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PubMed1992

Endothelin-converting enzymes.

The putative endothelin-converting enzyme (ECE) has been the focus of intense research, both within academia and the pharmaceutical industry. Interest in ECE stems mainly from the hypothesis that development of inhibitors of ECE will provide an effective means of preventing production of endothelin in circumstances where it may play a pathogenic role. Both an aspartic and a metalloprotease have been identified that have characteristics of this putative enzyme. Evidence suggests that the metalloprotease, which is inhibited by phosphoramidon, may be the physiologically relevant converting enzyme. However, it remains to be demonstrated conclusively that any inhibitor of an ECE activity directly alters endogenous endothelin production and/or the pathogenesis of a disease condition in which endothelin is thought to play a primary role.

Amino Acid Sequence↗
PubMed1992

The endothelin-converting enzyme from human umbilical vein is a membrane-bound metalloprotease similar to that from bovine aortic endothelial cells.

A phosphoramidon-sensitive, membrane-bound metalloprotease that cleaves big endothelin 1 (big-ET-1) to ET-1 was obtained from human umbilical vein endothelial cells and also from bovine aortic endothelial cells by isolation of plasma-membrane vesicles free of lysosomes. The enzyme was characterized by RIA with an antibody specific for ET-1 and also by reverse-phase HPLC. For both sources, the pH rate profile of the membrane fraction had a very sharp maximum at pH 7.0; little or no activity was seen at more acidic pH values. In contrast, the cytosolic fraction had a major peak at acidic pH values, as well as a broad peak in the neutral region. The activity at pH 7.0 in the membrane fraction was shown by reverse-phase HPLC to produce ET-1 and C-terminal fragment as products. This activity was abolished by phosphoramidon, EDTA, and 1,10-phenanthroline but was not inhibited by pepstatin A, phenylmethylsulfonyl fluoride, soybean trypsin inhibitor, leupeptin, or E-64--consistent with the characteristics of a metalloprotease. These results suggest that this activity is from the physiologically relevant, phosphoramidon-inhibitable, endothelin-converting enzyme. The activity found at neutral pH values in the cytosolic fraction was only partially inhibited by EDTA and 1,10-phenanthroline but was not inhibited by phosphoramidon. The membrane-bound endothelin-converting enzyme from human umbilical vein endothelial cells and bovine aortic endothelial cells showed marked similarities, including IC50 values for phosphoramidon of 2.7 and 1.8 microM and Km values for big-ET-1 of 45.4 and 20.9 microM, respectively. The apparent molecular mass by gel filtration was approximately 300-350 kDa for the enzyme from either source. This report characterizes human endothelin-converting enzyme, which may be an important therapeutic target for cardiovascular disease.

Animals↗
PubMed1992

Identification and partial purification of a thiol endothelin-converting enzyme from porcine aortic endothelial cells.

Endothelin is a potent peptide vasoconstrictor. The final step in the processing of endothelin has been postulated to be the cleavage of the Trp21-Val22 peptide bond in proendothelin by a putative endothelin-converting enzyme. A soluble extract of primary porcine aortic endothelial cells was found to contain an enzyme activity that converted proendothelin-1 (proET-1) to an endothelin-1 (ET-1)-like peptide as determined by the rabbit aortic ring contraction assay. This enzyme was partially purified by DE52 ion-exchange chromatography. Incubation of proET-1 with the partially purified enzyme generated a product which had a retention time on HPLC identical to that of authentic ET-1. Further analysis of the product showed that it caused contraction of rabbit aortic rings, had a molecular weight identical to ET-1 as measured by fast atom bombardment mass spectrometry, and competed for [125I]ET-1 binding in an RIA using specific antibodies which recognize the carboxy terminal tryptophan of ET-1. The enzyme activity could be inhibited by thiol protease inhibitors such as Z-phe-pheCHN2 and p-hydroxymercuribenzoate, but not by serine- or metalloprotease inhibitors. The optimal pH for the enzymatic activity was between 7.0 and 7.5, and no activity was detected at pH 4.0. These results demonstrate that this thiol protease is a potential endothelin-converting enzyme.

Animals↗
PubMed1992

Characterization of endothelin-converting enzyme from endothelial cells and rat brain: detection of the formation of biologically active endothelin-1 by rapid bioassay.

