicated for therapeutic intervention.