Excess zinc ions are a competitive inhibitor for carboxypeptidase A

Investigating the Cell Entry Mechanism, Disassembly, and Toxicity of the Nanocage PCC-1: Insights into Its Potential as a Drug Delivery Vehicle

The porous coordination cage PCC-1 represents a new platform potentially useful for the cellular delivery of drugs with poor cell permeability and solubility. PCC-1 is a metal-organic polyhedron constructed from zinc metal ions and organic ligands through coordination bonds. PCC-1 possesses an internal cavity that is suitable for drug encapsulation. To better understand the biocompatibility of PCC-1 with human cells, the cell entry mechanism, disassembly, and toxicity of the nanocage were investigated. PCC-1 localizes in the nuclei and cytoplasm within minutes upon incubation with cells, independent of endocytosis and cargo, suggesting direct plasma membrane translocation of the nanocage carrying its guest in its internal cavity. Furthermore, the rates of cell entry correlate to extracellular concentrations, indicating that PCC-1 is likely diffusing passively through the membrane despite its relatively large size. Once inside cells, PCC-1 disintegrates into zinc metal ions and ligands over a period of several hours, each component being cleared from cells within 1 day. PCC-1 is relatively safe for cells at low micromolar concentrations but becomes inhibitory to cell proliferation and toxic above a concentration or incubation time threshold. However, cells surviving these conditions can return to homeostasis 3-5 days after exposure. Overall, these findings demonstrate that PCC-1 enters live cells by crossing biological membranes spontaneously. This should prove useful to deliver drugs that lack this capacity on their own, provided that the dosage and exposure time are controlled to avoid toxicity.

Highly dynamic metal exchange in anthrax lethal factor involves the occupation of an inhibitory metal binding site

Rapid metal exchange in anthrax lethal factor and potentially other zinc enzymes proceeds via the formation of transient bimetallic species.

[Coordination chemical studies on the zinc enzymes]

The metal dissociation constants of bovine carbonic anhydrase II, bovine carboxypeptidase A, rat aminopeptidase B, and rat dipeptidyl peptidase III were measured using metal buffer solutions. The zinc dissociation constants of bovine carbonic anhydrase II, bovine carboxypeptidase A, rat aminopeptidase B, and rat dipeptidyl peptidase III were 5.8×10(-14), 3.5×10(-12), 3.7×10(-13), and 1.9×10(-13) M, respectively. The ternary complex between metal derivatives of bovine carbonic anhydrase and various chelating agents were characterized using the kinetic method and visible and magnetic circular dichroism spectra. The coordination geometry of the ternary complex was in the equilibrium state between the five and the tetrahedral coordination geometry. The equilibrium state depends on the character of ligands. Dipeptidyl peptidase III which has an abnormal zinc binding motif (HEXXXH) was characterized using the point mutation and computer simulation methods. The abnormal zinc binding motif (HEXXXH) of rat dipeptidyl peptidase III has a large helix part. It is generally known that the cupric derivatives of the zinc peptidase loses enzyme activity, but the cupric derivative of dipeptidyl peptidase III surprisingly has enzyme activity. The measurement of the electron paramagnetic resonance spectra of the cupric rat dipeptidyl peptidase III in the presence of the substrate showed that the coordination geometry is very flexible. The flexibility of the coordination geometry in the cupric rat dipeptidyl peptidase III is important for the expression of enzyme activity. Docking simulation was used to identify the substrate binding site of aminopeptidase B, which is a powerful tool to estimate substrate binding residues in enzymes.

A structural and functional analysis of Nna1 in Purkinje cell degeneration (pcd) mice

The axotomy-inducible enzyme Nna1 defines a subfamily of M14 metallocarboxypeptidases, and its mutation underlies the Purkinje cell degeneration (pcd) mouse. However, the relationship among its catalytic activity, substrate specificities, and the critical processes of neurodegeneration/axon regeneration is incompletely understood. Here we used a transgenic rescue strategy targeting expression of modified forms of Nna1 to Purkinje cells in pcd mice to determine structure-activity relationships for neuronal survival and in parallel characterized the enzymatic properties of purified recombinant Nna1. The Nna1 subfamily uniquely shares conserved substrate-determining residues with aspartoacylase that, when mutated, cause Canavan disease. Homologous mutations (D1007E and R1078E) inactivate Nna1 in vivo, as does mutation of its catalytic glutamate (E1094A), which implies that metabolism of acidic substrates is essential for neuronal survival. Consistent with reports that Nna1 is a tubulin glutamylase, recombinant Nna1-but not the catalytic mutants-removes glutamate from tubulin. Recombinant Nna1 metabolizes synthetic substrates with 2 or more C-terminal glutamate (but not aspartate) residues (V(max) for 3 glutamates is ∼7-fold higher than 2 glutamates although K(M) is similar). Catalysis is not ATP/GTP dependent, and mutating the ATP/GTP binding site of Nna1 has no effect in vivo. Nna1 is a monomeric enzyme essential for neuronal survival through hydrolysis of polyglutamate-containing substrates.

