The hemoregulatory peptide N-acetyl-Ser-Asp-Lys-Pro is a natural and specific substrate of the N-terminal active site of human angiotensin-converting enzyme

Angiotensin I-converting enzyme (ACE) is a zinc-dipeptidyl carboxypeptidase, which contains two similar domains, each possessing a functional active site. Respective involvement of each active site in the degradation of the circulating peptide N-acetyl-seryl-aspartyl-lysyl-proline (AcSDKP), a negative regulator of hematopoietic stem cell proliferation, was studied by using wild-type recombinant ACE and two full-length mutants containing a single functional site. Both the N- and C-active sites of ACE exhibit dipeptidyl activity toward AcSDKP, with Km values of 31 and 39 microM, respectively. However, the N-active site hydrolyzes the peptide 50 times faster compared with the C-active site, with kcat/Km values of 0.5 and 0.01 microM-1.s-1, respectively. The predominant role of the N-active site in AcSDKP hydrolysis was confirmed by the inhibition of hydrolysis using a monoclonal antibody specifically directed against the N-active site. The N-domain specificity for AcSDKP will aid the identification of specific inhibitors for this domain. This is the first report of a highly specific substrate for the N-active site of ACE, with kinetic constants in the range of physiological substrates, suggesting that ACE might be involved via its N-terminal active site in the in vivo regulation of the local concentration of this hemoregulatory peptide.

Iron supplementation for unexplained fatigue in non-anaemic women: double blind randomised placebo controlled trial

OBJECTIVE

To determine the subjective response to iron therapy in non-anaemic women with unexplained fatigue.

DESIGN

Double blind randomised placebo controlled trial.

SETTING

Academic primary care centre and eight general practices in western Switzerland.

PARTICIPANTS

144 women aged 18 to 55, assigned to either oral ferrous sulphate (80 mg/day of elemental iron daily; n=75) or placebo (n=69) for four weeks.

MAIN OUTCOME MEASURES

Level of fatigue, measured by a 10 point visual analogue scale.

RESULTS

136 (94%) women completed the study. Most had a low serum ferritin concentration; <or= 20 microg/l in 69 (51%) women. Mean age, haemoglobin concentration, serum ferritin concentration, level of fatigue, depression, and anxiety were similar in both groups at baseline. Both groups were also similar for compliance and dropout rates. The level of fatigue after one month decreased by -1.82/6.37 points (29%) in the iron group compared with -0.85/6.46 points (13%) in the placebo group (difference 0.95 points, 95% confidence interval 0.32 to 1.62; P=0.004). Subgroups analysis showed that only women with ferritin concentrations <or= 50 microg/l improved with oral supplementation.

CONCLUSION

Non-anaemic women with unexplained fatigue may benefit from iron supplementation. The effect may be restricted to women with low or borderline serum ferritin concentrations.

Antiandrogenic effect of spirolactones: mechanism of action

Spirolactones are aldosterone antagonists which inhibit the binding of aldosterone to the renal mineralocorticoid receptor. These molecules also possess an antiandrogenic effect which could be due, among other possibilities, to a peripheral antagonism of androgens. This hypothesis has been tested in the present study. From in vivo experiments, spironolactone K+ canrenoate appear to inhibit the binding of [3H]5alpha-dihydrotestosterone [3H]DHT to the cytosolic and nuclear receptor of the rat ventral prostate. The doses used are in the same range as those used for demonstrating the antimineralocorticoid effect of these molecules. In vitro incubations and in vitro displacement studies show that spironolactone and K+ canrenoate are respectively about 20 and 100 times less effective than DHT in displacing 50 percent of 5 times 10- minus 10 M [3H]DHT from its receptor. Spirolactones are also able to compete with [3H]DHT for the specific 8 S cytosolic receptor. Neither spironolactone nor K+ canrenoate decreases prostatic 5alpha-reductase activity, even at a concentration as high as 10- minus 5 M. It seems likely that spirolactones, besides their action on testosterone biosynthesis, exert their antiandrogenic activity via a peripheral androgen antagonism.

