Pig kidney particulate aminopeptidase. A zinc metalloenzyme

The mercapturic acid pathway

The mercapturic acid pathway is a major route for the biotransformation of xenobiotic and endobiotic electrophilic compounds and their metabolites. Mercapturic acids (N-acetyl-l-cysteine S-conjugates) are formed by the sequential action of the glutathione transferases, γ-glutamyltransferases, dipeptidases, and cysteine S-conjugate N-acetyltransferase to yield glutathione S-conjugates, l-cysteinylglycine S-conjugates, l-cysteine S-conjugates, and mercapturic acids; these metabolites constitute a "mercapturomic" profile. Aminoacylases catalyze the hydrolysis of mercapturic acids to form cysteine S-conjugates. Several renal transport systems facilitate the urinary elimination of mercapturic acids; urinary mercapturic acids may serve as biomarkers for exposure to chemicals. Although mercapturic acid formation and elimination is a detoxication reaction, l-cysteine S-conjugates may undergo bioactivation by cysteine S-conjugate β-lyase. Moreover, some l-cysteine S-conjugates, particularly l-cysteinyl-leukotrienes, exert significant pathophysiological effects. Finally, some enzymes of the mercapturic acid pathway are described as the so-called "moonlighting proteins," catalytic proteins that exert multiple biochemical or biophysical functions apart from catalysis.

Inhibition, reactivation, and determination of metal ions in membrane metalloproteases of bacterial origin using high-performance liquid chromatography coupled on-line with inductively coupled plasma mass spectrometry

High-performance liquid chromatography coupled on-line with inductively coupled plasma mass spectrometry (HPLC-ICP-MS) was used for the characterization of metal ions in several metalloproteases of bacterial origin. The different components of the bacterial extracts were separated on a size-exclusion column. The eluent of the HPLC system was continuously transported to the ICP-MS system for rapid, reproducible, and sensitive analyses of trace elements in the metalloproteases. Two different membrane proteases from Bacillus cereus and Pseudomonas aeruginosa were characterized to be zinc metalloproteases using enzymological methods and HPLC-ICP-MS. The zinc content was determined to be three molecules of zinc per protein molecule for the B. cereus protease and one molecule of zinc per protein molecule for the P. aeruginosa protease. For another purified protease, a periplasmic alanyl aminopeptidase of P. aeruginosa, the lack of protein-bound metal ions could be clearly determined-a confirmation that this main aminopeptidase of P. aeruginosa belongs to the cysteine protease family. The presence of nonionic detergents can influence the distribution of trace elements during the HPLC separation. Therefore, the use of these substances should be avoided during enzyme purification for metal analyses or they should be exchanged later for zwitterionic and ionic detergents with more strongly dissociating properties.

T cell responses affected by aminopeptidase N (CD13)-mediated trimming of major histocompatibility complex class II-bound peptides

Endocytosed protein antigens are believed to be fragmented in what appears to be a balance between proteolysis and MHC-mediated epitope protection, and the resulting peptide-MHC complexes are transported to the surface of the antigen-presenting cells (APC) and presented to T cells. The events that lead to antigenic peptide generation and the compartments where antigen processing takes place remains somewhat enigmatic. The importance of intracellular antigen processing has been well established; however, it is unclear whether additional processing occurs at the APC surface. To follow antigen processing, we have identified a pair of T cell hybridomas that recognize a long vs. a short version of the same epitope. We have used prefixed APC and various protease inhibitors to demonstrate that the APC surface has a considerable potential for antigen processing. Specific antibodies further identified the exopeptidase Aminopeptidase N (APN, CD13) as one of the enzymes involved in the observed cell-surface antigen processing. The NH2-terminal end of the longer peptide could, even while bound to major histocompatibility complex (MHC) class II molecules, be digested by APN with dramatic consequences for T cell antigen recognition. This could be demonstrated both in cell-free systems using purified reagents and in cellular systems. Thus, MHC class II and APN may act in concert to generate the final T cell epitopes.

