Immunoelectrophoretic analysis of vascular, membrane-bound angiotensin I converting enzyme, aminopeptidase M, and dipeptidyl(amino)peptidase IV

Soluble CD26: From Suggested Biomarker for Cancer Diagnosis to Plausible Marker for Dynamic Monitoring of Immunotherapy

Soluble CD26 (sCD26), a glycoprotein with dipeptidyl peptidase (DPP4) enzymatic activity, can contribute to early diagnosis of colorectal cancer and advanced adenomas and has been studied, including for prognostic purposes, across various other types of cancer and disease. The latest research in this field has confirmed that most, though not all, serum/plasma sCD26 is related to inflammation. The shedding and/or secretion of sCD26 from different immune cells are being investigated, and blood DPP4 activity levels do not correlate very strongly with protein titers. Some of the main substrates of this enzyme are key chemokines involved in immune cell migration, and both soluble and cell-surface CD26 can bind adenosine deaminase (ADA), an enzyme involved in the metabolism of immunosuppressor extracellular adenosine. Of note, there are T cells enriched in CD26 expression and, in mice tumor models, tumor infiltrating lymphocytes exhibited heightened percentages of CD26+ correlating with tumor regression. We employed sCD26 as a biomarker in the follow-up after curative resection of colorectal cancer for the early detection of tumor recurrence. Changes after treatment with different biological disease-modifying antirheumatic drugs, including Ig-CTLA4, were also observed in rheumatoid arthritis. Serum soluble CD26/DPP4 titer variation has recently been proposed as a potential prognostic biomarker after a phase I trial in cancer immunotherapy with a humanized anti-CD26 antibody. We propose that dynamic monitoring of sCD26/DPP4 changes, in addition to well-known inflammatory biomarkers such as CRP already in use as informative for immune checkpoint immunotherapy, may indicate resistance or response during the successive steps of the treatment. As tumor cells expressing CD26 can also produce sCD26, the possibility of sorting immune- from non-immune-system-originated sCD26 is discussed.

Potential of soluble CD26 as a serum marker for colorectal cancer detection

Colorectal cancer is characterized by a low survival rate even though the basis for colon cancer development, which involves the evolution of adenomas to carcinoma, is known. Moreover, the mortality rates continue to rise in economically transitioning countries although there is the opportunity to intervene in the natural history of the adenoma–cancer sequence through risk factors, screening, and treatment. Screening in particular accounted for most of the decline in colorectal cancer mortality achieved in the USA during the period 1975-2000. Patients show a better prognosis when the neoplasm is diagnosed early. Among the variety of screening strategies, the methods range from invasive and costly procedures such as colonoscopy to more low-cost and non-invasive tests such as the fecal occult blood test (guaiac and immunochemical). As a non-invasive biological serum marker would be of great benefit because of the performance of the test, several biomarkers, including cytologic assays, DNA and mRNA, and soluble proteins, have been studied. We found that the soluble CD26 (sCD26) concentration is diminished in serum of colorectal cancer patients compared to healthy donors, suggesting the potential utility of a sCD26 immunochemical detection test for early diagnosis. sCD26 originates from plasma membrane CD26 lacking its transmembrane and cytoplasmic domains. Some 90%–95% of sCD26 has been associated with serum dipeptidyl peptidase IV (DPP-IV) activity. DPP-IV, assigned to the CD26 cluster, is a pleiotropic enzyme expressed mainly on epithelial cells and lymphocytes. Our studies intended to validate this test for population screening to detect colorectal cancer and advanced adenomas are reviewed here.

On the origin of serum CD26 and its altered concentration in cancer patients

Dipeptidyl peptidase IV (DPP-IV), assigned to the CD26 cluster, is expressed on epithelial cells and lymphocytes and is a multifunctional or pleiotropic protein. Its peptidase activity causes degradation of many biologically active peptides, e.g. some incretins secreted by the enteroendocrine system. DPP-IV has, therefore, become a novel therapeutic target for inhibitors that extend endogenously produced insulin half-life in diabetics, and several reviews have appeared in recent months concerning the clinical significance of CD26/DPP-IV. Biological fluids contain relatively high levels of soluble CD26 (sCD26). The physiological role of sCD26 and its relation, if any, to CD26 functions, remain poorly understood because whether the process for CD26 secretion and/or shedding from cell membranes is regulated or not is not known. Liver epithelium and lymphocytes are often cited as the most likely source of sCD26. It is important to establish which tissue or organ is the protein source as well as the circumstances that can provoke an abnormal presence/absence or altered levels in many diseases including cancer, so that sCD26 can be validated as a clinical marker or a therapeutic target. For example, we have previously reported low levels of sCD26 in the blood of colorectal cancer patients, which indicated the potential usefulness of the protein as a biomarker for this cancer in early diagnosis, monitoring and prognosis. Through this review, we envisage a role for sCD26 and the alteration of normal peptidase capacity (in clipping enteroendocrine or other peptides) in the complex crosstalk between the lymphoid lineage and, at least, some malignant tumours.