Using the endothelin-1 (ET-1)-stimulated elevation in cGMP in LLC-PK1 cells as a biological detector system for the conversion of big ET-1 (bET-1) to ET-1, we detected bET-1-converting activities in subcellular fractions from bovine aortic cultured endothelial cells (BAE) and rat brain. Within the particulate fraction of BAE, we detected two activities, at pH 3.4 and pH 5.4-7.4. The latter but not the former activity was inhibited in a concentration-dependent manner by phosphoramidon (approximate IC50, 1 microM) and converted bET-1 to ET-1 at a rate of 0.6 nmol/hr/mg of protein. It could be solubilized from the particulate fraction by detergent treatment. Phosphoramidon-inhibitable converting activity was also detected in the cytosolic fraction of BAE. Within the rat brain, phosphoramidon-inhibitable conversion of bET-1 to ET-1 was detected principally in the cytoskeletal fraction, i.e., that fraction from the membrane that was not solubilized by detergent treatment. These results show the presence of at least two different endothelin-converting enzyme activities in endothelial cells and a third within the rat brain. They also demonstrate the use of LLC-PK1 cells as a rapid assay that permits the sensitive detection and measurement of the formation of biologically active ET-1 from its precursor bET-1.

Animals↗
PubMed1992

Phosphoramidon-sensitive endothelin-converting enzyme in vascular endothelial cells converts big endothelin-1 and big endothelin-3 to their mature form.

Incubation of big endothelin-3 (big ET-3(1-41)) with the membrane fraction obtained from cultured endothelial cells (ECs) resulted in an increase in immunoreactive-ET (IR-ET). This increasing activity was markedly suppressed by phosphoramidon, which is known to inhibit the conversion of big ET-1(1-39) to ET-1(1-21). Reverse-phase HPLC of the incubation mixture of the membrane fraction with big ET-3 revealed one major IR-ET component corresponding to the elution position of synthetic ET-3(1-21). When the cultured ECs were incubated with big ET-3, a conversion to the mature ET-3, as well as an endogenous ET-1 generation, was observed. Both responses were markedly suppressed by phosphoramidon. By the gel filtration of 0.5% CHAPS-solubilized fraction of membrane pellets of ECs, the molecular mass of the proteinase which converts big ET-1 and big ET-3 to their mature form was estimated to be 300-350 kDa. Phosphoramidon almost completely abolished both converting activities of the proteinase. We conclude that the above type of phosphoramidon-sensitive metalloproteinase functions as an ET-converting enzyme to generate the mature form from big ET-1 and big ET-3 in ECs.

Animals↗
PubMed1992

Detection by bioassay and specific enzyme-linked immunosorbent assay of phosphoramidon-inhibitable endothelin-converting activity in brain and endothelium.

The endothelin-converting enzyme (ECE) activity present in endothelial cells and rat and human brains was characterized using a selective and rapid bioassay for endothelin-1 (ET-1) or endothelin-3 (ET-3) together with a sensitive enzyme-linked immunosorbent assay. We found that ECE activity was predominantly in the membrane fraction of endothelial cells from which it could be extracted by treatment with detergent. In rat brain tissue, the ECE activity was in the membrane fraction and was not solubilized by detergent treatment. Further dissection of the brain revealed that there was a strong localization of ECE activity in the hypothalamus, midbrain, and medulla oblongata in agreement with earlier observations of ET-like immunoreactivity and binding sites. Experiments with human brain tissue also showed the presence of ECE activity. In conclusion, our studies confirmed the presence of ECE activity within endothelial cells, and showed ECE to be localized in brain tissue in sites consistent with the selective distribution of the ET-1 synthetic pathway. In all tissues studied, the ECE activity was significantly inhibited by phosphoramidon or ethylenediaminetetra-acetate.

Animals↗
PubMed1992

The comparative effects of big endothelin-1, endothelin-1, and endothelin-3 in the human fetal-placental circulation.