RXP 407, a phosphinic peptide, is a potent inhibitor of angiotensin I converting enzyme able to differentiate between its two active sites

The human somatic angiotensin converting enzyme (ACE) contains two homologous domains, each bearing a zinc-dependent active site. All of the synthetic inhibitors of this enzyme used in clinical applications interact with these two active sites to a similar extent. Recently, several lines of evidence have suggested that the N-terminal active site of ACE might be involved in specific hydrolysis of some important physiological substrates, like Acetyl-Seryl-Aspartyl-Lysyl-Proline, a negative regulator of hematopoietic stem cell differentiation and proliferation. These findings have stimulated studies aimed at identifying new ACE inhibitors able to block only one of the two active sites of this enzyme. By screening phosphinic peptide libraries, we discovered a phosphinic peptide Ac-Asp-(L)Phepsi(PO2-CH2)(L)Ala-Ala-NH2, called RXP 407, which is able to differentiate the two ACE active sites, with a dissociation constant three orders of magnitude lower for the N-domain of the enzyme. The usefulness of a combinatorial chemistry approach to develop new lead structures is underscored by the unusual chemical structure of RXP 407, as compared with classical ACE inhibitors. As a highly potent and selective inhibitor of the N-terminal active site of wild ACE (Ki = 12 nM), RXP 407, which is metabolically stable in vivo, may lead to a new generation of ACE inhibitors able to block in vivo only a subset of the different functions regulated by ACE.

Skeletal muscle microbiopsy: a validation study of a minimally invasive technique

The study of the peripheral skeletal muscle function in patients with chronic obstructive pulmonary disease (COPD) is of growing interest, but often requires biopsies, usually with the Bergström technique. The current study was designed to test the validity of a minimally invasive technique: the microbiopsy. In 17 patients with COPD and four normal subjects, two specimens of the vastus lateralis were taken percutaneously under local anaesthesia, one with a 16-gauge needle (microbiopsy) and the other with the Bergström needle. The enzymatic activity of citrate synthase (CS) and phosphofructokinase (PFK), and the myosin heavy chain (MyoHC) composition were measured for both techniques. The subjects reported no pain or much less with the microbiopsy compared with the Bergström biopsy. The microbiopsy sample weight reached 55+/-17 mg. The two techniques showed excellent agreement for CS activity and MyoHC composition. The PFK activity did not differ statistically between the techniques, but the agreement was moderate. The agreement between both biopsy techniques was stable over time. The median (range) fibre number within the microbiopsy specimens was 144 (38-286). In conclusion, the current study shows the feasibility and validity of a minimally invasive muscle biopsy technique that appears more comfortable for subjects, compared with the Bergström technique.

Hypoxaemia enhances peripheral muscle oxidative stress in chronic obstructive pulmonary disease

BACKGROUND

Because oxidative stress affects muscle function, the underlying mechanism to explain exercise induced peripheral muscle oxidative stress in patients with chronic obstructive pulmonary disease (COPD) is clinically relevant. This study investigated whether chronic hypoxaemia in COPD worsens peripheral muscle oxidative stress and whether an abnormal muscle inflammatory process is associated with it.

METHODS

Nine chronically hypoxaemic and nine non-hypoxaemic patients performed repeated knee extensions until exhaustion. Biopsy specimens were taken from the vastus lateralis muscle before and 48 hours after exercise. Muscle oxidative stress was evaluated by lipid peroxidation (lipofuscin and thiobarbituric acid reactive substances (TBARs)) and oxidised proteins. Inflammation was evaluated by quantifying muscle neutrophil and tumour necrosis factor (TNF)-alpha levels.

RESULTS

When both groups were taken together, arterial oxygen pressure was positively correlated with quadriceps endurance time (n = 18, r = 0.57; p < 0.05). At rest, quadriceps lipofuscin inclusions were significantly greater in hypoxaemic patients than in non-hypoxaemic patients (2.9 (0.2) v 2.0 (0.3) inclusions/fibre; p < 0.05). Exercise induced a greater increase in muscle TBARs and oxidised proteins in hypoxaemic patients than in non-hypoxaemic patients (40.6 (9.1)% v 10.1 (5.8)% and 51.2 (11.9)% v 3.7 (12.2)%, respectively, both p = 0.01). Neutrophil levels were significantly higher in hypoxaemic patients than in non-hypoxaemic patients (53.1 (11.6) v 21.5 (11.2) counts per fibre x 10(-3); p < 0.05). Exercise did not alter muscle neutrophil levels in either group. Muscle TNF-alpha was not detected at baseline or after exercise.