Ontogenic and adult whole body distribution of aminopeptidase N in rat investigated by in vitro autoradiography

Aminopeptidase N (APN), which is widely distributed in mammalian tissues, is able to cleave numerous regulatory peptides. The selective inhibitor of APN, [(125)I] RB129, has been used to study the distribution of this exopeptidase during rat prenatal development and adult life by in vitro whole-body autoradiography. In the central nervous system, APN shows a weak labeling compared to the major part of the non-nervous tissues in the embryo and in the adult. APN is progressively expressed in kidney, intestine, heart, lung, sensory organs, eye, and thymus. In organs such as the liver, the cartilages and the bones, altered levels of APN expression are observed during the development, or in the embryo compared to the adult, suggesting a role of APN during the liver haematopoiesis and bone growth. At this time, all the physiological functions of APN are still incompletely known, however its developmental pattern of expression strongly suggests a function of modulation of this enzyme during the development, next in physiological and/or pathological situations in adult. In this way, APN could represent a new therapeutic target in pathological processes, such as tumoral proliferation and/or angiogenesis associated with cancer development, where an increase in the level of this enzyme has been observed.

Dipeptidyl-peptidase IV from bench to bedside: an update on structural properties, functions, and clinical aspects of the enzyme DPP IV

Dipeptidyl-peptidase IV/CD26 (DPP IV) is a cell-surface protease belonging to the prolyloligopeptidase family. It selectively removes the N-terminal dipeptide from peptides with proline or alanine in the second position. Apart from its catalytic activity, it interacts with several proteins, for instance, adenosine deaminase, the HIV gp120 protein, fibronectin, collagen, the chemokine receptor CXCR4, and the tyrosine phosphatase CD45. DPP IV is expressed on a specific set of T lymphocytes, where it is up-regulated after activation. It is also expressed in a variety of tissues, primarily on endothelial and epithelial cells. A soluble form is present in plasma and other body fluids. DPP IV has been proposed as a diagnostic or prognostic marker for various tumors, hematological malignancies, immunological, inflammatory, psychoneuroendocrine disorders, and viral infections. DPP IV truncates many bioactive peptides of medical importance. It plays a role in glucose homeostasis through proteolytic inactivation of the incretins. DPP IV inhibitors improve glucose tolerance and pancreatic islet cell function in animal models of type 2 diabetes and in diabetic patients. The role of DPP IV/ CD26 within the immune system is a combination of its exopeptidase activity and its interactions with different molecules. This enables DPP IV/CD26 to serve as a co-stimulatory molecule to influence T cell activity and to modulate chemotaxis. DPP IV is also implicated in HIV-1 entry, malignant transformation, and tumor invasion.

Analysis of responses to angiotensin IV in the pulmonary vascular bed of the cat

Responses to angiotensin IV were investigated and compared with responses to angiotensin II in the pulmonary vascular bed of the intact-chest cat under constant flow conditions. Under low resting tone conditions, intralobar injections of angiotensin IV caused dose-related increases in lobar arterial pressure. Angiotensin II also increased lobar arterial pressure and was 100 fold more potent than angiotensin IV. Dose-response curves for both peptides were parallel, and the time-to-peak increase in lobar arterial pressure in response to angiotensin IV and angiotensin II was similar whereas the duration of the response to angiotensin IV was significantly shorter. Following administration of the angiotensin receptor subtype 1 (AT1) antagonist, DuP 532 (2-n-butyl-4-chloro-5-hydroxymethyl-1-[(2'-(1H-tetrazol-5-yl)biphe nyl-4- yl)-methyl]imidazole), responses to angiotensin IV and angiotensin II were reduced in a similar manner, whereas pulmonary pressor responses to serotonin were not altered. In contrast to the inhibitory effects of the angiotensin AT1 receptor antagonist, administration of PD 123,319 ((S)-1-[[4-(dimethylamino)-3-methyl-phenyl]methyl-5-(diphenylacetyl++ +)- 4,5,6,7-tetrahydro-1H-imidazol[4,5-c]pyridine-6-carboxylic acid), an angiotensin AT2 receptor antagonist, did not change responses to angiotensin II and angiotensin IV. The results of the present study demonstrate that angiotensin IV has significant vasoconstrictor activity in the pulmonary vascular bed, and suggest that pressor responses to angiotensin IV are mediated by the activation of angiotensin AT1 receptors. These data indicate that angiotensin IV is 100-fold less potent than angiotensin II and suggest that the hexapeptide may have a lower apparent affinity for the angiotensin AT1 receptor than does angiotensin II in the pulmonary vascular bed of the cat.

