Purification of a membrane-bound neutral endopeptidase from rat kidney and its activation by polyamines

Validation of the Target Protein of Insecticidal Dihydroagarofuran Sesquiterpene Polyesters

A series of insecticidal dihydroagarofuran sesquiterpene polyesters were isolated from the root bark of Chinese bittersweet (Celastrus angulatus Max). A previous study indicated that these compounds affect the digestive system of insects, and aminopeptidase N3 and V-ATPase have been identified as the most putative target proteins by affinity chromatography. In this study, the correlation between the affinity of the compounds to subunit H and the insecticidal activity or inhibitory effect on the activity of V-ATPase was analyzed to validate the target protein. Results indicated that the subunit H of V-ATPase was the target protein of the insecticidal compounds. In addition, the possible mechanism of action of the compounds was discussed. The results provide new ideas for developing pesticides acting on V-ATPase of insects.

Effects of a protease inhibitor protein from Xenorhabdus bovienii on physiology of pea aphid (Acyrthosiphon pisum)

The effects of a protease inhibitor protein from Xenorhabdus bovienii strain xbi1 (Xbpi-1) on biochemistry and physiology of pea aphid (Acyrthosiphon pisum) were studied, and the effects of Xbpi-1 on the activities of aphid aminopeptidase and the concentrations of carbohydrates, free amino acids and lipids in A. pisum were determined. The results showed that the activity of aminopeptidase was reduced 18.30% by Xbpi-1 in vitro assays. In addition, the total sugar, glycogen, and lipid concentrations per aphid fed on an artificial diet with Xbpi-1 were reduced by 39.41%, 24.61% and 26.25%, respectively, compared to the controls. The total free amino acids in the aphids on the diet with Xbpi-1 was significantly decreased by 20.78% compared to the control. The mortalities of A. pisum were significantly higher when the concentrations of Xbpi-1 were 400 and 800 μg/ml in the artificial diet. The results from this study indicated that Xbpi-1 had adverse effects on the physiology and biochemistry of A. pisum.

Cytotoxic T lymphocytes carrying a pattern recognition protein Tag7 can detect evasive, HLA-negative but Hsp70-exposing tumor cells, thereby ensuring FasL/Fas-mediated contact killing

Within the broad problem of host immune surveillance versus tumor immune evasion, a most intriguing question is how the cellular immunity can cope with cancerous cells that have gotten rid of the classical antigen-presenting machinery. One such option stems from (1) the fact that HLA loss is often attended with expression of Hsp70 on the tumor cell surface, and (2) our findings that human lymphocytes express a protein Tag7 (also known as PGRP-S) capable of tight and specific interaction with cognate Hsp70. Here we show that a subpopulation of human CD4(+)CD25(+) lymphocytes, obtained either in culture as lymphokine-activated killers or directly from healthy donors, carry Tag7 and FasL on their surface and can indeed kill the HLA-negative tumor-derived cells K562 and MOLT-4 that expose Hsp70 and Fas. The primary binding of lymphocyte Tag7 to target-cell Hsp70 is very specific (eg, it is blocked by preincubating either cell with minimal peptides from the "partner" protein), and secures cell contact indispensable for subsequent FasL/Fas-triggered apoptosis. Unrelated to natural killer cell action or the putative role of Hsp as an antigen-presenting substitute, this novel mechanism is rather a backup analog of orthodox (CD8(+)) target recognition (Tag7 acting as built-in T-cell receptor and Hsp70 itself as ligand).

