Physical and chemical characterization of pig kidney particulate aminopeptidase

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.

Synthesis and structure activity relationships of novel non-peptidic metallo-aminopeptidase inhibitors

Racemic derivatives of 3-amino-2-tetralone were synthesised and evaluated for their ability to inhibit metallo-aminopeptidase activities. New compounds substituted in position 2 by methyl ketone, substituted oximes or hydroxamic acids as well as heterocyclic derivatives were evaluated against representative members of zinc-dependent aminopeptidases: leucine aminopeptidase (E.C. 3.4.11.1), aminopeptidase-N (E.C. 3.4.11.2), Aeromonas proteolytica aminopeptidase (E.C. 3.4.11.10), and the aminopeptidase activity of leukotriene A(4) hydrolase (E.C. 3.3.2.6). Several compounds showed K(i) values in the low micromolar range against the 'one-zinc' aminopeptidases, while most of them were rather poor inhibitors of the 'two-zinc' enzymes. This interesting selectivity profile may guide the design of new, specific inhibitors of target mammalian aminopeptidases with one active site zinc.

3-Amino-2-hydroxy-propionaldehyde and 3-amino-1-hydroxy-propan-2-one derivatives: new classes of aminopeptidase inhibitors

3-Amino-2-hydroxy-propionaldehydes [H2NCH(R)CHOHCHO with R = H, i-Bu, CH2Ph] were designed as metallo-aminopeptidase inhibitors based on the metal active site chelation concept. These compounds were found to be micromolar inhibitors of aminopeptidase-M (AP-M, EC 3.4.11.2) with potencies similar to bestatin (Ki = 3.5 microM). Notably, compound 5a (R = H) is a selective inhibitor of AP-M (Ki = 7 microM) with respect to cytosolic leucine aminopeptidase (LAPc, EC 3.4.11.1) (Ki = 385 microM). However, due to their easy oligomerization, these compounds are low practical value. In contrast, the corresponding isomeric 3-amino-1-hydroxy-propan-2-one derivatives [H2NCH(R)COCH2OH with R = H, i-Bu, CH2Ph, i-Pr, CH2Biph] are well defined structures. These hydroxymethylketones also exhibit micromolar affinities on AP-M. Compound 6c (R = CH2Ph) was the most potent (Ki = 1 microM). Selectivity studies of 6a (R = H) and 6b (R=i-Bu) show a preference for AP-M. Compound 6a is moderately active on AP-M (Ki = 25 microM) and inactive on LAPc. This new class of inhibitors is proposed to bind as bidentates, analogous to hydroxamates.

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.

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.

Mechanism of mercurial inhibition of sodium-coupled alanine uptake in liver plasma membrane vesicles from Raja erinacea

In mammalian hepatocytes the L-alanine carrier contains a sulfhydryl group that is essential for its activity and is inhibited by mercurials. In hepatocytes of the evolutionarily primitive little skate (Raja erinacea), HgCl2 inhibits Na(+)-dependent alanine uptake and Na+/K(+)-ATPase and increase K+ permeability. To distinguish between direct effects of HgCl2 on the Na(+)-alanine cotransporter and indirect effects on membrane permeability, [3H]alanine transport was studied in plasma membrane vesicles. [3H]Alanine uptake was stimulated by an "out-to-in" Na+ but not K+ gradient and was saturable confirming the presence of Na(+)-alanine cotransport in liver plasma membranes from this species. Preincubation of the vesicles with HgCl2 for 5 min reduced initial rates of Na(+)-dependent but not Na(+)-independent alanine uptake in a dose-dependent manner (10-200 microM). In the presence of equal concentrations of NaCl or KCl inside and outside of the vesicles, 75 microM HgCl2 directly inhibited sodium-dependent alanine-[3H]alanine exchange, demonstrating that HgCl2 directly affected the alanine cotransporter. Inhibition of Na(+)-dependent alanine uptake by 30 microM HgCl2 was reversed by dithiothreitol (1 mM). HgCl2 (10-30 microM) also increased initial rates of 22Na uptake (at 5 sec), whereas 22Na uptake rates were decreased at HgCl2 concentrations greater than 50 microM. Higher concentrations of HgCl2 (100-200 microM) produced nonspecific effects on vesicle integrity. These studies indicate that HgCl2 inhibits Na(+)-dependent alanine uptake in skate hepatocytes by three different concentration-dependent mechanisms: direct interaction with the transporters, dissipation of the driving force (Na+ gradient), and loss of membrane integrity. Inactivation of the Na(+)-coupled alanine carrier by mercury in hepatocytes of this evolutionarily primitive vertebrate, as in mammals, suggests that the sulfhydryl groups on this transport protein are highly conserved.