Amastatin interferes with the antagonist properties of MEN 10,208 in the rabbit pulmonary artery but not in the hamster trachea

The tachykinin peptide agonists neurokinin A and [beta Ala8]neurokinin A-(4-10), and the NK2 tachykinin receptor-selective antagonists MEN 10,208, MEN 10,207, MEN 10,282, MEN 10,376 and R396 were assayed in the isolated rabbit pulmonary artery and isolated hamster trachea in the absence and in the presence of the aminopeptidase inhibitor amastatin (10 microM for 30 min). The affinity of MEN 10,208 in the rabbit pulmonary artery was markedly reduced in the presence of amastatin (pKB values from 7.47 to 5.94), while it was unchanged in the hamster trachea. Neither neurokinin A, [beta Ala8]neurokinin A-(4-10), nor the other antagonists were affected by pretreatment with amastatin in either bioassay. The results obtained in the rabbit pulmonary artery show that MEN 10,208 is degraded by local amastatin-sensitive enzymes (possibly aminopeptidase M), which may convert the linear octapeptide MEN 10,208 to the heptapeptide MEN 10,207 by removing the N-terminal Thr from the amino acid sequence of MEN 10,208. The present results are discussed in relation to a previously reported heterogeneity between NK2 receptors of the rabbit and bovine species, and show amastatin to be a new tool for the classification of tachykinin receptors with peptide ligands.

Alteration of 5'-nucleotidase properties in rat adipose and liver plasma membranes after 1 Gy whole-body gamma-irradiation

5'-Nucleotidase activities in rat adipose and liver plasma membranes (PM) were examined at various times after 1 Gy whole-body gamma-irradiation. Changes in the enzyme kinetic parameters were recorded 6 and 14 weeks after exposure: Km decreased while Vmax increased. The enzyme in control PM showed temperature-dependent Arrhenius plots with a lower activation energy at temperatures above a break point at approximately 28 degrees C, whereas 5'-nucleotidase in irradiated PM showed a constant activation energy between 10 and 40 degrees C which was reasonably close to the value observed controls at temperatures below the break point. Furthermore, the interaction of [3H]-AMP with the 5'-nucleotidase catalytic site was studied on adipose PM using a rapid filtration technique. gamma-Irradiation was associated with an increased number of specific AMP-binding sites and decreased ligand affinity. Comparison of the presented data with previous work (Yegutkin et al. 1993a,b) indicate significant functional and structural modification of rat adipose and liver PM following 1 Gy gamma-irradiation and reveals interrelation of these changes.

Cultured human synovial fibroblasts rapidly metabolize kinins and neuropeptides

Kinins and substance P have been implicated in the pathogenesis of inflammatory arthritis by virtue of their abilities to induce vasodilation, edema, and pain. The relative biological potencies of these peptides in vivo would depend at least in part upon their rates of catabolism in the joint. We hypothesized that human synovial lining cells may regulate intraarticular levels of kinins and neuropeptides via degradation by cell surface-associated peptidases. We exposed intact human synovial fibroblasts to kinins and substance P, in the presence or absence of specific peptidase inhibitors, and measured the amount of intact substrate remaining and degradation product(s) generated over time. Aminopeptidase M (AmM; EC 3.4.11.2), neutral endopeptidase-24.11 (NEP-24.11; EC 3.4.24.11), and dipeptidyl(amino)peptidase IV (DAP IV; EC 3.4.14.5) were identified on the cell surface of synovial cells. Bradykinin degradation was due entirely to NEP-24.11 (1.39 +/- 0.29 nmol/min per well). Lysylbradykinin was also degraded by NEP-24.11 (0.80 +/- 0.19 nmol/min per well); however, in the presence of phosphoramidon, AmM-mediated conversion to bradykinin (3.74 +/- 0.46 nmol/min per well) could be demonstrated. The combined actions of NEP-24.11 (0.93 +/- 0.15 nmol/min per well) and DAP IV (0.84 +/- 0.18 nmol/min per well) were responsible for the degradation of substance P. AmM (2.44 +/- 0.33 nmol/min per well) and NEP-24.11 (1.30 +/- 0.45 nmol/min per well) were responsible for the degradation of the opioid peptide, [Leu5]enkephalin. The identity of each of the three peptidases was confirmed via synthetic substrate hydrolysis, inhibition profile, and immunological identification. The profiles of peptidase enzymes identified in cells derived from rheumatoid and osteoarthritic joints were identical. These data demonstrate the human synovial fibroblast to be a rich source of three specific peptidases and suggest that it may play a prominent role in regulating peptide levels in the joint.