Purification and characterization of aminopeptidase M from muscle and mucosa of the pig intestine

The aim of this investigation was to purify aminopeptidase M (APM) from the muscle layer of the small intestine, to compare it with APM of the mucosa and kidney, and to examine the degradation of gastrointestinal neural and hormonal peptides by muscle APM. APM was purified from the muscle and mucosa of the pig small intestine by DEAE-Sepharose and immuno-affinity chromatography. The specific activity of APM from muscle, mucosa, and kidney was 3900, 3000, and 3800 nmol/min per mg protein, respectively (substrate [Leu5]enkephalin). Muscle and mucosa APM contained four protein bands with apparent molecular weights of 150, 110, 73, and 52 kDa. Kidney APM contained three protein bands of 150, 110, and 56 kDa. The 150, 110, and 52/56 kDa bands cross-rected with an APM antiserum. APM from each tissue degraded [Leu5]enkephalin and [Met5]enkephalin, but not cholecystokinin-8, gastrin releasing peptide-10, somatostatin-14, substance P, and vasoactive intestinal peptide. The enzymes were identically inhibited by APM antiserum, amastatin, bestatin, actinonin, and 1, 10 phenanthroline. Non-mucosal APM may degrade enkephalins and terminate their biological actions.

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.

Identification of the thyrotropin-releasing-hormone-degrading ectoenzyme as a metallopeptidase

A time-dependent inhibition of the thyrotropin-releasing-hormone (TRH)-degrading ectoenzyme (EC 3.4.19.-) from rat brain by the metal-complexing agents imidazole, NaCN, EDTA and 1,10-phenanthroline could be demonstrated. In contrast, the inhibition by the non-chelating analogues 4,7- and 1,7-phenanthroline was not time-dependent. At a concentration of 100 microM EDTA the enzymic activity decreased by 50% only after pretreatment for 6 h. It could be restored by addition of Zn2+, Ni2+ and Co2+ but not by other transition metal ions and also not by Ca2+ and Mg2+. Without pretreatment, the enzyme was activated by Co2+ and inhibited by Cu2+, Cd2+ and Hg2+ in a time-dependent manner, but remained unaffected by Ni2+ and Mn2+, as well as by Ca2+ and Mg2+. Compatible with the His-Glu-Xaa-Xaa-His consensus sequence of most zinc-containing metallopeptidases, chemical modification studies with carbodiimide revealed the presence of an essential acidic amino acid residue, probably located at the active site of the enzyme. The catalytically active metal ion could be exchanged for 65Zn and the enzyme could be effectively inhibited by L-pyroglutamyl hydroxamic acid, the chelating derivative of the TRH cleavage product pyroglutamic acid. The TRH-degrading ectoenzyme thus classifies as a member of the zinc-dependent metallopeptidase family.

Immunochemical localization of aminopeptidase M in the alimentary tract of the guinea pig and rat

Aminopeptidase M (APM) was localized in the kidney and alimentary tract of guinea pigs and rats by indirect immunohistochemistry. APM was detected in the brush border of the epithelium of the proximal convoluted tubule of the kidney and of the small intestine, and it was localized to cells scattered throughout lymphoid tissue in the small intestine and colon. The gastric mucosa was unstained. APM was localized to numerous fibers supplying the myenteric plexus of the stomach, small intestine, and colon. The submucosal plexus was sparsely supplied by immunoreactive fibers. Occasional cell bodies were stained in the myenteric plexus. Staining was abolished by preabsorption of the primary antibody with APM. APM was characterized in membranes prepared from the muscle and mucosa of the guinea pig and rat stomach, small intestine, and colon by Western blotting. The major immunoreactive protein identified in membranes prepared from all tissues had an apparent molecular weight of 140, corresponding to the monomer of APM. In the brush border APM has a digestive function, whereas in neural tissue it may degrade and inactivate neuropeptides.

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.

Inhibition of aminopeptidase M by alkyl D-cysteinates

Ethyl D-cysteinate is a potent competitive inhibitor (Ki = 3.5 x 10(-7) M) of aminopeptidase M. D-cysteine and ethyl L-cysteinate inhibit more than two orders of magnitude less effectively. Inhibition studies on several n-alkyl esters of D-cysteine reveal an optimum at the n-butyl ester (Ki = 1.8 x 10(-7) M). The results are consistent with the hypothesis that the thiol group coordinates to Zn+2 at the active site and the alkyl group occupies the hydrophobic binding site for the side chain of the amino-terminal residue of substrates. Cytosolic leucine aminopeptidase is not significantly inhibited by ethyl D-cysteinate.

Complete amino acid sequence of human intestinal aminopeptidase N as deduced from cloned cDNA

The complete primary structure (967 amino acids) of an intestinal human aminopeptidase N (EC 3.4.11.2) was deduced from the sequence of a cDNA clone. Aminopeptidase N is anchored to the microvillar membrane via an uncleaved signal for membrane insertion. A domain constituting amino acid 250-555 positioned within the catalytic domain shows very clear homology to E. coli aminopeptidase N and contains Zn2+ ligands. Therefore these residues are part of the active site. However, no homology of the anchor/junctional peptide domain is found suggesting that the juxta- and intra-membraneous parts of the molecule have been added/preserved during development. It is speculated that this part carries the apical address.

