Purification and Amino Acid Sequence of Aminopeptidase P from Pig Kidney

Characterization of Large Immune Complexes with Size Exclusion Chromatography and Native Mass Spectrometry Supplemented with Gas Phase Ion Chemistry

Large immune complexes formed by the cross-linking of antibodies with polyvalent antigens play critical roles in modulating cell-mediated immunity. While both the size and the shape of immune complexes are important determinants in Fc receptor-mediated signaling responsible for phagocytosis, degranulation, and, in some instances, autoimmune pathologies, their characterization remains extremely challenging due to their large size and structural heterogeneity. We use native mass spectrometry (MS) supplemented with limited charge reduction in the gas phase to determine the stoichiometry of immune complexes formed by a bivalent (homodimeric) antigen, a 163 kDa aminopeptidase P2 (APP2), and a monoclonal antibody (mAb) to APP2. The observed (APP2·mAb)n complexes populate a wide range of stoichiometries (n = 1-4) with the largest detected species exceeding 1 MDa, although the gas-phase dissociation products are also evident in the mass spectra. While frequently considering a nuisance that complicates interpretation of native MS data, limited dissociation provides an additional dimension for characterization of the immune complex quaternary structure. APP2/mAb associations with identical composition but slightly different elution times in size exclusion chromatography exhibit notable differences in their spontaneous fragmentation profiles. The latter indicates the presence of both extended linear and cyclized (APP2·mAb)n configurations. The unique ability of MS to distinguish between such isomeric structures will be invaluable for a variety of applications where the biological effects of immune complexes are determined by their ability to assemble Fc receptor clusters of certain density on cell surfaces, such as platelet activation by clustering the low-affinity receptors FcγRIIa on their surface.

Identification and characterization of the metal ion-dependent L-alanoyl-D-glutamate peptidase encoded by bacteriophage T5

Although bacteriophage T5 is known to have lytic proteins for cell wall hydrolysis and phage progeny escape, their activities are still unknown. This is the first report on the cloning, expression and biochemical characterization of a bacteriophage T5 lytic hydrolase. The endolysin-encoding lys gene of virulent coliphage T5 was cloned in Escherichia coli cells, and an electrophoretically homogeneous product of this gene was obtained with a high yield (78% of total activity). The protein purified was shown to be an L-alanoyl-D-glutamate peptidase. The enzyme demonstrated maximal activity in diluted buffers (25-50 mM) at pH 8.5. The enzyme was strongly inhibited by EDTA and BAPTA, and fully reactivated by calcium/manganese chlorides. It was found that, along with E. coli peptidoglycan, peptidase of bacteriophage T5 can lyse peptidoglycans of other Gram-negative microorganisms (Pectobacterium carotovorum, Pseudomonas putida, Proteus vulgaris, and Proteus mirabilis). This endolysin is the first example of an L-alanoyl-D-glutamate peptidase in a virulent phage infecting Gram-negative bacteria. There are, however, a great many sequences in databases that are highly similar to that of bacteriophage T5 hydrolase, indicating a wide distribution of endolytic L-alanoyl-D-glutamate peptidases. The article discusses how an enzyme with such substrate specificity could be fixed in the process of evolution.

Aminopeptidase P isozyme expression in human tissues and peripheral blood mononuclear cell fractions

Aminopeptidase P (APP) isoforms specifically remove the N-terminal amino acid from peptides that have a proline residue in the second position. The mRNA levels of three different isoforms, each coded by a different gene, were determined in 16 human tissues and in peripheral blood mononuclear cell (PBMC) fractions by RT-PCR. The cytosolic isoform, APP1, and the cell surface membrane-bound isoform, APP2, are expressed in all of the human tissues and PBMC fractions examined. The very high expression of APP2 mRNA in kidney compared to other tissues was confirmed by enzyme activity measurements. Among the PBMC fractions, APP2 expression is highest in resting CD8(+) T cells, but decreases in these cells following their activation with phytohemagglutinin; in contrast, expression of APP2 increases in CD4(+) T cells upon activation. The third isoform, APP3, is a hypothetical protein identified by nucleotide sequencing. A detailed analysis of its amino acid sequence confirmed that the protein is an aminopeptidase P-like enzyme with greater similarity to Escherichia coli APP than to either APP1 or APP2. Two splice variants of APP3 exist, one of which is predicted to have a mitochondrial localization (APP3m) while the other is cytosolic (APP3c). Both forms are variably expressed in all of the human tissues and PBMC fractions examined.