Possible implication of peptidase activity in different potency of angiotensins II and III for displacing [125I]angiotensin II binding in pig aorta

A single class of [125I]angiotensin II ([125I]AII) binding sites was found in porcine aortic smooth muscle membranes. Des-Asp1-AII (AIII) and des-Asp1-[Ile8]AII were 20 times less potent than AII or [Sar1,Ile8]AII to displace [125I]AII binding. In contrast, AII and AIII equipotently induced an increase in cytosolic free Ca2+ concentration in cultured porcine aortic smooth muscle cells. Des-Asp1-[Ile8]AII and [Sar1,Ile8]AII equipotently inhibited the increase induced by either AII or AIII. In order to explain this discrepancy, we studied [125I]AIII binding. In the presence of amastatin or a potent inhibitor of aminopeptidase M, [125I]AII binding remained stable for 90 min, but [125I]AIII binding decreased gradually after the peak at 30 min. The decrease was completely blocked by the presence of amastatin and phenylmethylsulfonylfluoride. Consequently, AIII was as potent as AII to displace [125I]AII binding in the presence of these two protease inhibitors. These results suggest that the lower potency of AIII to displace [125I]AII binding is related primarily to AIII-specific degradation by proteases.

Use of aminopeptidase M as a hypotensive agent in spontaneously hypertensive rats

The present investigation determined that a commercially available aminopeptidase M (AmM, Sigma Chemical) can be utilized to lower blood pressure in normotensive and hypertensive rats. In vitro analyses indicated that the predominant peptidase present in this preparation was AmM; however, it also contained some aminopeptidase A (AmA) and less DAP IV. Although no DAP IV-mediated metabolism of angiotensin II (AII) or angiotensin III (AIII) was measured, both AmM and AmA metabolized AII and AIII. Upon further examination, it appeared that AII could be converted to AIII by either AmM or AmA; however, Arg was cleaved from the N-Terminal of AIII predominantly by AmM. The aminopeptidase inhibitors actinonin (AC), amastatin (AM), and bestatin (BE) effectively blocked the AmM-induced hydrolysis of the Asp-Arg bond of AII, and the Arg-Val bond of AIII. The activity of AmA was inhibited by AM but was relatively resistant to inhibition by AC and BE. Next, exogenous aminopeptidase replacement was employed in the anesthetized spontaneously hypertensive rat (SHR) in an attempt to temporarily correct a hypothesized brain deficiency of receptor-associated peptidases and lower blood pressure. Third-ventricle infusion of AmM produced significant drops in blood pressure and heart rate in both SHRs and Wistar-Kyoto normotensive controls. Pretreatment with AC or BE was particularly effective at interfering with the subsequent AmM-induced hypotensive effect, while AM was less effective. The central mechanisms underlying these effects are in need of further investigation; however, they are at least partially dependent upon the brain angiotensin system.

Differential processing of substance P and neurokinin A by plasma dipeptidyl(amino)peptidase IV, aminopeptidase M and angiotensin converting enzyme

In addition to plasma metabolism of substance P (SP) by angiotensin converting enzyme (ACE; EC 3.4.15.1) (less than 1.0 nmol/min/ml), the majority of SP hydrolysis by rat and human plasma was due to dipeptidyl(amino)peptidase IV (DAP IV; EC 3.4.14.5) (3.15-5.91 nmol/min/ml), which sequentially converted SP to SP(3-11) and SP(5-11). In turn, the SP(5-11) metabolite was rapidly hydrolyzed by rat and human plasma aminopeptidase M (AmM; EC 3.4.11.2) (24.2-25.5 nmol/min/ml). The Km values of SP for DAP IV and of SP(5-11) for AmM ranged from 32.7 to 123 microM. In contrast, neurokinin A (NKA) was resistant to both ACE and DAP IV but was subject to N-terminal hydrolysis by AmM (3.76-10.8 nmol/min/ml; Km = 90.7 microM). These data demonstrate differential processing of SP and NKA by specific peptidases in rat and human plasma.

Metabolism of vasoactive peptides by plasma and purified renal aminopeptidase M

Aminopeptidase M (AmM; EC 3.4.11.2) is a membrane-bound peptidase present on renal brush border and vascular plasma membrane. In the present study, AmM, purified from rabbit kidney cortex, produced a single immunoprecipitin line against AmM antisera, hydrolyzed alanyl-, leucyl- and arginyl-beta-naphthylamides at rates of 5.1 +/- 0.5, 3.9 +/- 0.5 and 2.6 +/- 0.3 mumol/min/mg, respectively, exhibited little or no alpha-glutamyl-, aspartyl- or glycyl-prolyl-naphthylamidase activities (less than or equal to 0.14 mumol/min/mg), and was inhibited by o-phenanthroline, amastatin (IC50 = 400 nM) and bestatin (IC50 = 6 microM). The alanyl-naphthylamidase activity of unfractionated rabbit plasma was found to be identical to purified AmM regarding relative rates of hydrolysis of alanyl-, leucyl- and arginyl-naphthylamides (100:79:42), pH optimum, and inhibition profile. In comparative studies with the purified enzyme, immunoreactive AmM accounted for essentially all of the alanyl-2-naphthylamidase activity of rabbit plasma. N-Terminal metabolism of (Met5)enkephalin by purified renal AmM was 3.92 +/- 0.69 mumol/min/mg, followed by somatostatin (1.25 mumol/min/mg), hepta(5-11)substance P (1.14 +/- 0.13 mumol/min/mg), (Asn1)angiotensin II (1.11 +/- 0.06 mumol/min/mg), angiotensin III (0.45 +/- 0.04 mumol/min/mg) and des(Asp1)-angiotensin I (0.36 +/- 0.04 mumol/min/mg). In contrast, substance P, bradykinin, (Sar1,Ala8)angiotensin II and neurokinin analogs containing modified N-termini (e.g. Ac-Arg) were resistant to hydrolysis by AmM. Peptide degradation was optimal at neutral pH and was inhibited by amastatin (IC50 = 200 nM) and bestatin (IC50 = 5 microM). Apparent Km values ranged from 15.7 +/- 0.4 microM for angiotensin III to 102 +/- 2 microM for (Met5)enkephalin. These data support a significant role for vascular and plasma AmM in the metabolism of circulating vasoactive peptides.