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.

Cryptic Na+,K(+)-ATPase activity in rat liver canalicular plasma membranes: evidence for its basolateral origin

Controversy exists concerning the localization of the enzyme Na+,K(+)-ATPase to canalicular membranes in hepatocytes. Most studies find enzyme activity only at the basolateral plasma membrane domain of the hepatocyte. However, Na+,K(+)-ATPase activity has been detected recently in a canalicular membrane fraction prepared by Mg++ precipitation, suggesting that differences in membrane domain fluidity account for these discrepancies. To reinvestigate this question, we used free-flow electrophoresis to further purify canalicular liver plasma membranes originally separated by sucrose density centrifugation. With this technique, canalicular membranes devoid of Na+,K(+)-ATPase activity by routine assay were separated into six subfractions. More than 80% of the activities of canalicular marker enzymes was recovered in two subfractions closest to the anode, which were totally devoid of Na+,K(+)-ATPase activity. However, Na+,K(+)-ATPase activity could now be detected in the four other fractions that contained only small amounts of canalicular marker enzymes. The basolateral marker enzyme, glucagon-stimulated adenyl cyclase, comigrated with this cryptic Na+,K(+)-ATPase activity. Furthermore, addition of 6 mumol/L [12-(2-methoxyethoxy)-ethyl-8-(cis-2-n-octylcyclopropyl)-octanoate ], a membrane-fluidizing agent, to the original canalicular membrane preparation and to all subfractions did not stimulate or unmask latent Na+,K(+)-ATPase activity. Finally, when canalicular membranes isolated by Mg++ precipitation were subjected to free-flow electrophoresis, they could not be separated from the more positively charged Na+,K(+)-ATPase-containing fractions, probably because of alterations in surface charge. Together these findings suggest that Na+,K(+)-ATPase is a basolateral enzyme, that represents a small contaminant when present in canalicular liver plasma membranes and that methodological differences may account for previous discrepancies.

Characteristics of aminopeptidase activity from bovine brain microvessel endothelium

Blood-brain barrier (BBB) aminopeptidase activity was investigated using an in vitro model consisting of primary cultures of brain microvessel endothelium. Using two different substrates, both membrane-bound and soluble aminopeptidases were found to be associated with brain endothelium. That the enzyme activity was aminopeptidase activity was confirmed with the competitive inhibition of substrate degradation by typical aminopeptidase inhibitors puromycin and bestatin. The aminopeptidase activity was also competitively inhibited by enkephalin, met-enkephalin, and leu-enkephalin. Results from parallel experiments with cerebral gray matter and kidney confirm assay conditions. This report supports previous suggestions that aminopeptidases of the enzymatic BBB may play a role in regulating levels of circulating neuropeptides in the cerebrovasculature.

L-lysinethiol: a subnanomolar inhibitor of aminopeptidase B

L-Lysinethiol was found to be an extremely potent inhibitor of aminopeptidase B (AP-B) with a Ki = 9.1 X 10(-10) M. L-leucinethiol was also a potent inhibitor of AP-B (kj = 1.3 X 10(-7) M), while the D-isomer was much less effective (Ki = 9.8 X 10(-5) M). A thiol-zinc interaction at the active site is postulated for AP-B.

Spectral and kinetic studies of metal-substituted Aeromonas aminopeptidase: nonidentical, interacting metal-binding sites

Apoenzyme prepared by removal of the 2 mol of Zn2+/mol from Aeromonas aminopeptidase is inactive. Addition of Zn2+ reactivates it completely, and reconstitution with Co2+, Ni2+, or Cu2+ results in a 5.0-, 9.8-, and 10-fold more active enzyme than native aminopeptidase, respectively. Equilibrium dialysis and spectral titration experiments with Co2+ confirm the stoichiometry of 2 mol of metal/mol. The addition of only 1 mol of metal/mol completely restores activity characteristic of the particular metal. Interaction between the two sites, however, causes hyperactivation; thus, addition of 1 mol of Zn2+/mol subsequent to 1 mol of Co2+, Ni2+, or Cu2+ per mole increases activity 3.2-, 42-, or 59-fold, respectively. The cobalt absorption spectrum has a peak of 527 nm with a molar absorptivity of 53 M-1 cm-1 for 1 mol of cobalt/mol, which increases to 82 M-1 cm-1 for a second cobalt atom and is unchanged by further addition of Co2+. Circular dichroic (CD) and magnetic CD spectra indicate that the first Co2+ binding site is tetrahedral-like and that the second is octahedral-like. Stoichiometric quantities of 1-butylboronic acid, a transition-state analogue inhibitor of the enzyme [Baker, J. O., & Prescott, J. M. (1983) Biochemistry 22, 5322], profoundly affects absorption, CD, and MCD spectra, but n-valeramide, a substrate analogue inhibitor, has no effect. These findings suggest that the tetrahedral-like site is catalytic and the other octahedral-like site is regulatory or structural.

