The angiotensin I-converting enzyme (kininase II): molecular organization and regulation of its expression in humans

Protein sequencing and molecular cloning of human endothelial angiotensin I-converting enzyme (ACE; kininase II), have led to a description of the structure of the enzyme and to several questions concerning the intracellular maturation of ACE and the mechanisms of enzyme action. With the help of recombinant ACE expression in mammalian cells and site-directed mutagenesis, a model for the maturation of ACE in endothelial cells has been proposed. This model comprises transmembrane anchoring of the membrane-bound ACE near its carboxyterminal extremity, and post-translational cleavage of the anchor in the secreted form. The endothelial ACE displays a high degree of internal homology between two large peptidic domains that each bears a consensus sequence for zinc binding and therefore a putative active site. The testicular ACE, however, encoded from the same gene by a shorter mRNA, contains only the carboxyterminal half of endothelial ACE and therefore a single active site. Expression of ACE mutants with only one intact homologous domain, however, indicates that in endothelial ACE both domains are enzymatically active. Further characterization of these two active sites of endothelial ACE is in progress. In humans, population studies have indicated that the large interindividual variability in plasma ACE levels is partly genetically determined and under the influence of a major gene effect. This was later confirmed and extended by the observation of an insertion-deletion polymorphism of the ACE gene that is associated with the level of ACE in plasma. The clinical implications of these observations are discussed.

Pentobarbital produces dissimilar changes in glucose influx and utilization in brain

The effects of pentobarbital sodium on local cerebral glucose utilization (LCGU) and 3-O-methylglucose (3-MG) influx were measured by quantitative autoradiography in 52 brain areas of control and treated rats. Pentobarbital (50 mg/kg ip) lowered LCGU to a relatively uniform rate (approximately 35 mumol.100 g-1.min-1) in 24 of 25 forebrain areas. Among the 18 hindbrain areas, LCGU was decreased by pentobarbital by 15-55% (range 50-157 and 28-110 mumol.100 g-1.min-1 in control and treated rats, respectively). In contrast, pentobarbital lowered the 3-MG influx rate constant and permeability-surface area product by 20-30% in nearly all brain structures. The 3-MG results fit a model in which both the half-saturation constant and the maximal velocity of the glucose carrier are decreased by pentobarbital. After pentobarbital treatment, the ratio of local cerebral plasma flow (LCPF) to LCGU was the same as in controls for brain areas in which LCGU was less than 35 mumol.100 g-1.min-1 but was higher in brain areas where LCGU exceeded 35 mumol.100 g-1.min-1. Pentobarbital produced dissimilar changes in LCGU, 3-MG influx, and LCPF; these processes may thus not be closely linked during pentobarbital anesthesia.

Variation in local cerebral blood flow response to high-dose pentobarbital sodium in the rat

Microvascular bed structure and functions are known to vary throughout the brain. Microvascular responses to high doses of pentobarbital sodium might therefore differ among brain areas. This possibility was examined by measuring local cerebral blood flow (LCBF) with [14C]iodoantipyrine in 52 brain areas at 5, 10, 25, and 60 min after intraperitoneal administration of pentobarbital (50 mg/kg). From 5 to 60 min, LCBF was significantly lowered in 17 of 25 forebrain gray matter areas but in only 1 of 18 hindbrain gray matter structures, the pontine nuclei. Smaller, shorter duration lowering of LCBF was also observed in ten other brain areas. In both control and treated rats, LCBF was found to vary within individual brain structures. The pattern of these LCBF variations was columnar in the cerebral cortex and the hippocampus but was patchy in the caudate-putamen, thalamus, and inferior colliculus. These results indicate that pentobarbital anesthesia more strongly alters LCBF in the forebrain than in the hindbrain and produces different patterns of changes in LCBF than in local cerebral glucose utilization, which was measured with 2-deoxyglucose in a companion study.

Functional variations in parenchymal microvascular systems within the brain

Variations in microvascular system functions were observed among a number of brain areas. The rates of local blood flow varied 18-fold among areas and were extremely high in neuroendocrine structures. Marked differences in blood flow were also found within some brain structures. The volume of radiolabeled blood in perfused parenchymal microvessels ranged from 5 to 70 microliters/g and correlated closely with local cerebral blood flow. The hematocrits within parenchymal microvessels were 45-75% of the arterial hematocrit, which indicates that red cells more rapidly traverse brain microvessels than do plasma proteins. The mean transit times of blood through parenchymal microvessels were extremely short and ranged from 0.3 to 0.6 s.