Zinc regulation of aminopeptidase B involved in neuropeptide production

Aminopeptidase B (AP-B) is a metallopeptidase that removes basic residues from the N-termini of neuropeptide substrates in secretory vesicles. This study assessed zinc regulation of AP-B activity, since secretory vesicles contain endogenous zinc. AP-B was inhibited by zinc at concentrations typically present in secretory vesicles. Zinc effects were dependent on concentration, incubation time, and the molar ratio of zinc to enzyme. AP-B activity was recovered upon removal of zinc. AP-B with zinc became susceptible to degradation by trypsin, suggesting that zinc alters enzyme conformation. Zinc regulation demonstrates the metallopeptidase property of AP-B.

The sigma(70) transcription factor TyrR has zinc-stimulated phosphatase activity that is inhibited by ATP and tyrosine

The TyrR protein of Escherichia coli (513 amino acid residues) is the chief transcriptional regulator of a group of genes that are essential for aromatic amino acid biosynthesis and transport. The TyrR protein can function either as a repressor or as an activator. The central region of the TyrR protein (residues 207 to 425) is similar to corresponding polypeptide segments of the NtrC protein superfamily. Like the NtrC protein, TyrR has intrinsic ATPase activity. Here, we report that TyrR possesses phosphatase activity. This activity is subject to inhibition by L-tyrosine and its analogues and by ATP and ATP analogues. Zinc ion (2 mM) stimulated the phosphatase activity of the TyrR protein by a factor of 57. The phosphatase-active site of TyrR was localized to a 31-kDa domain (residues 191 to 467) of the protein. However, mutational alteration of distant amino acid residues at both the N terminus and the C terminus of TyrR altered the phosphatase activity. Haemophilus influenzae TyrR (318 amino acid residues), a protein with a high degree of sequence similarity to the C terminus of the E. coli TyrR protein, exhibited a phosphatase activity similar to that of E. coli TyrR.

Interaction of cobalt ions with carboxypeptidase A

The interaction of cobalt(II) with native and cobalt(II)-substituted carboxypeptidase has been investigated, at pH 7.5, by electronic absorption and 1H NMR spectroscopies. The reaction of the cobalt(II) uptake by the metalloenzyme [MCPA] (M = Zn or Co) occurs very slowly and a bimetallic complex, [MCPA(Co)], is formed. On the basis of the 1H NOE experiments, the isotropically shifted proton resonances were assigned as belonging to a coordinated histidine residue. 1H NMR titrations of [ZnCPA(Co)] with zinc(II) show that the zinc ion does not compete with cobalt for binding to the noncatalytic site. The temperature dependence of the isotropic shifts, molar absorbance, and longitudinal relaxation time values are indicative of a five-coordinated geometry for the cobalt ion. The identification of the noncatalytic cobalt binding site is also discussed.

Rat kidney endopeptidase 24.16. Purification, physico-chemical characteristics and differential specificity towards opiates, tachykinins and neurotensin-related peptides

Endopeptidase 24.16 was purified from rat kidney homogenate on the basis of its ability to generate the biologically inactive degradation products neurotensin (1-10) and neurotensin (11-13). On SDS gels of the proteins pooled after the last purification step, the enzyme appeared homogeneous and behaved as a 70-kDa monomer. The peptidase was not sensitive to specific inhibitors of aminopeptidases, pyroglutamyl aminopeptidase I, endopeptidase 24.11, endopeptidase 24.15, proline endopeptidase and angiotensin-converting enzyme but was potently inhibited by several metal chelators such as o-phenanthroline and EDTA and was blocked by divalent cations. The specificity of endopeptidase 24.16 towards peptides of the tachykinin, opioid and neurotensin families was examined by competition experiments of tritiated neurotensin hydrolysis as well as HPLC analysis. These results indicated that endopeptidase 24.16 could discriminate between peptides belonging to the same family. Neurotensin, Lys8-Asn9-neurotensin(8-13) and xenopsin were efficiently hydrolysed while neuromedin N and kinetensin underwent little if any proteolysis by the peptidase. Analogously, substance P and dynorphins (1-7) and (1-8) were readily proteolysed by endopeptidase 24.16 while neurokinin A, amphibian tachykinins and leucine or methionine enkephalins totally resisted degradation. By Triton X-114 phase separation, 15-20% of endopeptidase 24.16 partitioned in the detergent phase, indicating that renal endopeptidase 24.16 might exist in a genuine membrane-bound form. The equipotent solubilization of the enzyme by seven detergents of various critical miscellar concentrations confirmed the occurrence of a membrane-bound counterpart of endopeptidase 24.16. Furthermore, the absence of release elicited by phosphatidylinositol-specific phospholipase C suggested that the enzyme was not attached by a glycosyl-phosphatidylinositol anchor in the membrane of renal microvilli. Finally, endopeptidase 24.16 could not be released from these membranes upon trypsinolysis.