OBJECTIVE: This study compared the effects of big endothelin-1, endothelin-1, and endothelin-3 and whether endothelin-converting enzyme was present in the human fetal-placental circulation. STUDY DESIGN: Single cotyledons of term placentas were dually perfused in vitro, and increases in fetal-placental perfusion pressure to bolus injections of big endothelin-1, endothelin-1, and endothelin-3 (8 x 10(-10) to 1 x 10(-7) mol/L) were recorded. Responses to big endothelin-1 (10(-7) mol/L) were measured in the same placenta before and after perfusion of the fetal-placental circulation with the neutral metalloprotease inhibitor phosphoramidon (10(-5) mol/L), which acts as an endothelin-converting enzyme inhibitor. All experiments were performed in at least five separate placentas. RESULTS: Significant concentration-dependent increases in fetal-placental perfusion pressure were seen with endothelin-1 (p < 0.0005), endothelin-3 (p < 0.0256), and big endothelin-1 (p < 0.0034, analysis of variance). Big endothelin-1 always elicited transient vasodilatation before constriction. Phosphoramidon significantly inhibited the vasoconstrictor effect of big endothelin-1 (p < 0.039, paired t test). CONCLUSION: The three endothelins tested are vasoconstrictors, and endothelin-converting enzyme is present in the fetal-placental circulation.

Blood Circulation↗
PubMed1992

Evidence for a direct vasoconstrictor effect of big endothelin-1 in the rat kidney.

Inhibition of endothelin-1 (ET-1)-converting enzyme has been suggested as a strategy for blocking ET-1-mediated vasoconstriction. However, it is unclear whether its putative substrate, bigET-1, is an inactive precursor. Thus, we compared in the rat the effects of ET-1 and bigET-1 on renal vascular resistance (RVR) in vitro (isolated perfused kidney, n = 15) and in vivo (Doppler shift technique, n = 23) when injected i.v. or in the rat renal artery (i.a.), before and after metalloprotease inhibition with phosphoramidon (30 mg/kg i.v.). In vitro, the ET-1/bigET-1 potency ratio for the RVR increase was 175; in vivo (i.v.) it was approximately 7 (ED50: 99 and 692 pmol/kg, respectively; P < 0.01). Unlike that of ET-1, the bigET-1 effect started slowly (peak effect at 15 min). On i.a. injection, the ED50 of ET-1 was lower but that of big ET-1 was unchanged (ED50: 28 and 706 pmol/kg, respectively). Moreover, the effect of i.a. bigET-1 on RVR was biphasic, with a dose-related rapid increase followed by a slowly developing further rise. Phosphoramidon completely inhibited the hemodynamic effects of i.v. bigET-1, but abolished only the second phase of the response when given i.a. It also significantly enhanced the effect of ET-1. We conclude that in the rat: (1) bigET-1 may affect RVR by both a direct effect and through phosphoramidon-sensitive conversion to ET-1; (2) the direct vasoconstrictor effect of bigET-1 might be expressed during endothelin-converting enzyme inhibition; (3) metalloproteases are involved in ET-1 degradation.

Animals↗
PubMed1992

Role of endothelin-1 and big endothelin-1 in modulating coronary vascular tone, contractile function and severity of ischemia in rat hearts.

The effect of endothelin-1 (ET-1) and big ET-1 on coronary flow and contractile function was determined in isolated nonischemic and ischemic rat hearts. Both ET-1 (IC50 = 12 pMol) and big ET-1 (IC50 = 2 nMol) reduced coronary flow in a concentration-dependent manner, although ET-1 was > 100-fold more potent. Both compounds decreased contractility, an effect which was lost when coronary flow was held constant, indicating that ET-1 and big ET-1 decrease contractility secondary to reducing coronary flow. Mechanical reduction in coronary flow to levels equivalent to those seen for ET-1 or big ET-1 caused similar reductions in contractility. Both 30 pMol ET-1 and 10 nMol big ET-1 pretreatment significantly reduced the time to contracture in globally ischemic rat hearts, suggesting a proischemic effect. Phosphoramidon (100 microM, endothelin-converting enzyme inhibitor) and BQ-123 (0.3 microM, ETA receptor antagonist) abolished the preischemic increase in coronary perfusion pressure induced by big ET-1 as well as its proischemic effect, whereas only BQ-123 abolished the cardiac effect of ET-1. Neither phosphoramidon nor BQ-123 had an effect on severity of ischemia when given alone. Phosphoramidon was also given i.v. to rats subjected to coronary occlusion and reperfusion and was found to significantly reduce infarct size 24 hr postischemia. Thus, in isolated rat hearts, big ET-1 appears to be converted to ET-1 and is a potent coronary constrictor.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Regional differences in endothelin converting enzyme activity in rat brain: inhibition by phosphoramidon and EDTA.