[Analysis of volatile oil in pericarpium Zanthoxyli]

Essential oils were extracted from the pericarp of Zanthoxylum bungeanum and Z. schinifolium. The chemical constituents were analysed by means of GC-MS and 33 compounds were identified. Their contents in the essential oil were also determined.

The two homologous domains of human angiotensin I-converting enzyme are both catalytically active

Molecular cloning of human endothelial angiotensin I-converting enzyme (kininase II; EC 3.4.15.1) (ACE) has recently shown that the enzyme contains two large homologous domains (called here the N and C domains), each bearing a putative active site, identified by sequence comparisons with the active sites of other zinc metallopeptidases. However, the previous experiments with zinc or competitive ACE inhibitors suggested a single active site in ACE. To establish whether both domains of ACE are enzymatically active, a series of ACE mutants, each containing only one intact domain, were constructed by deletion or point mutations of putative critical residues of the other domain, and expressed in heterologous Chinese hamster ovary cells. Both domains are enzymatically active and cleave the C-terminal dipeptide of hippuryl-His-Leu or angiotensin I. Moreover, both domains have an absolute zinc requirement for activity, are activated by chloride and are sensitive to competitive ACE inhibitors, and appear to function independently. However, the two domains display different catalytic constants and different patterns of chloride activation. At high chloride concentrations, the C domain hydrolyzes the two substrates tested faster than does the N domain. His-361,365 and His-959,963 are established as essential residues in the N and C domains, respectively, most likely involved in zinc binding, and Glu-362 in the N domain and Glu-960 in the C domain are essential catalytic residues. These observations provide strong evidence that ACE possesses two independent catalytic domains and suggest that they may have different functions.

Expression and characterization of recombinant human angiotensin I-converting enzyme. Evidence for a C-terminal transmembrane anchor and for a proteolytic processing of the secreted recombinant and plasma enzymes

Chinese hamster ovary (CHO) cells have been transfected with either a full-length cDNA encoding human angiotensin I-converting enzyme (kininase II; EC 3.4.15.1) (ACE) or a mutated cDNA, in which the last C-terminal 47 amino acids, including the putative transmembrane domain, are not translated. Cell lines expressing high levels of the wild-type ACE or the mutant were established. The cells transfected with the wild-type cDNA (CHO-ACE) express a membrane-bound ectoenzyme with an intracellular C terminus, as shown by indirect immunofluorescence using an antiserum (28A7) raised against a synthetic peptide corresponding to the deduced C terminus of ACE. This enzyme is structurally, immunologically, and enzymatically identical to human kidney ACE. In addition, CHO-ACE cells also produce a secreted form of the enzyme. Neither this secreted form nor the enzyme purified from human plasma is recognized by the antiserum 28A7, indicating that they undergo a truncation in the C-terminal region. On the other hand, the transfected cells expressing the C-terminally truncated mutant (CHO-ACE delta COOH) do not retain ACE in the plasma membrane, but secrete it into the medium. These results indicate that ACE is anchored to the plasma membrane by the predicted C-terminal transmembrane domain, and the secreted form is derived from the membrane-bound form by a post-translational proteolytic cleavage of the C-terminal region.

18-substituted steroids--Part 17. 2 alpha-hydroxylated liver metabolites of aldosterone identified by high-field [1H]NMR spectroscopy

11 beta,18-Epoxy-2 alpha,3 alpha,18,21-tetrahydroxy-5 alpha,17 alpha- pregnan-20-one (2 alpha-hydroxy-3 alpha,5 alpha-tetrahydro-17-isoaldosterone) and its apo isomer have been identified by high-field NMR studies, supported by thermospray HPLC/MS, to be among the major polar metabolites formed from incubation of aldosterone with rat liver microsomal fraction. Indications that unreduced 2 alpha-hydroxy-aldosterone is also present among the metabolites have still to be confirmed.

Regulation of angiotensinogen gene

The development of recombinant DNA technology has introduced new directions for the study of the angiotensinogen molecule. The cloning and sequencing of the human and rat cDNAs demonstrate the similarity of angiotensinogen to various serine protease inhibitors produced by the liver and was the beginning of studies looking for new physiological roles of angiotensinogen, in addition to the substrate for renin. The determination of the nucleotide sequence of these cDNAs also allowed the identification of angiotensinogen mRNA in many tissues in addition to the liver that is the major site of synthesis. This multilocalization of angiotensinogen is one of the arguments for the presence and the function of local renin-angiotensin systems. Finally, the hepatic biosynthesis of angiotensinogen is regulated by many different hormonal factors including glucocorticoid, estrogen, thyroid hormone, insulin, and angiotensin II. The cloning of the angiotensinogen gene offers the opportunity to study this regulation at the transcriptional level. We present in this paper a review of the literature concerning the new aspects of angiotensinogen using molecular biological tools and its regulation together with the characterization of the human angiotensinogen gene.