CONCLUSION

Chronic hypoxaemia was associated with lower quadriceps endurance time and worsened muscle oxidative stress at rest and after exercise. Increased muscle neutrophil levels could be a source of the increased baseline oxidative damage. The involvement of a muscle inflammatory process in the exercise induced oxidative stress of patients with COPD remains to be shown.

Recent advances in knowledge of the structure and function of the angiotensin I converting enzyme

AIM

To review the structure and function of the angiotensin I converting enzyme (ACE), focusing on recent results from studies using a wide range of molecular biological techniques.

ACE STRUCTURE AND FUNCTION

ACE is an ectoenzyme expressed as two isoenzymes in mammals, a larger somatic form found in endothelial, epithelial and neuronal tissues and a smaller form in germinal tissues. Both forms have similar enzymatic activities but differ in size and immunological properties. The somatic form of ACE is composed of two highly homologous domains (amino and carboxyl domains) while the germinal form contains only one domain. Somatic ACE has two functional catalytic sites, both dependent on a zinc cofactor. Each ACE domain has also been shown to interact differently with competitive inhibitors.

MECHANISM OF ACE ANCHORAGE AND SOLUBILIZATION

The mechanism for anchoring ACE to the cell membrane has also been reported, and the solubilization step outlined. The relationship between the membrane-bound and soluble forms has been investigated, and the physiological relevance of this mechanism discussed.

GENETIC STRUCTURE

The structure of the ACE gene has been determined and the distribution in cells and different tissues has been reported in various studies.

CONCLUSION

All results have indicated that there are important functional and structural differences between the two domains, but at present ACE cannot be considered a true bifunctional enzyme, even though an exclusive substrate has been identified for the amino domain.

The Drosophila melanogaster-related angiotensin-I-converting enzymes Acer and Ance--distinct enzymic characteristics and alternative expression during pupal development

Drosophila melanogaster express two distinct angiotensin-I-converting enzymes (ACEs) called Ance and Acer, which display a high level of primary structure similarity. We have expressed Acer in the yeast Pichia pastoris and purified the recombinant enzyme with a view to developing biochemical tools to distinguish between Acer and Ance. Purified Acer and Ance expressed in yeast were used to raise anti-Acer Ig and anti-Ance Ig that specifically cross-reacted with the respective enzyme on immunoblotting, but did not act as specific inhibitors. Acer cleaves the C-terminal dipeptides from benzoylglycyl-histidyl-leucine and [Leu5]enkephalin, and Acer and Ance are both able to act as endopeptidases, releasing the C-terminal dipeptideamide from [Leu5]enkephalinamide. However, Acer hydrolyses this substrate at a slightly faster rate than [Leu5]enkephalin, whereas Ance hydrolyses the peptide with a free C-terminus with a kcat 15-fold higher than [Leu5]enkephalinamide. In addition, Acer did not cleave angiotensin I. In contrast, Ance hydrolysed 25% of this substrate at an 8-fold lower enzyme concentration. Furthermore, Acer did not hydrolyse the synthetic substrates Phe-Ser-Pro-Arg-Leu-Gly-Arg-Arg and Phe-Ser-Pro-Arg-Leu-Gly-Lys-Arg, two partially processed putative locustamyotropin precursors, under conditions where Ance produced 82% substrate hydrolysis. Acer was inhibited by captopril, trandolaprilat and enalaprilat, with apparent Ki values in the nanomolar range, whereas lisinopril and fosinoprilat were less potent. We show that the two Drosophila ACEs are alternatively expressed in stages P1 (white puparium)-P15 (eclosion) of pupal development; Ance is expressed predominantly during stages P4-P7, whereas the ACE activity expressed during stages P9-P12 is mainly due to Acer suggesting different roles for the two enzymes during pupal development.

Profiles of a healthful diet and its relationship to biomarkers in a population sample from Mediterranean southern France

OBJECTIVES

The failure of single-nutrient supplementation to prevent disease in intervention studies underlines the necessity to develop a holistic view of food intake. The objectives of this study were to devise a diet quality index (DQI) and identify biomarkers of multidimensional dietary behavior.