Human recombinant membrane-bound aminopeptidase P: production of a soluble form and characterization using novel, internally quenched fluorescent substrates

APP (aminopeptidase P) has the unique ability to cleave the N-terminal amino acid residue from peptides exhibiting a proline at P(1)'. Despite its putative involvement in the processing of bioactive peptides, among them the kinins, little is known about the physiological roles of both human forms of APP. The purpose of the present study is first to engineer and characterize a secreted form of hmAPP (human membrane-bound APP). Our biochemical analysis has shown that the expressed glycosylated protein is fully functional, and exhibits enzymic parameters similar to those described previously for mAPP purified from porcine or bovine lungs or expressed from a porcine clone. This soluble form of hmAPP cross-reacts with a polyclonal antiserum raised against a 469-amino-acid hmAPP fragment produced in Escherichia coli. Secondly, we synthesized three internally quenched fluorescent peptide substrates that exhibit a similar affinity for the enzyme than its natural substrates, the kinins, and a higher affinity compared with the tripeptide Arg-Pro-Pro used until now for the quantification of APP in biological samples. These new substrates represent a helpful analytical tool for rapid and reliable screening of patients susceptible to adverse reactions associated with angiotensin-converting enzyme inhibitors or novel vasopeptidase (mixed angiotensin-converting enzyme/neprilysin) inhibitors.

Rat and mouse membrane aminopeptidase P: structure analysis and tissue distribution

Membrane-bound aminopeptidase P (mAPP) is a highly specific exopeptidase that removes the N-terminal amino acid only from a peptide (three amino acids or longer) that has a prolyl residue in the second position. mAPP can inactivate bradykinin, a potent vasodilating and cardioprotective peptide hormone, by hydrolyzing the Arg(1)-Pro(2) bond. Studies on the rat have shown that the metabolism of bradykinin is an important physiological role of this enzyme. We report here the complete coding sequences for rat and mouse mAPP determined from mRNA isolated from lung tissue. Key structural features that determine post-translational processing and substrate recognition and catalysis were identified based on sequence homologies and the crystal structure of Escherichia coli aminopeptidase P complexed with Pro-Leu. The tissue-specific expression of mAPP was studied using the polymerase chain reaction. The mAPP gene is widely, but variably, expressed in adult tissues of the rat and mouse and in mouse embryos.

Disruption of six novel ORFs on the left arm of chromosome XII reveals one gene essential for vegetative growth of Saccharomyces cerevisiae

Deletion via PCR-mediated gene replacement, together with basic functional and bioinformatic analyses, have been performed on six novel open reading-frames (ORFs) on the left arm of chromosome XII of Saccharomyces cerevisiae(YLL033w, YLL032c, YLL031c, YLL030c, YLL029w and YLL028w). ORF deletion was realized using either a short-flanking homology (SFH) or a long-flanking homology (LFH) replacement cassette in the diploid strain FY1679. Sporulation and tetrad analysis showed that YLL031c is the only essential gene of the six. Microscopic examination of the non-growing spores carrying a disrupted copy of the essential gene showed that most of them were blocked after one or two cell divisions with heterogeneous bud size. The standard EUROFAN growth tests failed to reveal any obvious phenotype resulting from the deletion of each the five non-essential ORFs. Bioinformatic analysis revealed that YLL029w is probably an aminopeptidase for mitochondrial or nuclear protein processing and YLL028w may be involved in drug resistance in S. cerevisiae. Replacement cassettes, comprising the promoter and terminator regions of each of the six ORFs, were cloned into pUG7 and demonstrated to efficiently mediate gene replacement in an alternative diploid strain, W303. All the cognate gene clones were constructed, using either PCR products amplified from genomic DNA, or gap-repair. All clones and strains generated have been deposited in the EUROFAN genetic stock centre (EUROSCARF, Frankfurt).