Identification and characterization of the major stilbene-disulphonate- and concanavalin A-binding protein of the porcine renal brush-border membrane as aminopeptidase N

A 130 kDa glycoprotein (GP 130) was purified from porcine renal brush-border membranes by affinity chromatography using immobilized 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonate (SITS)- and concanavalin A-Sepharose. GP 130 was the major concanavalin A-binding protein in porcine renal brush-border membranes and also bound Ricinus communis (castor-bean) and wheat-germ agglutinins. Endo-beta-N-acetylglucosaminidase F reduced the molecular mass of GP 130 by 20 kDa as determined by SDS/PAGE, whereas endo-beta-N-acetylglucosaminidase H reduced the molecular mass by 5 kDa, showing that GP 130 contained both complex and high-mannose carbohydrate structures. Western-blot analyses using an antibody raised against GP 130 showed that it was localized to the brush-border membrane fraction and was present in a membrane fraction of the pig kidney cell line LLC-PK1. The N-terminal sequence and amino acid composition of GP 130 showed that GP 130 is similar to rat kidney zinc peptidase and human intestinal aminopeptidase N. GP 130 had aminopeptidase N enzymic activity and was inhibited by bestatin (Ki = 36 microM), 1,10-phenanthroline (Ki 30 microM), Zn2+ (Ki 26 microM), Cu2+ (Ki 260 microM), pre-incubation with EDTA and by a polyclonal antibody against GP 130. Bicarbonate and iodide blocked the binding of GP 130 to the SITS-affinity resin, showing that GP 130 has an anion-binding site. Neither these anions nor stilbene disulphonates affected the aminopeptidase N activity of GP 130.

Studies of zinc transport into brush-border membrane vesicles isolated from pig small intestine

Zinc transport into brush-border membrane vesicles was investigated by measuring uptake rates at a very short incubation time (2 seconds), during the initial linear uptake. A divalent cation chelator (EGTA) was added to the stop and washout solutions in order to remove the zinc bound to the external surface of the vesicles. Under these conditions, we showed that zinc enters the vesicles by (1) a saturable carrier-mediated process, and (2) an unsaturable pathway. The kinetic parameters we calculated were an affinity of 0.215 +/- 0.039 mM, a Jmax of 17.2 +/- 1.7 nmol.min-1.(mg protein)-1 and an unsaturable constant of 0.025 +/- 0.006 (n = 6). The imposition of an outwardly directed K+ gradient (negative inside) did not affect the Jmax value of the zinc uptake but increased the Km value significantly. This suggests that, at least a portion of zinc which crosses the membrane does not do so in a cationic form. Zinc uptake was decreased or increased according to the nature of accompanying anions (Cl-, SO4(2)-, SCN-) in the absence of any membrane potential. With highly permeant anions such as thiocyanates, zinc uptake was considerably augmented, suggesting a movement of zinc in a complexed form involving the presence of negative species. We also showed that cadmium competitively inhibited the zinc uptake; we measured a Ki value of 0.21 mM, indicating a similar affinity of cadmium for the carrier as zinc itself. By contrast, the presence of calcium had little effect on zinc entry into vesicles. The calcium ionophore A23187 had only a slight stimulating effect on zinc uptake. These results indicate that zinc and calcium transports are probably independent of each other.

Ionization states of the complex formed between 2-benzyl-3-phosphonopropionic acid and carboxypeptidase A

The binding to carboxypeptidase A of two phosphonic acid analogues of 2-benzylsuccinate, 2-DL-2-benzyl-3-phosphonopropionic acid (inhibitor I) and 2-DL-2-benzyl-3-(-O-ethylphosphono)propionic acid (inhibitor II) was studied by observing their 31P resonances when free and bound to the enzyme in the range of pH from 5 to 10. The binding of I by co-ordination to the active-site Zn(II) lowered the highest pKa of I from a value of 7.66(+/- 0.10) to a value of 6.71(+/- 0.17). No titration of any protons on II occurred over the pH range studied. The enzyme-bound inhibitor II also did not titrate over the pH range 6.17-7.60. The pH-dependencies of the apparent inhibition constants for I and II were also investigated by using N-(-2-(furanacryloyl)-L-phenylalanyl-L-phenylalanine as substrate. Two enzymic functional groups with pKa values of 5.90(+/- 0.06) and 9.79(+/- 0.14) must be protonated for binding of inhibitor I, and two groups with pKa values of 6.29(+/- 0.10) and 9.19(+/- 0.15) for binding of inhibitor II. Over the pH range from 6.71 to 7.66, inhibitor I binds to the enzyme in a complex of the enzyme in a more protonated form, and the inhibitor in a less protonated form than the predominant unligated forms at this pH. Mock & Tsay [(1986) Biochemistry 25, 2920-2927] made a similar finding for the binding of L-2-(1-carboxy-2-phenylethyl)-4-phenylazophenol over a pH range of nearly 4 units. The true inhibition constant for the dianionic form of inhibitor I (racemic) was calculated to be 54.0(+/- 5.9) nM and that of the trianionic form to be 5.92(+/- 0.65) nM. The true inhibition constant of the fully ionized II (racemic) was calculated to be 79.8(+/- 6.4) nM.