Expression and characterization of recombinant human angiotensinogen in a heterologous eukaryotic cell line

Transfection of Chinese hamster ovary cells with an expression plasmid containing a full length human angiotensinogen cDNA has provided cell lines that secrete recombinant angiotensinogen in large quantities. This angiotensinogen is immunologically identical to plasma angiotensinogen and can be cleaved by human kidney renin (EC 3.4.23.15.). The peptide liberated by renin cleavage is immunologically identical to standard angiotensin I and shows a retention time on isocratic reversed-phase high-pressure liquid chromatography identical to that of standard angiotensin I. The heterogeneity of recombinant angiotensinogen on sodium dodecyl sulfate-polyacrylamide gel electrophoresis differs from that of plasma angiotensinogen. Treatment with endoglycosidases demonstrated that this difference is restricted to that of N-glycans and that N-glycans correspond to the quasi-totality of the carbohydrate content of both recombinant and plasma angiotensinogens. The development of a system capable of expressing human angiotensinogen cDNA in mammalian cells and the ability to obtain the corresponding angiotensinogen in large quantities will allow new studies on structure-function relationships of this protein.

Diphtheria toxin resistance in human lymphocytes and lymphoblasts in the in vivo somatic cell mutation test

It has been shown that circulating peripheral blood lymphocytes can be used for the enumeration of 6-thioguanine-resistant cells that presumably arise by mutation in vivo. This somatic cell mutation test has been studied in lymphocytes from human populations exposed to known mutagens and/or carcinogens. The sensitivity of the test could be further enhanced by including other gene markers, since there is evidence for locus-specific differences in response to mutagens. Resistance to diphtheria toxin (Dipr) seemed like a potential marker to incorporate into the test because the mutation acts codominantly, can readily be selected in human diploid fibroblasts and Chinese hamster cells with no evidence for cell density or cross-feeding effects, and can be assayed for in nondividing cells by measuring protein synthesis inhibition. Blood samples were collected from seven individuals, and fresh, cryopreserved, or Epstein-Barr virus (EBV)-transformed lymphocytes were tested for continued DNA synthesis (3H-thymidine, autoradiography) or protein synthesis (35S-methionine, scintillation counting). Both fresh and cryopreserved lymphocytes, stimulated to divide with phytohemagglutinin (PHA), continued to synthesize DNA in the presence of high doses of diphtheria toxin (DT). Similarly, both dividing (PHA-stimulated) and nondividing fresh lymphocytes carried on significant levels of protein synthesis even 68 hr after exposure to 100 flocculating units (LF)/ml DT. The findings were confirmed in cord blood lymphocytes, ruling out the possibility that diphtheria immunization could have led to a selection of Dipr lymphocytes. One lymphoblast line (EBV-transformed lymphocytes) showed a reduction in protein synthesis to 0.2% of controls only at 192 hr after exposure to 100 LF/ml. The results suggest that human T and B lymphocytes may not be as sensitive to DT protein synthesis inhibition as human fibroblast and Chinese hamster cells. For this reason, Dipr may not be a suitable marker for the somatic cell mutation test.

Environmental chemodynamic studies with terbufos (Counter) insecticide in soil under laboratory and field conditions

The effect of temperature, soil moisture, soil type, adsorption, and formulation on terbufos persistence in soil was studied under laboratory conditions. Temperature appeared to be more important than moisture in influencing the dissipation rate of terbufos. More terbufos was recovered from a granular formulation than a technical formulation one month after incubation. Soil adsorption coefficient and calculated first order rate constant were well correlated. Terbufos was relatively immobile on soil thin layers and in field-located soil columns. Laboratory data agreed reasonably well with measurements of persistence in the field. Terbufos degradation kinetics were qualitatively analyzed and the toxicological significance of studying parent terbufos alone was discussed.