Proteinases and their inhibitors in cells and tissues

A large body of evidence has been assembled to indicate the substantial importance of proteolytic processes in various physiological functions. It has recently become clear too that endo-acting peptide bond hydrolases provisionally characterized and classified at present as serine, cysteine, aspartic and metallo together with unknown catalytic mechanism proteinases sometimes act in cascades. They are controlled by natural proteinase inhibitors present in cells and body fluids. In the first part of the present monograph the author was concerned to present an overview on the morphological and physiological approach to localization, surveying reaction principles and methods suitable for visualization of proteolytic enzymes and their natural and synthetic inhibitors. In the second part the roles played by proteinases have been summarized from the point of view of cell biology. The selection of earlier and recent data reviewed on the involvement of proteolysis in the behavior of individual cells reveals that enzymes, whether they be exogeneous or intrinsic, can be effective and sensitive modulators of cellular growth and morphology. There exists a close correlation between malignant growth and degradation of cells. It appears likely that as yet unknown or at least so far inadequately characterized factors that influence the survival or the death of cells may turn out to be proteinases. The causal role of extracellular proteolysis in cancer cell metastases, in stopping cancer cell growth and in cytolysis remains for further investigated. Ovulation, fertilization and implantation are basic biological functions in which proteolytic enzymes play a key role. The emergence of new approaches in reproductive biology and a growing factual basis will inevitably necessitate a reevaluation of present knowledge of proteolytic processes involved. The molecular aspects of intracellular protein catabolism have been discussed in terms of the inhibition of lysosomal and/or non-lysosomal protein breakdown. Peptide and protein hormone biosynthesis and inactivation are still at the centre of interest in cell biology, and a number of proteinases have been implicated in both processes. A number of conjectures partly based on the author's own work have been discussed which suggest the possibility of the involvement of proteolysis in exocytosis and endocytosis. The author's optimistic conclusion is that through the common action of biochemists, cell biologists, cytochemists, and pharmacologists the mystery of cellular proteolysis is beginning to be solved.

Metabolism of vasoactive peptides by vascular endothelium and smooth muscle aminopeptidase M

The cellular localization of vascular plasma membrane aminopeptidase M (AmM; EC3.4.11.2) was examined in cultured porcine aorta endothelium and smooth muscle cells. AmM was 14-fold higher on smooth muscle (117 +/- 16 units/mg) than on endothelium (8.4 +/- 0.2). Proportional to its cellular distribution, AmM hydrolyzed the N-terminus of kallidin to produce bradykinin, and degraded des(Asp1)angiotensin I, angiotensin III, hepta(5-11)substance P and Met5-enkephalin. In contrast, bradykinin, angiotensin II and substance P were resistant to AmM-mediated hydrolysis. Peptide metabolism was optimal at pH 7.0 and was inhibited by o-phenanthroline, bestatin (Ki = 2.2 +/- 0.1 microM) and amastatin (Ki = 25 +/- 5 nM). Des(Asp1)angiotensin I and angiotensin III had the highest affinity (lowest Km) for AmM (Km = 2.2 +/- 0.5 and 2.0 +/- 0.4 microM respectively), followed by hepta(5-11)substance P (53.9 +/- 1.7 microM) and Met5-enkephalin (75.7 +/- 3.5 microM). In contrast, maximal velocities of hydrolysis were higher for Met5-enkephalin (313 +/- 2 nmol/min/mg) than for hepta(5-11)substance P (109 +/- 18 nmol/min/mg) or angiotensin III (26.5 +/- 1.0 nmol/min/mg). As expected for hydrolysis by a common enzyme, AmM-mediated enkephalin degradation was inhibited competitively by angiotensin III (Ki = 0.34 +/- 0.04 microM), hepta(5-11)substance P (43.7 +/- 6.3 microM) and kallidin (62 microM). These data suggest that vascular AmM may modulate vasoactive peptide levels in vivo, particularly within the microenvironment of endothelial and smooth muscle cell surface receptors.