DESIGN

A nutrition survey was conducted in Mediterranean southern France by means of a food frequency questionnaire. The DQI was based on current dietary recommendations for prevention of diet-related diseases such as cardiovascular disease and some cancers. A second DQI included tobacco use.

STATISTICAL ANALYSES

performed Spearman rank correlations, cross-classifications and intraclass correlations were computed between the DQI and biomarkers.

RESULTS

Of the 146 subjects, 10 had a healthful diet and 18 had a poor diet. Erythrocyte omega-3 fatty acids-eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)-beta carotene, and vitamin E concentrations were lower and cholesterol concentrations were higher in the poor diet; the difference was significant for EPA and DHA and borderline significant for vitamin E. Significant correlation was found between the DQI and vitamin E (-0.12), EPA (-0.30), and DHA (-0.28), and beta carotene (-0.17) when tobacco use was considered, but not between the DQI and cholesterol. The correlation coefficient reached 0.58 (P0.01) for a composite index based on all biomarkers except cholesterol.

CONCLUSIONS

Subjects with a beta carotene levels greater thanl micromol/L, vitamin E greater than 30 micromol/L and EPA greater than 0.65% and DHA greater than 4% of fatty acids in erythrocytes were likely to have a healthful diet. Each biomarker indicated the quality of diet, but correlation was higher with a composite index.

Identification and Characterization of Conjugated Fatty Acid Methyl Esters of Mixed Double Bond Geometry by Acetonitrile Chemical Ionization Tandem Mass Spectrometry

Fatty acids with conjugated double bonds have attracted great interest because of their reported potent bioactivities. However, there are currently no rapid methods for their structural characterization. We report here a convenient mass spectrometry-based strategy to establish double bond geometry by analysis of collisional dissociation products of cis/trans and trans/cis conjugated linoleic acids (CLAs), as methyl esters, and to distinguish CLAs from homoallylic (methylene-interrupted) fatty acids in a single-stage mass spectrum. A series of CLA standards with double bond positions 6,8; 7,9; 8,10; 9,11; 10,12; 11,13; 12,14; and 13,15, with all four possible geometries (cis/trans; trans/cis; cis/cis; trans/trans) were analyzed. The m/z 54 (1-methyleneimino)-1-ethenylium ion, generated by self-reaction of acetonitrile under chemical ionization conditions, reacts with unsaturated fatty acids to yield an [M + 54]+ ion, which decomposes in the single-stage mass spectrum by loss of neutral methanol to form [M + 54 - 32]+. The ratio of [M + 54]+/[M + 54 - 32]+ in the single-stage mass spectra of CLA isomers is 1 order of magnitude less than for homoallylic diene FAME. Collisional dissociation of the [M + 54]+ ion yields two diagnostic ions that contain the alpha- and omega-carbon atoms and is characteristic of double bond position in the analyte. The fragment vinylic to the trans double bond is significantly more abundant than that for the cis double bond, revealing double bond geometry. The ratio of alpha to we diagnostic ion abundances is >4.8 for cis/trans isomers, <0.5 for trans/cis isomers, and 0.7-3.2 for cis/cis and trans/trans isomers. This method provides a rapid alternative to conventional conjugated fatty acid analysis and, together with complementary elution time information provided by gas chromatography, enables rapid, positive identification of double bond position and geometry in most CLA FAME.

Double bond localization in minor homoallylic fatty acid methyl esters using acetonitrile chemical ionization tandem mass spectrometry