1. It has been demonstrated previously that conversion of big endothelin-1 (bET-1) to endothelin-1 (ET-1) is inhibited in vitro and in vivo by phosphoramidon. In addition, ET-1 binding sites and mRNA have been shown within the brain. Here we expand upon our previous observation that rat brain contains phosphoramidon-inhibitable endothelin converting enzyme (ECE) and show that this activity is not uniformly distributed throughout the brain. 2. ECE activity was detected by a bioassay which depended upon the 10,000 fold difference in potency between bET-1 and ET-1 as stimulants of guanosine 3':5'-cyclic monophosphate (cyclic GMP) accumulation in kidney epithelial (PK1) cells of the pig. Data were confirmed by specific enzyme-linked immunosorbent assay (ELISA) employing antibody directed against ET-1/3(17-21). 3. Following homogenization of the whole brain and ultracentrifugation the 100,000 g pellet contained greater than 4 times more ECE activity than the cytosol. Washing of the pellet with KCl (1 M) and extraction with the detergent CHAPS (20 mM) revealed a phosphoramidon-inhibitable ECE within the residual particulate fraction (nominally classified as the cytoskeletal fraction). Phosphoramidon (IC50, approx. 5 microM) or EDTA inhibited the conversion of bET-1 to ET-1 by the cytoskeletal fraction of rat brain by more than 60%.2+ 4. Following dissection of rat brain into olfactory bulb, cerebral cortex, striatum, hippocampus, cerebellum, midbrain (including thalamus), hypothalamus and medulla oblongata (including pons) the greatest ECE was detected in the hypothalamus and medulla oblongata.After fractionation, the ECE-activities in the cytoskeletal fractions prepared from the hypothalamus or medulla oblongata were inhibited concentration-dependently by phosphoramidon or EDTA, with maximum inhibitions of>80% and >70%, respectively.5. These data show that rat brain contains a phosphoramidon- and EDTA-inhibitable ECE which maybe similar to that present in endothelial cells. The localization of this enzyme correlates with published reports of immunoreactive-ET-l, ET-1-binding sites, and messenger RNA for ET-1 in the rat brain, and suggests the presence of the entire synthetic pathway for ET-1.

Animals↗
PubMed1992

Partial purification of phosphoramidon-sensitive endothelin converting enzyme in porcine aortic endothelial cells: high affinity for Ricinus communis agglutinin.

A membrane-bound endothelin converting enzyme (ECE) of porcine aortic endothelial cells (ECs) was solubilized with Lubrol PX with high efficiency and stability. The solubilized ECE was bound to Ricinus communis agglutinin (RCA) but not to peanut agglutinin (PNA) or wheat germ agglutinin (WGA), suggesting that the ECE has a galactosylated structure possessing a high affinity for RCA. The sequential chromatography on RCA-agarose, PNA-agarose and a TSKgel DEAE-5PW column attained 2,100-fold purification for the ECE over the membrane fractions. The purified ECE was sensitively inhibited by phosphoramidon but not by thiorphan. The present RCA and PNA affinity column procedures may be a powerful approach to isolation of ECE of EC origin.

Animals↗
PubMed1992

Rhamnose moiety of phosphoramidon is not required for in vivo inhibition of endothelin converting enzyme.

Experiments were conducted to determine the importance of the carbohydrate moiety of phosphoramidon in the inhibition of the pressor response to big endothelin-1 in anesthetized rats. Big endothelin-1 produced a 42% increase in mean arterial pressure which was nearly abolished by co-infusion of phosphoramidon. Similarly, when an analog of phosphoramidon lacking the rhamnose group (phosphoryl-leu-trp-OH) was co-infused, a significant attenuation of the pressor response was observed. These findings indicate that the rhamnose moiety of phosphoramidon is not responsible for the distinguishing this compound as an inhibitor of the response to big endothelin-1 in the rat.

Animals↗
PubMed1992

Biochemical properties of endothelin converting enzyme in renal epithelial cell lines.

This is the first report clearly demonstrating the presence of endothelin (ET) converting enzyme (ECE) in non-vascular cells (renal epithelial cell lines, MDCK and LLC-PK1). ECEs derived from these epithelial cells were very similar to the endothelial ECE in the following biochemical properties: 1) The optimum pH was 7.0; 2) the Km value for big ET-1 was approximately 30 microM; 3) the enzyme was potently inhibited by EDTA, o-phenanthroline and phosphoramidon; and 4) the enzyme did not convert big ET-2 or big ET-3. These data suggest that phosphoramidon-sensitive ECE is involved in the processing of big ET-1 to ET-1 in the renal tubule.