Proteins of the kidney microvillar membrane. Purification and properties of carboxypeptidase P from pig kidneys

Carboxypeptidase P has been purified by immunoaffinity chromatography from pig kidneys. A single-step assay with Z-Pro-Met (where Z represents benzyloxycarbonyl) as substrate was used, methionine being determined by using L-amino acid oxidase and horseradish peroxidase. The enzyme constitutes about 1.5% of the kidney microvillar proteins. Triton X-100-solubilized and papain-released forms of the enzyme were isolated. The former had an apparent subunit Mr of 135 000, and the latter form contained two polypeptide chains of Mr 128 000 and 95 000. The undenatured forms were dimeric proteins. In common with other microvillar hydrolases, carboxypeptidase P was a glycoprotein and each subunit contained one Zn atom. MnCl2 (1 mM) in the assay was necessary for maximum activity; in its absence, 0.5 mM-ZnSO4 produced a limited activation, but was inhibitory at higher concentrations. The Km for Z-Pro-Met, in the presence of MnCl2, was 4.1 mM, and the kcat. for freshly prepared enzyme was 1230 min-1. The enzyme lost activity during storage at -20 degrees C. In a limited survey of peptides, hydrolysis was observed only with substrates containing a proline, alanine or glycine residue in the P1 position, and these included angiotensins II and III. The best substrate in this series was Val-Ala-Ala-Phe.

Microsomal methionine aminopeptidase: properties of the detergent-solubilized enzyme

A methionine aminopeptidase (MAP) found in rat liver microsomes behaves as membrane-bound enzyme. Triton-solubilized MAP when chromatographed on DEAE-cellulose columns was separated from other microsomal arylamidases. The enzyme hydrolyzes N-terminal methionine from methionyl-lysyl-bradykinin (Met-Lys-Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg) being then characterized as a typical aminopeptidase. It also shows preferential arylamidase activity upon Met-2-naphthylamide. MAP was activated by 2-mercaptoethanol and inhibited by p-hydroxymercuribenzoate. Contrarily to other well characterized aminopeptidases, MAP was not affected by EDTA, puromycin or bestatin. Altogether these data suggest that MAP is a unique microsomal enzyme distinct from other previously described aminopeptidases. It could be involved in the removal of methionine from nascent peptides during protein synthesis.

Aminopeptidase A is angiotensinase A. II. Biochemical studies on aminopeptidase A and M in rat kidney homogenate

Biochemical fluorometric methods were used to investigate aminopeptidase A (APA; E.C.3.4.11.7) in the rat kidney homogenate and glomeruli and to compare it with aminopeptidase M (APM; E.C.3.4.11.2). It is shown that APA is a calcium-ion-dependent enzyme, while APM is not. To clarify the functional importance of APA and APM in the kidney, their activities were measured under the influence of angiotensins. Fluorimetric measurements in renal homogenate (with 2-naphthylamide derivatives as substrates), which represents mixed-enzyme tissue preparations containing a variety of peptidases besides APA and APM, showed a Km of 0.13 mM for APA and competitive inhibition of ANG II (K1 = 0.015 mM), and a Km of 0.24 for APM and competitive inhibition by ANG III (K1 = 0.003 mM). The remaining two angiotensins showed non-competitive inhibition of APA (ANG I, III) and APM (ANG I, II) in this preparation. For comparison purposes, fluorometric measurements were performed in microdissected glomeruli which contain only APA. A Km of 0.23 mM for the APA and a competitive inhibition of APA by ANG I and II were determined. Thus it was possible to show biochemically that APA is equivalent to angiotensinase A and that both APA and APM participate in angiotensin degradation in the kidney. APA initiating the breakdown of ANG I and II, and APM possibly continuing it in sequential fashion.