Dipeptidyl aminopeptidase IV and aminopeptidase P, two proline specific enzymes from the cytoplasm of guinea-pig brain: their role in metabolism of peptides containing consecutive prolines

In this study the majority of dipeptidyl aminopeptidase IV and aminopeptidase P activities of guinea-pig brain are reported to reside in the cytoplasm. Both activities were purified and soluble dipeptidyl aminopeptidase IV was found to have a relative molecular mass of 194000 and to be comprised of two equal subunits of relative molecular mass 93000 while native soluble aminopeptidase P had a relative molecular mass of 140000. Both activities require proline or alanine in the penultimate position from the N-terminus. Dipeptidyl aminopeptidase IV removed the N-terminal dipeptide whereas aminopeptidase P removed only the N-terminal amino acid. Dipeptidyl aminopeptidase IV was inactive if proline was also present in the third position from the N-terminus whereas aminopeptidase P was unable to remove the N-terminal glycyl, pyroglutamyl or prolyl residues even though proline was present in the second position. Soluble dipeptidyl aminopeptidase IV was differentiated from the previously reported particulate form by its sensitivity to p-chloromercuribenzoate, N-ethyl maleimide and puromycin. The metabolism of Leu-Pro-Pro-Ser by guinea-pig cytoplasm was investigated in the presence of inhibitors to evaluate the contribution by dipeptidyl aminopeptidase IV and aminopeptidase P to the hydrolysis of a peptide containing two consecutive proline residues. The results indicated that either dipeptidyl aminopeptidase IV or prolyl oligopeptidase were required along with aminopeptidase P and prolidase to achieve complete hydrolysis of this tetrapeptide.

Cloning and functional expression of the cytoplasmic form of rat aminopeptidase P

A rat cytoplasmic aminopeptidase P was purified from liver cytosol with a procedure including an affinity elution step with 3 microM inositol 1,3,4-trisphosphate. Proteolytic fragments were generated, sequenced and the enzyme was cloned from a rat liver cDNA library. The structure shows high (87.8% and 95.5%, respectively) sequence identity at the nucleotide and amino acid levels with the previously described human putative cytoplasmic aminopeptidase P. The cloned rat enzyme was functionally expressed in Escherichia coli and also in COS-1 cells. Western blot analysis, using an antibody generated against the recombinant protein, and Northern blot hybridization showed ubiquitous expression of the protein in different tissues with the highest expression level in the testis.

Proline specific peptidases

Proline is unique among the 20 amino acids due to its cyclic structure. This specific conformation imposes many restrictions on the structural aspects of peptides and proteins and confers particular biological properties upon a wide range of physiologically important biomolecules. In order to adequately deal with such peptides, nature has developed a group of enzymes that recognise this residue specifically. These peptidases cover practically all situations where a proline residue might occur in a potential substrate. In this paper we endeavour to discuss these enzymes, particularly those responsible for peptide or protein hydrolysis at proline sites. We have detailed their discovery, biochemical attributes and substrate specificities and have provided information as to the methodology used to detect and manipulate their activities. We have also described the roles, or potential roles that these enzymes may play physiologically and the consequences of their dysfunction in varied disease states.

Cloning and tissue distribution of human membrane-bound aminopeptidase P

Complementary DNA clones encoding human membrane-bound aminopeptidase P (AmP) were isolated by reverse transcription-polymerase chain reaction (RT-PCR) of human kidney and lung poly (A)+ RNA. Comparison of the human AmP sequence to that of the pig shows significant evolutionary divergence with only 83% amino acid sequence identity between the two species. Northern hybridization analysis and RT-PCR suggests that the soluble and membrane-bound forms of human AmP are products of two distinct genes or, through alternative splicing, have different C-terminal sequences.

A novel mammalian high-molecular-weight aminopeptidase

Studies with the human lymphoma U937 cell line revealed the presence of two soluble aminopeptidase activities. Using specific antisera one of these was identified as the puromycin-specific aminopeptidase, while the other appeared to be a novel approximately 200-kDa activity. The kinetic properties of this high-molecular-weight aminopeptidase, referred to as Ap200, were similar to those of the puromycin-sensitive aminopeptidase, but showed quantitative differences. Ap200 is relatively insensitive to inhibition by both puromycin, K(i) = 27 microM, and bestatin, K(i) = 1.6 microM. Among the synthetic beta-naphthylamides, Ap200 is more specific for alanine-beta-naphthylamide compared to the puromycin-sensitive aminopeptidase. Similarly, this enzyme cleaves a more limited number of physiological peptides exhibiting a preference for the enkephalins. Ammonium sulfate, but not sodium chloride at the same ionic strength, was able to dissociate the high-molecular-weight aminopeptidase to a approximately 100-kDa active form. The high-molecular-weight aminopeptidase is found as a low abundant protein in a number of tissues including intestine, kidney, liver, lung, muscle, spleen, and testes, but could not be detected in adrenal, heart, or brain. Thus, it has a tissue distribution which differs from the puromycin-sensitive aminopeptidase.