Double bond position in natural fatty acids is critical to biochemical properties, however, common instrument-based methods cannot locate double bonds in fatty acid methyl esters (FAME), the predominant analysis form of fatty acids. A recently described mass spectrometry (MS) method for locating double bonds in FAME is reported here for the analysis of minor (<1%) components of real FAME mixtures derived from three natural sources; golden algae (Schizochytrium sp.), primate brain white matter, and transgenic mouse liver. Acetonitrile chemical ionization tandem MS was used to determine double bond positions in 39 FAME, most at concentrations well below 1% of all fatty acid methyl esters. FAME identified in golden algae are 14:1n-6, 14:3n-3, 16:1n-7, 16:2n-6, 16:3n-6, 16:3n-3, 16:4n-3, 18:2n-7, 18:3n-7, 18:3n-8, 18:4n-3, 18:4n-5, 20:3n-7, 20:4n-3, 20:4n-5, 20:4n-7, 20:5n-3, and 22:4n-9. Additional FAME identified in primate brain white matter are 20:1n-7, 20:1n-9, 20:2n-7, 20:2n-9, 22:1n-7, 22:1n-9, 22:1n-13, 22:2n-6, 22:2n-7, 22:2n-9, 22:3n-6, 22:3n-7, 22:3n-9, 22:4n-6, 24:1n-7, 24:1n-9, and 24:4n-6. Additional FAME identified in mouse liver are 26:5n-6, 26:6n-3, 28:5n-6, and 28:6n-3. The primate brain 22:3n-7 and algae 18:4n-5 are novel fatty acids. These results demonstrate the usefulness of the technique for analysis of real samples. Tables are presented to aid in interpretation of acetonitrile CIMS/MS spectra.

Drosophila melanogaster angiotensin I-converting enzyme expressed in Pichia pastoris resembles the C domain of the mammalian homologue and does not require glycosylation for secretion and enzymic activity

Drosophila melanogaster angiotensin I-converting enzyme (AnCE) is a secreted single-domain homologue of mammalian angiotensin I-converting enzyme (ACE) which comprises two domains (N and C domains). In order to characterize in detail the enzymic properties of AnCE and to study the influence of glycosylation on the secretion and enzymic activity of this enzyme, we overexpressed AnCE (expression level, 160 mg/l) and an unglycosylated mutant (expression level, 43 mg/l) in the yeast Pichia pastoris. The recombinant enzyme was apparently homogeneous on SDS/PAGE without purification and partial deglycosylation demonstrated that all three potential sites for N-linked glycosylation were occupied by oligosaccharide chains. Each N-glycosylation sequence (Asn-Xaa-Ser/Thr) was disrupted by substituting a glutamine for the asparagine residue at amino acid positions 53, 196 and 311 by site-directed mutagenesis to produce a single mutant. Expression of the unglycosylated mutant in Pichia produced a secreted catalytically active enzyme (AnCE delta CHO). This mutant displayed unaltered kinetics for the hydrolyses of hippuryl-His-Leu, angiotensin 1 and N-acetyl-Ser-Asp-Lys-Pro (AcSDKP) and was equally sensitive to ACE inhibitors compared with wild-type AnCE. However, AnCE delta CHO was less stable, displaying a half-life of 4.94 h at 37 degrees C, compared with AnCE which retained full activity under the same conditions. Two catalytic criteria demonstrate the functional resemblance of AnCE with the human ACE C domain: first, the kcat/Km of AcSDKP hydrolysis and secondly, the kcat/Km and optimal chloride concentration for hippuryl-His-Leu hydrolysis. A range of ACE inhibitors were far less potent towards AnCE compared with the human ACE domains, except for captopril which suggests an alternative structure in AnCE corresponding to the region of the S1 subsite in the human ACE active sites.

Substrate dependence of angiotensin I-converting enzyme inhibition: captopril displays a partial selectivity for inhibition of N-acetyl-seryl-aspartyl-lysyl-proline hydrolysis compared with that of angiotensin I

Angiotensin I-converting enzyme (ACE) is composed of two highly similar domains (referred to here as the N and C domains) that play a central role in blood pressure regulation; ACE inhibitors are widely used in the treatment of hypertension. However, the negative regulator of hematopoiesis, N-acetyl-seryl-aspartyl-lysyl-prolyl (AcSDKP), is a specific substrate of the N domain-active site; thus, in addition to the cardiovascular function of ACE, the enzyme may be involved in hematopoietic stem cell regulation, raising the interest of designing N domain-specific ACE inhibitors. We analyzed the inhibition of angiotensin I and AcSDKP hydrolysis as well as that of three synthetic ACE substrates by wild-type ACE and the N and C domains by using a range of specific ACE inhibitors. We demonstrate that captopril, lisinopril, and fosinoprilat are potent inhibitors of AcSDKP hydrolysis by wild-type ACE, with K(i) values in the subnanomolar range. However, of the inhibitors tested, captopril is the only compound able to differentiate to some degree between AcSDKP and angiotensin I inhibition of hydrolysis by wild-type ACE: the K(i) value with AcSDKP as substrate was 16-fold lower than that with angiotensin I as substrate. This raises the possibility of using captopril to enhance plasma AcSDKP levels with the aim of normal hematopoeitic stem cell protection during chemotherapy and a limited effect on the cardiovascular function of ACE.