Dipeptidyl(amino)peptidase IV and post proline cleaving enzyme in cultured endothelial and smooth muscle cells

Dipeptidyl(amino)peptidase IV (DAP IV; EC 3.4.14.5) and post proline cleaving enzyme (PPCE; EC 3.4.21.26) can convert or degrade vasoactive peptides and have been identified in isolated vessels. The present study examined the cellular (endothelial/smooth muscle) localization of vascular DAP IV and PPCE. Membrane-bound DAP IV was higher on cultured hog aorta smooth muscle (11.7 +/- 1.7 nmol/min/mg) than on endothelium (1.5 +/- 0.3 nmol/min/mg). In contrast, comparable levels of cytosolic PPCE were found in endothelium and smooth muscle (1.5 +/- 0.3 and 1.8 +/- 0.3 nmol/min/mg, respectively). DAP IV was specifically inhibited by diprotin A (Ile-Pro-Ile) (IC50 = 6 microM) while PPCE was inhibited by TPCK. Neither enzyme was affected by o-phenanthroline or inhibitors of aminopeptidase M (amastatin, bestatin), neutral endopeptidases (phosphoramidon), carboxypeptidases N (MERGETPA) or ACE (captopril). DAP IV may play a role in the extracellular metabolism of peptides at or near endothelial and smooth muscle cell surface receptors. In contrast, the cytosolic localization of PPCE may limit its participation to intracellular peptide metabolism.

Systemic absorption of ocularly administered enkephalinamide and inulin in the albino rabbit: extent, pathways, and vehicle effects

The systemic absorption of ocularly applied tritiated [D-Ala2]metenkephalinamide (YAGFM) and inulin was studied in the albino rabbit with respect to rate, extent, pathways, and vehicle effects and compared with epinephrine. Peak concentration was achieved within 20 min except for inulin, for which absorption was still ongoing at 120 min. For YAGFM, the apparent absorption rate was slower than the elimination rate, thus obeying "flip-flop" pharmacokinetics. Based on the area under the plasma concentration curve from zero to 120 min, the percent of dose systemically absorbed was 36.1 +/- 4.4% for YAGFM, at least 3.3 +/- 0.2% for inulin, and 58.5 +/- 4.4% for epinephrine. This suggests that loss of drug to the systemic circulation is a more important factor in reducing the ocular absorption of YAGFM than for inulin. The conjunctival mucosa played as important a role as the nasal mucosa in the systemic absorption of YAGFM, while playing a secondary role in the case of inulin. Unlike nonpeptide drugs, the systemic absorption of ocularly administered YAGFM and inulin was not adversely affected by incorporation in 5% polyvinyl alcohol. Overall, the contact time of the instilled dose with the conjunctival and the nasal mucosae, their intrinsic permeability, and the extent of dilution of the instilled dose are key factors determining the vehicle effects on the extent of systemic absorption of ocularly applied peptides.

Angiotensin metabolism by cerebral microvascular aminopeptidase A

Porcine cerebral microvessels were isolated by differential sieving and centrifugation and were characterized by microscopic examination and marker enzyme enrichment (gamma-glutamyltransferase; EC 2.3.2.2). Purified microvessels contained a membrane-bound enzyme immunologically indistinguishable from renal aminopeptidase A (AmA; EC 3.4.11.7). AmA hydrolyzed both alpha-glutamyl- and alpha-aspartyl-2-naphthylamide, and hydrolysis was competitively inhibited by angiotensin II. Micro-vessel AmA hydrolyzed the N-terminal Asp1-Arg2 bond of both angiotensin I and angiotensin II, whereas the angiotensin II antagonist saralasin [(Sar1, Ala8)angiotensin II] was resistant to N-terminal hydrolysis. Angiotensin metabolism was optimal at pH 8.5 and was inhibited by EDTA, o-phenanthroline and amastatin. Conversely, inhibitors of neutral endopeptidase (phosphoramidon), post-proline cleaving enzyme (Z-Pro-Prolinal), carboxypeptidase N [D-L-mercaptomethyl-3-guanidinoethylthiopropanoic acid (MERGETPA)] and angiotensin I converting enzyme (captopril) had no effect. The Km values of angiotensin I, angiotensin II and (Asn1, Val5)angiotensin II for microvessel AmA were 40.1 +/- 8.2, 35.3 +/- 4.3 and 156 +/- 22 microM respectively. Cerebral microvascular aminopeptidase A may play a role in vivo in modulating angiotensin-mediated local cerebral blood flow, and in preventing circulating angiotensins from crossing the blood-brain barrier.

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.