Characterization of aminopeptidase N from the brush border membrane of the larvae midgut of silkworm, Bombyx mori as a zinc enzyme

Three GPI-anchored proteins, aminopeptidase N, alkaline phosphatase and alkaline phosphodiesterase I were released from the midgut brush border membrane of Bombyx mori by phosphatidylinositol-specific phopholipase C and the aminopeptidase N was purified to a homogeneous state. N-terminus and 6 internal sequences, one of which possessed part of zinc-binding motif, showed homology with those from other species. The zinc content in purified aminopeptidase N was estimated as approximately 0.72 mol/mol of the protein and 1,10-phenanthroline completely inhibited the enzyme activity, suggesting zinc requirement for the activity. The aminopeptidase N activity was inhibited not only by probestin and actinonin, but also strongly depressed by amastatin, while leuhistin and bestatin were less inhibitory. These suggest that the active site of aminopeptidase N might be structurally different from those of mammals. Calcium and magnesium ions stimulated the aminopeptidase N activity, but copper ion was rather inhibitory. Zinc ion showed bi-modal effect on the activity, i.e., stimulatory at low concentration, but inhibitory at higher than 100 microM. This inhibition was completely restored by EDTA. These results suggest that the aminopeptidase N possesses two zinc ion-binding sites with high and low affinity as essential and inhibitory one, as well as some regulatory metal-binding sites.

Aminopeptidase N in sera of healthy subjects is a different N-terminal processed derivative from the one obtained from maternal serum

A major aminopeptidase present in normal human serum was purified to homogeneity as a 150-kDa molecular species. Western blotting confirmed the binding of an anti-aminopeptidase N antibody to the protein. The N-terminal amino acid sequence of the enzyme was determined. The first 13 amino acids of the enzyme completely matched amino acids 59-71 of the sequence predicted from the human intestinal aminopeptidase N cDNA nucleotide sequence. As reported previously, aminopeptidase N from maternal serum had 68 fewer amino acid residues at the N-terminus than the enzyme obtained from detergent-solubilized membranes. The results indicate that aminopeptidase N in normal serum is a different N-terminal processed derivative from that obtained from maternal serum.

Identification of an alanine aminopeptidase in human maternal serum as a membrane-bound aminopeptidase N

In addition to cystine aminopeptidase (oxytocinase) alanine aminopeptidase is present at high levels in the serum of pregnant women. In this study we compared the enzyme with membrane-bound aminopeptidase N purified from human placenta. Comparison of catalytic and immunological properties and N-terminal sequence analyses revealed that the enzymes were differentially processed derivatives of the same protein, and that the N-terminal 68 residues of aminopeptidase N were deleted in the alanine aminopeptidase. The deleted sequence contains a small cytoplasmic region, a hydrophobic transmembrane domain and a junctional domain. These results suggest that the enzyme may be released into the maternal circulation as a result of lacking these three domains.

Neutral endopeptidase activity in the interaction of N-formyl-L-methionyl-L-leucyl-L-phenylalanine with human polymorphonuclear leukocytes

Human polymorphonuclear leukocytes (PMN) hydrolyze the synthetic chemoattractant N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMet-Leu-Phe) at nanomolar concentrations in an autocatalytic-like manner that deviates from classical Michaelis-Menten kinetics [Yuli, I. & Snyderman, R. (1986) J. Biol. Chem. 261, 4902-4908]. By using inhibitors of distinct classes of endoproteases, this particular fMet-Leu-Phe degradation was attributed exclusively to an exoplasmic metalloendoprotease that matches the ubiquitous neutral endopeptidase (NEP). Membrane-bound NEP hydrolyzes non-chemotactic substrates according to a classic Michaelis-Menten mechanism. By competitive inhibition with non-chemotactic substrates, fMet-Leu-Phe was found to interact with membrane NEP through a single active site, in a non-cooperative mode with an apparent Km in the order of 1 mM. The discrepancy between the ordinary hydrolysis of the micromolar and millimolar concentrations of fMet-Leu-Phe, reported by others, and the particular degradation of the nanomolar fMet-Leu-Phe, could not be accounted for by any coherent correlation between NEP activity/inhibition and modulation of fMet-Leu-Phe binding to its receptor, and/or induction of fMet-Leu-Phe-receptor-mediated inflammatory responses. Based on these and previously reported results, a novel model is proposed in which the fMet-Leu-Phe-induced inflammatory stimulation of PMN involves both NEP and the fMet-Leu-Phe receptor. By this model, NEP and the fMet-Leu-Phe receptor are distinct membrane entities which can form dynamic binary and tertiary complexes; thus accounting for the unusual kinetic features of fMet-Leu-Phe degradation, as well as the two receptor states. The complex of NEP and the fMet-Leu-Phe receptor might be conceived as a chemotactic-perception mechanism that combines the high affinity of the receptor and the rapid turnover of NEP.