Cell surface localization of proteolysis of human endothelial angiotensin I-converting enzyme. Effect of the amino-terminal domain in the solubilization process

Angiotensin-converting enzyme (ACE) belongs to the type I class of ectoproteins and is solubilized by Chinese hamster ovary cells transfected with the full-length human ACE cDNA. ACE release in Chinese hamster ovary cells involves a proteolytic cleavage occurring in the carboxyl-terminal region, between Arg-1137 and Leu-1138. The subcellular localization of ACE proteolysis was established by pulse-chase experiments, cell surface immunolabeling, and biotinylation of radiolabeled mature proteins. The proteolysis of ACE takes place primarily at the plasma membrane. The solubilization of ACE is less than 2% within 1 h, is increased 2.4-fold by phorbol esters, but is not influenced by ionophores. An ACE mutant lacking the transmembrane domain and the cytosolic part (ACE delta COOH), is secreted at a faster rate without a carboxyl-terminal cleavage, and phorbol esters or ionophores have no effect on its rate of production in the medium. Therefore, the proteolysis of ACE is dependent on the presence of the membrane anchor and suggests that the secretase(s) involved is also membrane-associated. An ACE mutant lacking the amino-terminal domain (ACECF) is secreted 10-fold faster compared with wild-type ACE. The solubilization of ACECF occurs at the plasma membrane and is stimulated 2.7-fold by phorbol esters, and the cleavage site is localized between Arg-1227 and Val-1228. The amino-terminal domain of ACE slows down the proteolysis and seems to act as a "conformational inhibitor" of the proteolytic process, possibly via interactions with the "stalk" of ACE and the secretase(s) itself.

Study of asparagine 353 in aminopeptidase A: characterization of a novel motif (GXMEN) implicated in exopeptidase specificity of monozinc aminopeptidases

Aminopeptidase A (EC 3.4.11.7, APA) is a 160 kDa membrane-bound zinc enzyme that contains the HEXXH consensus sequence found in members of the zinc metalloprotease family, the zincins. In addition, the monozinc aminopeptidases are characterized by another conserved motif, GXMEN, the glutamate residue of which has been shown to be implicated in the exopeptidase specificity of aminopeptidase A [Vazeux G. (1998) Biochem. J. 334, 407-413]. In carboxypeptidase A (EC 3.4.17.1, CPA), the exopeptidase specificity is conferred by an arginine residue (Arg-145) and an asparagine residue (Asn-144). Thus, we hypothesized that Asn-353 of the GXMEN motif in APA plays a similar role to Asn-144 in CPA and contributes to the exopeptidase specificity of APA. We investigated the functional role of Asn-353 in APA by substituting this residue with a glutamine (Gln-353), an alanine (Ala-353) or an aspartate (Asp-353) residue by site-directed mutagenesis. Expression of wild-type and mutated APAs revealed that Gln-353 and Ala-353 are similarly routed and glycosylated to the wild-type APA, whereas Asp-353 is trapped intracellularly and partially glycosylated. Kinetic studies, using alpha-L-glutamyl-beta-naphthylamide (GluNA) as a substrate showed that the K(m) values of the mutants Gln-353 and Ala-353 were increased 11- and 8-fold, respectively, whereas the k(cat) values were decreased (2-fold) resulting in a 24- and 14-fold reduction in cleavage efficiency. When alpha-L-aspartyl-beta-naphthylamide or angiotensin II were used as substrates, the mutations had a greater effect on k(cat), leading to a similar decrease in cleavage efficiencies as that observed with GluNA. We then measured the inhibitory potencies of several classes of inhibitors, glutamate thiol, glutamine thiol and two isomers (L- or D-) of glutamate phosphonate to explore the functional role of Asn-353. The data indicate that Asn-353 is critical for the integrity and catalytic activity of APA. This residue is involved in substrate binding via interactions with the free N-terminal part and with the P1 carboxylate side chain of the substrate. In conclusion, Asn-353 of the GXMEN motif, together with Glu-352, contributes to the exopeptidase specificity of APA and plays an equivalent role to Asn-144 in CPA.