Role of intestinal brush border membrane aminopeptidase N in dipeptide transport

The kinetics of uptake of radioactive label from [U-14C]Gly, L-[4,5-3H]Leu and the dipeptide [14C]Gly-L-[4,5-3H]Leu by the brush border membrane vesicles of porcine small intestine have been studied. The effect of aminopeptidase N inhibitors and leucine-binding protein on accumulation rates has also been tested. Comparison of the kinetic parameters for uptake and hydrolysis of Gly-L-Leu makes it possible to conclude that the dipeptide transfer includes two conjugated steps, viz., hydrolysis catalysed by aminopeptidase N and transport of the resultant free amino acids by a specific carrier.

Rabbit intestinal aminopeptidase N. Purification and molecular properties

The detergent and protease forms of rabbit intestinal aminopeptidase N were purfied for chemical investigations and future specific immunological labeling of the enzyme in situ. The purification of the detergent form required a special technique called 'reverse immunoabsorbant chromatography'. The specific activity of the detergent form finally obtained was identical to that of the protease form. A significant charge micro heterogeneity persisted in the most purified preparations, due probably to a certain level of variability in the sugar moiety. The major proteolytic cleavage which occurred at the hydrophilic-hydrophobic junction of the detergent form during its conversion into the protease form was well defined. But additional splittings probably in C-terminal region of the molecules led to several protease forms differing by their size. The molecular weight assigned to the peptide liberated during the above conversion was overestimated due to preferential detergent binding to hydrophobic structures. The correct value, estimated by a new isotopic dilution method, was 3800 (36-38 residues) for the peptide originating from the rabbit enzyme. The real anchor plunging into the membrane core is possibly still shorter. Comparative N-terminal residue determinations in the detergent form, the protease form and the peptide difinitely confirmed that the enzyme is anchored to the bursh border membrane by its N-terminal region.

Intrinsic proteins of the intestinal microvillus membrane. Iodonaphthylazide labeling studies

Isolated brush border membranes of the intestinal epithelial cell were labeled with a hydrophobic photoactive compound [125U]iodonaphthylazide. High incorporation of the radioactive naphthylazide was noted for molecular weight bands of 99 000, 86 000, 65 000, 54 000 and 30 000. Minimal labeling occurred in the higher bands of 300 000, 135 000, 125 000 and 17 000. The iodonaphthylazide label was not removed by extensive papain digestion whereas chloramine T iodinated membranes released radioactivity under the same conditions. Neither enzymatic nor transport activities were inhibited by the presence of iodonaphthylazide or the irradiation process. On the basis of the presented data it is concluded that the iodonaphthylazide unspecifically labels those portions of membrane proteins which are inserted into the lipid bilayer matrix.

Structural and topological homology between porcine intestinal and renal brush border aminopeptidase

A method for the preparation of closed, right-side-out vesicles from the brush border membrane of the kidney proximal tubules is described. The aminopeptidase known to be bound to this membrane was investigated in order to compare its properties with those already reported for the intestinal enzyme. Both are composed of a hydrophilic, catalytically active part lying on the external side of the membrane and a short hydrophobic domain probably located in the N-terminal region of one of the subunits ensuring fixation to the lipid matrix. The enzyme were also found to be clinically similar. Moreover, a quantitative immunological technique showed that they contained 6 cross-reacting determinants, consistent with a very high degree of homology. Four of these determinants were accessible in the bound form of the enzymes in the region of the active site. The other two, probably related to the junction between the hydrophilic moiety and the hydrophobic anchor were completely masked in the bound form. The remainder (6 in the intestinal and 4 in the renal enzyme), were heterologous. The accessibility of two well determinants in this latter group was substantially reduced, perhaps by the proximity of the lipid and/or of other enzyme molecules.

