Generation of a 90 000 molecular weight fragment from human plasma angiotensin-I-converting enzyme by enzymatic or alkaline hydrolysis

Evidence for the negative cooperativity of the two active sites within bovine somatic angiotensin-converting enzyme

The somatic isoform of angiotensin-converting enzyme (ACE) consists of two homologous domains (N- and C-domains), each bearing a catalytic site. We have used the two-domain ACE form and its individual domains to compare characteristics of different domains and to probe mutual functioning of the two active sites within a bovine ACE molecule. The substrate Cbz-Phe-His-Leu (N-carbobenzoxy-L-phenylalanyl-L-histidyl-L-leucine; from the panel of seven) was hydrolyzed faster by the N-domain, the substrates FA-Phe-Gly-Gly (N-(3-[2-furyl]acryloyl)-L-phenylalanyl-glycyl-glycine) and Hip-His-Leu (N-benzoyl-glycyl-L-histidyl-L-leucine) were hydrolyzed by both domains with equal rates, while other substrates were preferentially hydrolyzed by the C-domain. The inhibitor captopril ((2S)-1-(3-mercapto-2-methylpropionyl)-L-proline) bound to the N-domain more effectively than to the C-domain, whereas lisinopril ((S)-N(alpha)-(1-carboxy-3-phenylpropyl)-L-lysyl-L-proline) bound to equal extent with all ACE forms. However, active site titration with lisinopril assayed by hydrolysis of FA-Phe-Gly-Gly revealed that 1 mol of inhibitor/mol of enzyme abolished the activity of either two-domain or single-domain ACE forms, indicating that a single active site functions in bovine somatic ACE. Neither of the k(cat) values obtained for somatic enzyme was the sum of k(cat) values for individual domains, but in every case the value of the catalytic constant of the hydrolysis of the substrate by the two-domain ACE represented the mean quantity of the values of the corresponding catalytic constants obtained for single-domain forms. The results indicate that the two active sites within bovine somatic ACE exhibit strong negative cooperativity.

Angiotensin converting enzymes from human urine of mild hypertensive untreated patients resemble the N-terminal fragment of human angiotensin I-converting enzyme

Angiotensin I-converting enzyme (ACE) activity was analyzed in human urine collected from mild hypertensive untreated patients. DEAE-cellulose chromatography using linear gradient elution revealed two forms of angiotensin I-converting enzyme, eluted in the conductivity of 0.75 and 1.25 mS. The fractions of each conductivity were pooled and submitted to direct gel filtration in an AcA-34 column, and the apparent molecular weights of urinary ACEs were estimated as 90 kDa (for ACE eluted in 0.75 mS) and 65 kDa (for ACE eluted in 1.25 mS). Both enzymes have a K(i) of the order of 10(-7) M for the specific inhibitors studied, and are able to hydrolyze luteinizing hormone-releasing hormone and N-acetyl-Ser-Asp-Lys-Pro as described for N-domain ACE. By Western blot analysis, both peaks were recognized by ACE-specific antibody Y4, confirming the molecular weight already described. A plate precipitation assay using monoclonal antibodies to the N-domain of ACE showed that both forms of ACE binds with all monoclonal antibodies to the active N-domain ACE, suggesting that these forms of human urine ACEs resemble the N-fragment of ACE. The HP2 ACE (65 kDa) is similar to low molecular weight (LMW) ACE from normal subjects, and the HP2 ACE (90 kDa) is different from high molecular weight (190 kDa) and LMW (65 kDa) normal ACEs. The 90 kDa ACE could have an important role in development of hypertension. It will be fundamental to elucidate the molecular mechanism responsible for the genesis of this isoform.