Neutral metalloendopeptidase associated with the smooth muscle cells of pregnant rat uterus

The pregnant rat uterus contains a membrane-bound metalloendopeptidase that is biochemically and immunologically similar to kidney enkephalinase (E.C.3.4.24.11). The uterus enzyme readily cleaved specific neutral endopeptidase substrates and oxytocin as well as the synthetic elastase substrate, Suc(Ala)3-pNA, yet did not digest native elastin. Using specific inhibitors, the uterus endopeptidase was identified as a metallopeptidase and not a serine protease, having an absolute requirement for zinc and perhaps calcium for maximal activity. The uterus endopeptidase cross-reacted with polyclonal antiserum to kidney microvillar endopeptidase and a monoclonal antibody to common acute lymphocytic leukemia antigen. Immunohistochemical localization of the enzyme in a 17 day pregnant uterus indicated that the enzyme was localized on the smooth muscle bundles of the myometrium and the endometrial epithelium. Total enzyme activity was 25 times higher in the late-term pregnant uterus (17th day of pregnancy) than in the nonpregnant uterus. Enzyme levels dropped rapidly prior to parturition and within 4 days after delivery the enzyme activity had returned to control levels. Inhibition of NEP in uterine strips with phosphoramidon resulted in a marked potentiation of oxytocin-induced contractions. Our results suggest that the uterine endopeptidase may have an important role in regulating uterine smooth muscle cell contraction during the later stages of pregnancy through its action on oxytocin and perhaps other biologically active peptides.

Deficiency of membrane-bound dipeptidyl aminopeptidase IV in a certain rat strain

The activity of membrane-bound dipeptidyl aminopeptidase IV (EC 3.4.14.5) was found to be markedly reduced in the Fischer 344 rat strain compared with that in the Wistar strain. Analysis of membrane proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Ouchterlony immunodiffusion revealed the specific absence of this enzyme molecule in the Fischer 344 strain.

Purification and characterization of a membrane-bound neutral endopeptidase from human placenta

A membrane-bound neutral endopeptidase which hydrolyzes Suc-(Ala)3-pNA to succinyl dialanine and Ala-pNA has been purified from human placenta. The enzyme was solubilized from membranes with DOC and papain, and was purified about 5000-fold by successive chromatographies on Sephadex G-200, DEAE-Sephacel, butyl-Toyopearl 650, and Sephacryl S-300. It was found to be homogeneous on SDS-polyacrylamide gel electrophoresis and to have a molecular weight of about 70,000. It was strongly inhibited by phosphoramidon, thiorphan, and metal-chelating agents, but was not affected by most other protease inhibitors. These findings indicate that it can be classified as a phosphoramidon-sensitive neutral endopeptidase. With biologically active peptides as substrates, the enzyme preferentially cleaved the bonds at the amino side of hydrophobic amino acid residues. The physiological significance of this enzyme is discussed with reference to the placental barrier.

Solubilization of trehalase from rabbit renal and intestinal brush-border membranes by a phosphatidylinositol-specific phospholipase C

Trehalase (EC 3.2.1.28) associated with renal and intestinal brush-border membranes was solubilized by highly purified phosphatidylinositol-specific phospholipase C (EC 3.1.4.10) from Bacillus thuringiensis, but not by phosphatidylcholine-hydrolyzing phospholipase C (EC 3.1.4.3) from Clostridium welchii or phospholipase D (EC 3.1.4.4) from cabbage. The solubilized trehalase was not adsorbed on phenyl-Sepharose, indicating that it was hydrophilic. Phosphatidylinositol-specific phospholipase C also converted Triton X-100-solubilized amphipathic trehalase into a hydrophilic form. These results suggest that trehalase is bound to the membrane through a direct and specific interaction with phosphatidylinositol.