Metabolism of Bradykinin by Peptidases in Health and Disease

This chapter provides an overview of the metabolism of bradykinin (BK) by peptidases in health and disease. The enzymatic breakdown of kinins affects the duration of their biological actions as the plasma half-life of intravenously injected BK is in the range of seconds. Kinins are cleaved in vitro and in vivo by enzymes that belong to families, such as zinc-metallopeptidases, astacin-like metallopeptidases, and catheptic enzymes. Vane noted the importance of the pulmonary circulation in the metabolism of vasoactive substances, such as BK as well as angiotensin 1 and 5- hydroxytryptamine. It is clear after decades of research that angiotensin 1-converting enzyme (ACE) on the vascular endothelial cell surface is the most important inactivator of blood-borne BK. BK may act primarily in an autocrine and paracrine fashion, establishing the importance of local regulation of its activity by enzymes on cell surfaces. Thus, the assortment of other enzymes that can inactivate BK is important in a variety of physiological and pathological situations. Most physiological systems have redundant pathways of metabolism so that the abolishment of one pathway is compensated for by the presence of others. This is demonstrated by the pharmacological inhibition of ACE in hypertension.

Inhibition and metal ion activation of pig kidney aminopeptidase P. Dependence on nature of substrate

Pig kidney aminopeptidase P (AP-P; EC 3.4.11.9) has been purified to homogeneity after its solubilisation from brush border membranes by phosphatidylinositol-specific phospholipase C. The effects of various activators and inhibitors of AP-P activity have been examined with a number of different substrates for the enzyme. The hydrolysis of bradykinin and ArgProPro is inhibited at Mn2+ concentrations above 10(-5) M, whereas the hydrolysis of other substrates (GlyProHyp, beta-casomorphin, substance P) is substantially activated, with 4-10 mM Mn2+ being optimal. The thiol reagent, p-chloromercuriphenylsulphonic acid, inhibits the hydrolysis of GlyProHyp but markedly activates the hydrolysis of bradykinin. A number of inhibitors of angiotensin converting enzyme (ACE; EC 3.4.15.1), previously reported to inhibit the hydrolysis of GlyProHyp, have no effect on the hydrolysis of bradykinin except in the presence of Mn2+. Differences were also observed in the degree of inhibition of GlyProHyp and bradykinin hydrolysis by EDTA and their reactivation by divalent cations. The hydrolysis of GlyProHyp follows Michaelis-Menten kinetics with a Km value of 2.7 mM. Bradykinin inhibits GlyProHyp hydrolysis with an I50 of 1.4 microM. The hydrolysis of bradykinin by AP-P reveals anomalous nonlinear kinetics indicative of negative cooperativity or the presence of more than one active site for this substrate. These results indicate that substrates for AP-P can be divided into 2 groups based on their responses to inhibitors and cation activators.

Chemical modification of porcine kidney aminopeptidase P indicates the involvement of two critical histidine residues

Aminopeptidase P (AP-P), purified to homogeneity from porcine kidney membranes, was completely inactivated by treatment with 0.2 mM diethylpyrocarbonate (DEP) at pH 7.0. Treatment of the modified enzyme with 20 mM hydroxylamine resulted in recovery of AP-P activity. The differential absorption of native and modified AP-P at 240 mm showed that DEP modified two histidyl residues per mol of AP-P. The substrates, bradykinin(1-5) and Gly-Pro-Hyp, and also the inhibitor, apstatin, could protect against DEP inactivation. These results suggest that histidine residues are critical for AP-P activity.

Characterization of protein glycosylation by mass spectrometry

Electrospray ionization, a natural interface with microbore and capillary high-pressure liquid chromatography, has become the method of choice for the reliable structural characterization of protein glycosylation by mass spectrometry at the picomole level. Its advantages include inherent sensitivity in the femtomole range, compatibility with collisional activation methods that both permit the detection and monitoring of structurally specific ions and enable the induction of glycopeptide fragmentation that facilitates determination of glycoform sequence and branching. Developments in high-performance electrospray mass spectrometry include sample introduction at nanoliter flow rates, tandem magnetic sector/orthogonal time-of-flight instruments, Fourier transform instruments, and new ion optical strategies, including ion traps. Although a sensitive and important complementary technique, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry suffers from matrix-dependent deposition of excess internal energies, which produce extensive metastable fragmentation and (photo)adduct formation. These metastable fragments may be focused into a mass spectrum by employing an ion mirror (reflectron) in time-of-flight instrumentation. In favorable cases, structural information may be obtained.