Limited proteolysis of human kidney angiotensin-converting enzyme and generation of catalytically active N- and C-terminal domains

The somatic form of angiotensin converting enzyme is a class I ectoenzyme that is bound to the surface of endothelial calls. It consists of two homologous, catalytic domains of approximately 600 residues each; a juxtamembrane "stalk" region; a transmembrane, hydrophobic sequence; and a 30 residue, C-terminal cytosolic domain. We have used limited proteolysis to probe the structural and functional properties of the enzyme. Endoproteinase Asp-N cleaves both the Thr615-Asp616 and the Leu1219-Asp1220 peptide bonds to generate the two catalytic domains which were isolated by a combination of immunoaffinity and lisinopril Sepharose affinity chromatography. The enzymatic characteristics of the N and C fragments were examined with angiotensin I, hippuryl-His-Leu, and luteinizing hormone-releasing hormone and indicate that both fragments contain catalytically active sites that retain their individual functional integrity.

Angiotensin-converting enzyme responses following enalapril in the sodium deficient rat

The relationship between blood pressure lowering activity and inhibition of plasma and tissue angiotensin-converting enzyme (ACE) has been studied in the sodium deficient normotensive rat at 24, 48 and 96 h after the administration for 21 days of enalapril (MK-421, 10 mg/kg per day p.o.). Blood pressure was reduced and plasma ACE activity inhibited at 24 and 48, but not 96 h, after cessation of dosing. Tissue ACE activity (aorta, lung, mesenteric bed) was inhibited up to 96 h post dose when blood pressure had returned to control values. ACE production (activity following removal of inhibitor) was increased in plasma at 24, 48 and 96 h post dose but in tissues (adrenal glands, renal arteries and mesenteric bed) only at 48 h post dose. There was no tendency for ACE production to increase in the lung, the largest source of the enzyme in the rat. Thus it appears that inhibition of ACE activity in both plasma and tissue contributes to the blood pressure lowering activity of enalapril in the sodium deficient normotensive rat.

Testicular angiotensin I-converting enzyme (E.C. 3.4.15.1)

In the two mammalian species (i.e., rabbit and rat) in which it has been studied to date, testicular angiotensin I-converting enzyme possesses distinct physicochemical and immunological properties, and a susceptibility to hormonal regulation that makes it a unique isozyme of the converting enzyme ordinarily distributed throughout the body. The testicular isozyme appears to be a lower molecular weight version of the pulmonary enzyme, with similar, although not identical, catalytic properties. The testicular isozyme is under androgenic control and is associated with germinal cells. Although its function has yet to be elaborated, the testicular isozyme provides an excellent model for the study of tissue-specific regulation of carboxypeptidases.

Thermal denaturation of rat pulmonary and testicular angiotensin-converting enzyme isozymes. Effects of chelators and CoCl2

In an attempt to assess the biochemical consequences resulting from structural differences between rat pulmonary and testicular angiotensin-converting enzyme, the thermal stability of crude and purified preparations of each enzyme was compared. Structural heterology was verified by molecular weight determinations and by peptide mapping after limited proteolysis with Staphylococcus V8 proteinase. Thermal stability was monitored by changes in catalytic activity following incubations at 55 degrees C in the presence of chelators and CoCl2. Purified pulmonary angiotensin-converting enzyme was more sensitive to inhibition by the chelators EDTA and 1,10-phenanthroline and by the site-directed inhibitor captopril than was the testicular isozyme. Although the pulmonary holoenzyme was unaffected by cobalt, the testicular holoenzyme was inhibited by cobalt in a concentration-dependent manner. Crude pulmonary angiotensin-converting enzyme was significantly more resistant to thermal denaturation than its crude testicular counterpart. The differences in the thermal lability of each isozyme were still present in purified preparations, although the purified enzymes appeared to be more thermally stable than their crude counterparts. Both chelators and cobalt markedly potentiated the thermal denaturation of each isozyme. These data suggest that the structural heterology of the pulmonary and testicular isozymes may affect the interaction of zinc with the respective enzymes and that zinc may contribute to the structural integrity and thermal stability of angiotensin-converting enzyme in each tissue.