Increased shedding of angiotensin-converting enzyme by a mutation identified in the stalk region

Angiotensin-converting enzyme (ACE), an enzyme that plays a major role in vasoactive peptide metabolism, is a type 1 ectoprotein, which is released from the plasma membrane by a proteolytic cleavage occurring in the stalk sequence adjacent to the membrane anchor. In this study, we have discovered the molecular mechanism underlying the marked increase of plasma ACE levels observed in three unrelated individuals. We have identified a Pro(1199) --> Leu mutation in the juxtamembrane stalk region. In vitro analysis revealed that the shedding of [Leu(1199)]ACE was enhanced compared with wild-type ACE. The solubilization process of [Leu(1199)]ACE was stimulated by phorbol esters and inhibited by compound 3, an inhibitor of ACE-secretase. The results of Western blot analysis were consistent with a cleavage at the major described site (Arg(1203)/Ser(1204)). Two-dimensional structural analysis of ACE showed that the mutated residue was critical for the positioning of a specific loop containing the cleavage site. We therefore propose that a local conformational modification caused by the Pro(1199) --> Leu mutation leads to more accessibility at the stalk region for ACE secretase and is responsible for the enhancement of the cleavage-secretion process. Our results show that different molecular mechanisms are responsible for the common genetic variation of plasma ACE and for its more rare familial elevation.

Mechanism of action of a new antialdosterone compound, prorenone

Two new aldosterone antagonists, K-prorenoate [potassium 3(17 beta-hydroxy-6 beta, 7 beta-methylen-3-oxo-4-androsten-17 alpha-yl)propionate] and prorenone [3(17 beta-hydroxy-6 beta, 7 beta-methylen-3-oxo-4-androsten-17 alpha-yl) propionic acid gamma-lactone], its lactonic form, were studied in rat kidney using in vitro systems. Study of [3H]prorenone binding by a recently developed computer method indicated a high affinity, low capacity class of sites which are, seemingly, mineralocorticoid receptors. In competition experiments performed on [3H]aldosterone- and [3H]dexamethasone-binding sites, prorenone appeared to be a good competitor for mineralocorticoid-binding sites and a poor competitor for glucocorticoid-binding sites. The specificity of this molecule was further confirmed by its poor ability to displace [3H]dihydrotestosterone from rat prostate androgenic receptors compared to spironolactone [3-(3-oxo-7 alpha-acetylthio-17 beta-hydroxy-4-androsten-17 alpha-yl) propionic acid gamma-lactone]. In the same experiments, K-prorenoate demonstrated a very low affinity for the two types of receptors. The behavior of [3H]prorenone cytosolic complex was also studied in kidney mince experiments, which showed that the [3H]prorenone complex was not able to translocate into the nucleus. Prorenone inhibited the binding of [3H]aldosterone to the receptor and, consequently, the nuclear binding of aldosterone was not observed.

Affinity of corticosteroids for mineralocorticoid and glucocorticoid receptors of the rabbit kidney: effect of steroid substitution

Corticosteroid derivatives coupled in the C3, C7 or C17 position with a long aliphatic chain were synthesized in order to select a suitable ligand for the preparation of a biospecific affinity adsorbent for mineralocorticoid receptor purification. The affinity of these derivatives for mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) was explored in rabbit kidney cytosol. In this model, aldosterone bound to a single class of receptors with high affinity (Kd 1 nM) and mineralocorticoid specificity. RU26988, a highly specific ligand for GR, did not compete for these sites. The C7 and C17 positions were found to be of crucial importance in the steroid's interaction with the mineralocorticoid receptors, since the linkage of a long side chain in these positions induced complete loss of affinity. Hence, deoxycorticosterone no longer bound to MR after 17 beta substitution with a 9-carbon aliphatic chain. This loss of affinity was not observed for glucocorticoids. The 17 beta nonylamide derivative of dexamethasone still competed for GR. Increasing the length of the C7 side of the spirolactone SC26304 suppressed its affinity for MR. Finally, C3 was an appropriate position for steroid substitution. The 3-nonylamide of carboxymethyloxime deoxycorticosterone bound to MR but not to GR, and therefore constitutes a suitable ligand for the preparation of a mineralocorticoid adsorbent.