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.

INtestinal sugar transport: studies with isolated plasma membranes

The luminal and the contraluminal plasma membranes of small intestinal epithelial cells were isolated separately and in the form of membrane vesciles. In the luminal membrane distinct transport systems exist for D-fructose and for D-glucose and glucalogs as evidenced by stereospecificity for the substrate, counter- and cotransport phenomena, and inhibitor specificity. D-Glucose is taken up against a concentration gradient when energy is supplied in the form of an electrochemical Na+ gradient between medium and intravesicular membrane space. The contraluminal membrane also possesses a glucose transport system. However, it is sensitive to different inhibitors than the glucose transport in the luminal membrane. D-Glucose uptake by the contraluminal membrane is not enhanced by Na+ gradients and can be characterized as "facilitated diffusion." The studies with the isolated plasma membranes allow a description of transcellular D-glucose transport in terms of the translocation events across single membranes.

Solubilization of brush borders of hamster small intestine and fractionation of some of the components

About 90% of the protein of hamster intestinal brush borders was solubilised in 0.25% (w/v) sodium dodecyl sulphate without total loss of biological activity. Detergent-polyacrylamide gel electrophoresis of the solubilised proteins separated 10-15 bands and partially resolved maltase, lactase, sucrase-maltase, trehalase and alkaline phosphatase activities. The disaccharidases, which were associated with the higher molecular weight proteins, were preferentially solubilised with 0.1%. (w/v) Triton X-100, butanol or papain, whereas Tris and NaI extracted only the lower molecular weight proteins, possible derived from the core filaments. Electrophoresis of brush border proteins metabolically labelled with [14-C] glucosamine suggested that many of the membrane-bound enzymes are glycoproteins. However, chromatography of a papain digest on Sephadex G-200 showed that the sucrase-maltase complex can be separated nearly free of carbohydrate without total loss of activity. The importance of characterizing membrane proteins solubilised by a number of techniques is discussed.

Dipeptide transport in isolated intestinal brush border membrane

Transport of glycyl-L-leucine into isolated brush border membrane vesicles was studied. On the basis of the following observations it was postulated that glycyl-L-leucine was transported intact by a specific dipeptide mechanism. (1) The differing time course and Na-+ stimulation of glycine, L-leucine and glycyl-L-leucine. (2) The failure of glycine and L-leucine to inhibit glycyl-L-leucine transport. (3) Initial presence of dipeptide within the vesicle. (4) Inhibition of glycyl-L-leucine uptake by other dipeptides. (5) The occurrence of accelerated amino acid uptake in the presence of the dipeptide.

The molecular weight and properties of a neutral metallo-endopeptidase from rabbit kidney brush border

1. Some properties of a brush-border neutral endopeptidase purified from rabbit kidney were investigated. The peptidase was assayed by its ability to hydrolyse [(125)I]iodoinsulin B chain. 2. The enzyme was found to be homogeneous when studied in the analytical ultracentrifuge and stained as a single glycoprotein band after electrophoresis in polyacrylamide gels. 3. The molecular weight was estimated by gel filtration in columns of Sephadex G-200, by polyacrylamide-gel electrophoresis in the presence of 2-mercapto-ethanol and sodium dodecyl sulphate and by sedimentation equilibrium in the ultra-centrifuge. The estimates fell within the range 87000-96000. The mean from two sedimentation equilibrium experiments was 93000, though this estimate may be slightly inflated because of the carbohydrate component of the enzyme. No evidence of dissociation into smaller subunits was obtained in the presence of thiol, sodium dodecyl sulphate or guanidine hydrochloride. 4. The endopeptidase was maximally active at pH6.0, although in phosphate buffer, which was strongly inhibitory, an optimum above pH8 was observed. 5. The enzyme was not affected by di-isopropyl phosphofluoridate nor by several thiol reagents. It was, however, strongly inhibited by many thiols and by EDTA and other chelating agents. 6. Although activity of the EDTA-treated enzyme could be partially restored by various bivalent metal ions, the optimum concentration for its reactivation by Zn(2+) was lower than that for other ions. This metal was detected in the enzyme preparation by atomic absorption spectrophotometry in an amount equivalent to approximately one atom/mol. 7. The enzyme is the only endopeptidase shown to be located in the kidney brush border and is the first mammalian example of a neutral Zn(2+)- activated endopeptidase to be characterized.