Animals↗
PubMed1992

Role of mast cell chymase in the extracellular processing of big-endothelin-1 to endothelin-1 in the perfused rat lung.

Previous studies of endothelin-1 (ET) synthesis have shown that some cultured endothelial cells secrete an intermediate product, big-endothelin-1 (bigET), suggesting that the processing of secreted bigET to ET may be physiologically significant. In this study, two pertinent ET converting enzyme activities, mast cell chymase I (EC 3.4.21.39) and a phosphoramidon-sensitive, neutral metalloprotease, were identified in a rat lung particulate fraction. We perfused rat lungs with bigET and chymostatin or phosphoramidon to study the relevance of these two proteases to the processing of extracellular bigET in vivo. Addition of compound 48/80 (a compound which activates mast cells, causing degranulation and release of chymase) to the perfusion buffer greatly increased hydrolysis of exogenously added bigET to ET. ET formation was inhibited completely by 32 microM chymostatin, whereas inhibition by 50 microM phosphoramidon was incomplete and variable. Perfusate histamine levels were used to monitor the extent of mast cell degranulation, and inhibition of ET production by phosphoramidon was attributed to inhibition of degranulation, per se. There was a direct correlation between perfusate ET and histamine levels in both control and phosphoramidon-treated (but not chymostatin-treated) lungs. Our results suggest that chymase from lung mast cells is capable of physiologically relevant extracellular processing by bigET to ET in the perfused rat lung.

Animals↗
PubMed1992

Effect of phosphoramidon on big endothelin-2 conversion into endothelin-2 in human renal adenocarcinoma (ACHN) cells. Analysis of endothelin-2 biosynthetic pathway.

The biosynthetic pathway of endothelin (ET)-2 was analyzed in cultured ACHN cells. In the supernatant, we detected three ET-2-related peptides, ET-2, big ET-2(1-38) and big ET-2(22-38). Phosphoramidon decreased the amount of ET-2 and increased that of big ET-2(1-38) dose-dependently. The amount of big ET-2(1-37) did not significantly change. These results suggest that big ET-2 is composed of 38 and not 37 amino acid residues, and that a putative ET-2-converting enzyme (ECE-2) should be classified as a phosphoramidon-sensitive neutral metalloprotease, bearing a resemblance to the putative ET-1-converting enzyme (ECE-1) in endothelial cells.

Adenocarcinoma↗
PubMed1992

Identification of endothelin converting enzyme in bovine lung membranes using a new fluorogenic substrate.

An enzyme partially purified from bovine lung membranes appears to be endothelin converting enzyme (ECE). This enzyme specifically cleaves big endothelin-1 (big ET-1) at the proper site, between Trp21 and Val22, with maximum activity at pH 7.5 and with a Km of roughly 3 microM, to produce endothelin-1 (ET-1) and C-terminal peptide (CTP). This same enzyme hydrolyzes the fluorogenic substrate succinyl-Ile-Ile-Trp-methylcoumarinamide to release the highly fluorescent 7-amino-4-methylcoumarin. The peptide derivative has the same amino acid sequence as big ET-1 and is a good substrate with a Km of about 27 microM. This enzyme is a metalloproteinase. It is not inhibited by five common proteinase inhibitors (pepstatin A, PMSF, NEM, E-64 and thiorphan) but it is inhibited by phosphoramidon and chelating compounds. The apoenzyme is restored to nearly full activity by a zinc-EDTA buffer with pZn = 13.

Animals↗
PubMed1992

A simple method for measurement of phosphoramidon-sensitive endothelin converting enzyme activity.

We have developed a rapid and convenient assay for measurement of the action of endothelin (ET) converting enzyme (ECE) using the scintillation proximity assay (SPA) principle. On incubation of [125I]big ET-1 at 37 degrees C for 0.5-6 hr with an enzyme preparation, the reaction was terminated by the addition of an ET-1-specific antibody formulated in a buffer designed to shift the pH to alkaline. The antibody was allowed to come to equilibrium for 1 hr at room temperature and the amount of ET-1 produced, detected in a single step by the addition of protein A SPA beads. Using this assay, ECE activities of enzyme preparations obtained from porcine cultured endothelial cells and rat lung were clearly detected. These activities were inhibited by phosphoramidon in a concentration-dependent manner. The SPA based assay is homogeneous requiring no separation steps and takes a half day to complete. This method is therefore suitable for the high throughput screening of potential ECE inhibitors.

Animals↗