Metabolism of opioid peptides by cerebral microvascular aminopeptidase M

Aminopeptidase M (EC 3.4.11.2), which can degrade low molecular weight opioid peptides, has been reported in both peripheral vasculature and in the CNS. Thus, we have studied the metabolism of opioid peptides by membrane-bound aminopeptidase M derived from cerebral microvessels of hog and rabbit. Both hog and rabbit microvessels were found to contain membrane-bound aminopeptidase M. At neutral pH, microvessels preferentially degraded low molecular weight opioid peptides by hydrolysis of the N-terminal Tyr1-Gly2 bond. Degradation was inhibited by amastatin (I50 = 0.2 microM) and bestatin (10 microM), but not by a number of other peptidase inhibitors including captopril and phosphoramidon. Rates of degradation were highest for the shorter peptides (Met5- and Leu5-enkephalin) whereas beta-endorphin was nearly completely resistant to N-terminal hydrolysis. Km values for the microvascular aminopeptidase also decreased significantly with increasing peptide length (Km = 91.3 +/- 4.9 and 28.9 +/- 3.5 microM for Met5-enkephalin and Met5-enkephalin-Arg6-Phe7, respectively). Peptides known to be present within or in close proximity to cerebral vessels (e.g., neurotensin and substance P) competitively inhibited enkephalin degradation (Ki = 20.4 +/- 2.5 and 7.9 +/- 1.6 microM, respectively). These data suggest that cerebral microvascular aminopeptidase M may play a role in vivo in modulating peptide-mediated local cerebral blood flow, and in preventing circulating enkephalins from crossing the blood-brain barrier.

Kinin and enkephalin conversion by an endothelial, plasma membrane carboxypeptidase

Utilizing both thin-layer chromatography and high pressure liquid chromatography, it was determined that a vascular plasma membrane preparation contains a carboxypeptidase capable of converting kinins (B2 agonists) to des(Arg)kinins (B1 agonists) by hydrolysis of C-terminal Arg. The plasma membrane carboxypeptidase also converted Leu5-enkephalin-Arg6 to Leu5-enkephalin. Carboxypeptidase activity was significantly higher in cultured endothelial (1.47 +/- 0.4 units/mg) than in cultured smooth muscle cells (0.16 +/- 0.4 units/mg). Both the vascular and endothelial activities had neutral pH optima and were activated 4- to 5-fold by 0.1 mM CoCl2. The carboxypeptidase N inhibitor MERGETPA (D-L-mercaptoethanol-3-guanidino-ethylthiopropanoic acid) inhibited the plasma membrane bound carboxypeptidase with an I50 of 0.3 microM. Conversion was also inhibited by o-phenanthroline and EDTA, whereas inhibitors of aminopeptidases (bestatin, puromycin), endopeptidases (phosphoramidon), "enkephalinase" (ZINCOV) or enkephalin convertase (PCMS) were without effect. The affinity of the endothelial plasma membrane carboxypeptidase for bradykinin (Km = 56.8 +/- 4.7 microM) was higher than that for Leu5-enkephalin-Arg6 (Km = 92.7 +/- 10.1 microM), whereas the maximal rates of conversion (calculated per mg of endothelial plasma membrane protein) were similar (17.1 and 21.3 nmoles/min/mg respectively). These results demonstrate that a carboxypeptidase is present on the cell surface of vascular endothelium which can convert kinins and enkephalins in the micro-environments of vascular cell surface receptors.

Degradation of low-molecular-weight opioid peptides by vascular plasma membrane aminopeptidase M

Since both aminopeptidases and angiotensin I-converting enzyme are reported to degrade circulating enkephalins, we have examined the degradation of low-molecular-weight opioid peptides by a vascular plasma membrane-enriched fraction previously shown to contain both angiotensin I-converting enzyme (EC 3.4.15.1) and aminopeptidase M (EC 3.4.11.2). Except for an enkephalin analog resistant to amino-terminal hydrolysis, [D-Ala2]enkephalin, the purified vascular plasma membrane preferentially degraded low-molecular-weight opioids by hydrolysis of the N-terminal Tyr-1--Gly-2 bond. Enkephalin degradation was optimal at pH 7.0 and was inhibited by the aminopeptidase inhibitors amastatin (I50 = 0.08 microM), bestatin (9.0 microM) and puromycin (80 microM). Maximal rates of hydrolysis, calculated per mg plasma membrane protein, were highest for the shorter peptides (18.3, 15.6 and 16.6 nmol/min per mg for Met5-enkephalin, Leu5-enkephalin and Leu5-enkephalin-Arg6, respectively) and decreased with increasing peptide length (0.7 nmol/min per mg for dynorphin (1-13)). No significant hydrolysis of beta- and gamma-endorphin was detected. Km values decreased significantly with increasing peptide length (Km = 72.9 +/- 2.7, 43.6 +/- 4.7 and 21.4 +/- 0.9 microM for Met5-enkephalin, Leu5-enkephalin-Arg6 and Met5-enkephalin-Arg6-Phe7, respectively). However, no further decreases were seen with even larger sequences, i.e., dynorphin(1-13). Other peptides hydrolyzed by the plasma membrane aminopeptidase (angiotensin III, kallidin and hepta(5-11)-substance P) inhibited enkephalin degradation in a competitive manner. Thus, localization, specificity and kinetic data are consistent with identification of aminopeptidase M as a vascular enzyme with the capacity to differentially metabolize low-molecular-weight opioid peptides within the microenvironment of vascular cell surface receptors. Such differential metabolism may play a role in modulating the vascular effects of peripheral opioids.