Ion excitation in a linear quadrupole ion trap with an added octopole field

Modeling of ion motion and experimental investigations of ion excitation in a linear quadrupole trap with a 4% added octopole field are described. The results are compared with those obtained with a conventional round rod set. Motion in the effective potential of the rod set can explain many of the observed phenomena. The frequencies of ion oscillation in the x and y directions shift with amplitude in opposite directions as the amplitudes of oscillation increase. Excitation profiles for ion fragmentation become asymmetric and in some cases show bistable behavior where the amplitude of oscillation suddenly jumps between high and low values with very small changes in excitation frequency. Experiments show these effects. Ions are injected into a linear trap, stored, isolated, excited for MS/MS, and then mass analyzed in a time-of-flight mass analyzer. Frequency shifts between the x and y motions are observed, and in some cases asymmetric excitation profiles and bistable behavior are observed. Higher MS/MS efficiencies are expected when an octopole field is added. MS/MS efficiencies (N(2) collision gas) have been measured for a conventional quadrupole rod set and a linear ion trap with a 4% added octopole field. Efficiencies are chemical compound dependent, but when an octopole field is added, efficiencies can be substantially higher than with a conventional rod set, particularly at pressures of 1.4 x 10(-4) torr or less.

Straight-chain acyl-CoA oxidase knockout mouse accumulates extremely long chain fatty acids from alpha-linolenic acid: evidence for runaway carousel-type enzyme kinetics in peroxisomal beta-oxidation diseases

Extremely long chain polyunsaturated fatty acids (ELCPs) with >24 carbons and four or more double bonds are normally found in excitatory tissues but have no known function, and are greatly increased in brain and other tissues of humans with peroxisomal disorders. Straight-chain acyl-CoA oxidase (AOX) catalyzes the first, rate-limiting step of peroxisomal beta-oxidation of very-long-chain saturated and unsaturated fatty acids. We have studied the polyunsaturated fatty acid metabolism of AOX knockout mice (AOX-/- as a model of human AOX deficiency (pseudo-neonatal adrenoleukodystrophy), and as a genetic tool to test the putative peroxisomal beta-oxidation involvement in polyunsaturated fatty acid synthesis. Liver lipids of 26-day-old weanling AOX-/- mice livers accumulate n-3 and n-6 ELCPs from C24 to C30 with 5 and 6 double bonds, have 356 +/- 66 microg/g docosahexaenoic acid (22:6n-3), similar to congenic (AOX -/* = AOX+/+ and AOX+/-) controls (401 +/- 96 microg/g), but increased 22:5n-6 (22.4 +/- 3.7 vs 6.4 +/- 1.5 microg/g). AOX+/* mice injected intraperitoneally at 23 days with [U-(13)C]-18:3n-3 show strong labeling of 22:6n-3 after 72 h, whereas AOX -/- mice display less labeling of 22:6n-3 but strong tracer incorporation into 24:6n-3, 26:6n-3, and 28:6n-3, after the same period. These data suggest that ELCPs are natural runaway elongation by-products of 22:6n-3 and 22:5n-6 synthesis, which are normally disposed of by peroxisomal beta-oxidation. Under conditions with impaired peroxisomal beta-oxidation, such as Zellweger syndrome and adrenoleukodystrophies, ELCPs accumulate due to increased synthesis and impaired disposal. Two mechanisms for the formation of these runaway elongation by-products and the involvement of secondary carnitine deficiency in this process are proposed: n-3 ELCPs are synthesized by a carnitine-dependent multifunctional mitochondrial docosahexaenoic acid synthase (mtDHAS) which normally synthesizes primarily 22:6n-3, while n-6 ELCPs are synthesized by independent elongation enzymes in the endoplasmic reticulum.