Localization of a highly antigenic human fibroblast surface glycoprotein (FSG) on fibroblasts and on epithelia involved in secretion or resorption

The cellular distribution of a highly antigenic fibroblast glycoprotein, identified previously as aminopeptidase M, was studied in vitro by immunofluorescence and immuno-electron microscopy. The antigen was found to be localized at the surface of live and fixed fibroblasts in suspension and in cell layer; clustering or patching on live cells could be observed. Immunofluorescence after permeabilization of the cells with acetone showed distribution of the antigen in cytoplasmic granules. The tissue distribution of this antigen was examined by immunofluorescence on frozen sections of various human organs. In addition to fibroblasts, renal tubules, liver, pancreas and gut were found to react selectively with the antibody preparation. Moreover, a specific localization of the reaction product was observed. In the kidney the epithelium and brush border of the proximal convoluted tubules were stained. In the liver, the bile canaliculi reacted; in the pancreas the acinar cells and in the gut the brush border of the mucosal cells of the villi and crypts were stained. The same cells did not stain with an antiserum prepared in a similar fashion against another fibroblast component, or with preimmune IgG. In most sections of the selectively stained tissues, a predominant localization at the apical pole of the cells was observed. The localization of the antigen in different tissues corresponds with the distribution of aminopeptidase M, thereby confirming its identity and the antigenic cross-reaction between the fibroblast and tissue enzyme.

On the subunit structure of particulate aminopeptidase from pig kidney

Solubilization of particulate aminopeptidase (EC 3.4.11.2) from pig kidney with Triton X-100 yields an aggregate (mol. wt. approx. 10(6)) that decomposes into "free" aminopeptidase (mol. wt. 280 000) either upon autolysis at pH 5 or after exposure to trypsin. Both procedures yield free enzymes that are identical with respect to electrophoretic mobility, enzymatic activity and zinc content. After dissociation, the enzyme resulting from autolysis yields a single subunit of 140 000 molecular weight while the trypsin-treated enzyme produces three fragments (140 000, 95 000 and 48 000 mol. wt.). As the aggregate is formed by subunits 10 000 daltons heavier than those of the free enzyme, the existence of a hydrophobic portion anchoring the enzyme to the membrane might be postulated. Reactivation experiments carried out on the three purified fragments of urea-denatured aminopeptidase show that the 140 000 molecular weight subunit is the only one able to yield an active enzyme (after spontaneous dimerization). It can be concluded that the smaller fragments are artefacts resulting from trypsin degradation during purification.

Enzyme histochemistry as a link between biochemistry and morphology

The presented paper describes the role of enzyme histochemistry in cell biological investigations. In the first chapter a general discussion has been given about enzyme histochemistry as a connecting link between biochemistry and morphology. The methods available for determination of enzymes in a particular cell or cell compartment have been reviewed. In this respect the characteristics of enzyme histochemistry have been discussed. Furthermore, attention has been paid to the possibilities and limitations of enzyme histochemistry. In chapter two a comparison has been made between histochemically judged and biochemically determined enzyme activities. Some fundamental differences between the biochemical and the histochemical approach in cell biological investigations are dealt with. To correlate histochemically and biochemically determined enzyme activities, a description has been given of the application of histochemical methods on isolated fractions and sucrose-ficoll gradients of these fractions. Several experimental results are described concerning the question whether a relation exists between histochemically and biochemically determined activities of respectively alkaline phosphatase, glucose-6-phosphatase, 5'-nucleotidase and 3ss-hydroxysteroid dehydrogenase. From these results the conclusion could be drawn that in general a good correlation exists between histochemically judged activity per volume (area X thickness) and biochemically determined activity per gram tissue. In chapter three the role of enzymes as markers of cellular particles and as parameters of metabolic pathways is described. Histochemical methods are available for most marker enzymes. Only activities of key enzymes can be regarded as parameters of metabolic pathways. The distribution in sucrose-ficoll gradients of enzymes, regarded as markers of mitochondria, lysosomes, endoplasmic reticulum and plasma membranes has been given. The changes occur ing under different experimental conditions for a number of marker enzymes in rat liver are described. Attention has been given to the contibution of enzyme histochemistry in the study of the heterogeneity of mitochondria, the dual localization of some (lysosomal) enzymes, the complexity of the microsomal fraction, the function of the Golgi apparatus and the heterogeneity and function of plasma membranes. Based on these results and on literature findings the possible role of some marker enzymes in cell metabolism has been discussed. In chapter four problems coherent with species and sex differences in enzyme activities are described. The interpretation of histochemical and biochemical results in view of these differences is discussed. Enzymes characteristic for a given cell type -3ss-hydroxysteroid dehydrogenase in steroid producing cells, ATP-ase in liver plasma membrane surrounding the bile canaliculi - do show less variations between species and sexes than enzymes not directly involved in specialized functions...