Vascular, post proline cleaving enzyme: metabolism of vasoactive peptides

Vasoactive peptides contain a high proportion of proline residues which make them resistant to hydrolysis by many peptidases. However, post proline cleaving enzyme (PPCE; EC 3.4.21.26), a proline specific endopeptidase which specifically hydrolyzes internal peptide bonds on the carboxyl side of proline residues, has been shown to inactivate numerous vasoactive peptides including angiotensins, kinins, substance P, vasopressin and oxytocin. In order to determine whether PPCE could be involved in vascular metabolism of vasoactive peptides, we carried out localization and characterization studies of PPCE-like activity in hog aorta and mesenteric artery. PPCE was assayed fluorometrically at pH 7.0 using the specific PPCE substrate CBZ-Gly-Pro-4-methyl-coumarinylamide. The subcellular distribution of vascular PPCE was essentially the same as that of the cytosolic marker enzyme lactic dehydrogenase (LDH). PPCE was enriched six-fold in the cytosolic fraction (11.4 +/- 2.7 units/mg) and unlike the plasma membrane-bound proline specific exopeptidase dipeptidyl-(amino)peptidase IV (DAP IV; EC 3.4.14.5), little or no activity could be detected in the microsomal or plasma membrane fractions. Similar to PPCE characterized from other sites, vascular PPCE was stabilized and activated by dithiothreitol and EDTA, and inhibited by DFP, p-chloromercuriphenyl sulfonic acid, L-1-tosylamido-2-phenylethylchloromethyl ketone, Cu++, Ca++, and Zn++. Vascular PPCE was unaffected by inhibitors of trypsin and kallikrein (Aprotinin, ABTI), aminopeptidase M (bestatin, amastatin), neutral endopeptidase (phosphoramidon), angiotensin I converting enzyme (captopril) or carboxypeptidase N (MERGETPA). These data demonstrate that PPCE is present in vascular endothelium and/or smooth muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Action and metabolism of des(Arg)kinins in mesenteric arteries

Kallidin and bradykinin can be hydrolyzed at their C-termini to produce des(Arg10)kallidin and des(Arg9)bradykinin respectively. These des(Arg)kinins, previously thought to be biologically inactive, are now known to have potent effects on B1 receptors. Although stimulation of B1 receptors has been reported to produce peripheral vasodepressor responses in certain experimental states, only constriction has been reported in isolated vessels (i.e., rabbit aorta, basilar artery, mesenteric vein). In the present study, we have investigated the biologic activity of des(Arg) kinins on a peripheral resistance vessel (rabbit mesenteric artery). We found that des(Arg)bradykinin relaxes mesenteric arteries, and that its potency relative to kallidin and bradykinin is consistent with the presence of B1 receptors. Further, intact mesenteric arteries, and a plasma membrane fraction purified from these arteries, contained a carboxypeptidase activity which was capable of producing des(Arg)kinins from both kallidin and bradykinin. Thus, these data demonstrate that the vasculature has the enzymatic capacity to form B1 kinins, and that stimulation of B1 receptors in resistance vessels can be associated with peripheral vasodilation.

Vascular, plasma membrane aminopeptidase M. Metabolism of vasoactive peptides

Aminopeptidase M (EC 3.4.11.2), an enzyme present on the cell surface of vascular endothelium and/or smooth muscle, rapidly hydrolyzes leucyl- and arginyl-2-naphthylamides and a number of vasoactive peptides at physiologic pH. Utilizing both thin-layer chromatography and high pressure liquid chromatography, it was found that vascular aminopeptidase M converted kallidin to bradykinin and inactivated des(Asp1)angiotensin I, angiotensin III, hepta(5-11)substance P and hexa(6-11)substance P. Aminopeptidase M did not, however, hydrolyze bradykinin, angiotensin I, angiotensin II, saralasin, vasopressin, oxytocin or any form of substance P containing a component of the Arg-Pro-Lys-Pro sequence. Both the naphthylamidase and peptidase activities were inhibited similarly by known amino-peptidase M inhibitors including o-phenanthroline, amastatin, bestatin and puromycin. However, inhibitors of angiotensin I converting enzyme (captopril), carboxypeptidase N (MERGETPA), neutral endopeptidase (phosphoramidon), post proline cleaving enzyme and dipeptidyl(amino)peptidase IV (diisopropylphosphofluoridate, DFP) were without effect. These results demonstrate that vascular, cell surface aminopeptidase M can selectively metabolize vasoactive peptides and may play a role in modulating their levels in the circulation and/or within the vessel wall.