Chromosome-damaging activity of ferritin and its relation to chelation and reduction of iron

Ferritin from horse spleen was found to cause severe chromosome aberrations in cultured Chinese hamster ovary cells. Ferritin at 15 to 170 microgram/ml was clastogenic and at higher doses was cytotoxic. At comparable concentrations of protein or iron, neither apoferritin nor complexed iron was clastogenic. Sulfhydryl compounds glutathione and cysteine reduced the cytotoxic and clastogenic activities of ferritin. Physiological concentrations of glutathione may normally be sufficient to protect cells from damage. The reducing agent ascorbate had little protective effect. Chelating agents varied in their inhibitory activity: ethylenediaminetetraacetic acid (hexadentate) greater than nitrilotriacetic acid (tetradentate) greater than salicylate (bidentate). 2,2'-Bipyridyl enhance the chromosome-damaging action of ferritin while histidine did not markedly alter the frequencies of aberrations. Catalase and superoxide dismutase showed no inhibitory activity. The mechanism of DNA damage may involve reduction of Fe(III) in the ferritin core to Fe(II), followed by reoxidation of Fe(II) with possible formation of free radicals.

Unscheduled DNA synthesis and chromosome aberrations induced by inorganic and organic selenium compounds in the presence of glutathione

Glutathione strongly enhanced the induction of unscheduled DNA synthesis (UDS) in cultured human cells by inorganic selenium compounds: sodium selenate, sodium selenite and sodium selenide. In the presence of 10(-3) M glutathione, high levels of UDS (74-114 grains per nucleus) were observed in cells treated with (i) selenate at 10(-3) M, (ii) selenite at 10(-5)-3 X 10(-4) 7, and (iii) selenide at 10(-5)-10(-3) M. Glutathione at 10(-3) M also enhanced the clastogenic and cytotoxic effects of selenite and selenate in Chinese hamster ovary (CHO) cells. Glutathione at 10(-4) M or 10(-2) M caused less enhancement of DNA damage and toxicity in both the UDS and chromosome aberration assays. In the absence of glutathione, these inorganic selenium compounds induced low levels of UDS (up to 13 grains per nucleus) and moderate frequencies of chromosome aberrations (up to 11%). 3 organic selenium compounds (selenocystine, selenocystamine and selenomethionine) were also examined for the induction of UDS. No unscheduled DNA synthesis was detected in cells treated with selenocystamine or selenomethione, with or without added glutathione. However, selenocystine alone at 10(-4)-10(-3) M induced a low level of UDS; glutathione enhanced the DNA-damaging effect of selenocystine. The maximum amount of UDS (22 grains/nucleus) occurred in the presence of 10(-2) M glutathione. This was about one-fifth of that detected in cells treated with inorganic selenium compounds and 10-fold lower concentrations of glutathione (10(-3) M). The results suggest that recution is involved in the conversion of selenium compounds to mutagenic forms. The active mutagens may be selenols, GS-Se- from inorganic selenium and R-Se- from organic selenium compounds.

Enhancement by transition metals of unscheduled DNA synthesis induced by isoniazid and related hydrazines in cultured normal and xeroderma pigmentosum human cells

In combination with transition metals (Mn(II), Cu(II), and Fe(III)), isoniazid and related hydrazine compounds induced unscheduled DNA synthesis (DNA repair) in cultured human fibroblasts. Manganese at 10(-5) and 10(-4) M strongly enhanced DNA repair induced by isoniazid, iproniazid, nialamide and hydrazine. Peak levels of DNA repair occurred at 5 x 10(-4)--10(-3) M of the 4 hydrazine compounds. Copper caused less enhancement of DNA repair while iron had no detectable effect. Without added metal, unscheduled DNA synthesis was not observed in cells treated with any of the 4 freshly-prepared hydrazine compounds. However, following preincubation in medium for 6--12 h, isoniazid alone at high concentrations (10(-2) M--10(-1) M) induced DNA repair. With isoniazid/manganese mixtures, preincubation did not further enhance DNA repair except at low concentrations of isoniazid (2--5 x 10(-4) M). Catalase reduced the DNA damage caused by preincubated isoniazid and by the isoniazid/metal mixtures. Exposure of repair-deficient xeroderma pigmentosum cells to isoniazid plus manganese resulted in a DNA-repair profile similar to that of normal cells. The results are consistent with hydrogen peroxide being a critical intermediate for the production of free radicals which cause the observed DNA damage.

Enhancement of the chromosome-damaging action of ascorbate by transition metals

Freshly prepared ascorbate inhibited mitosis and induced chromosome aberrations in cultured Chinese hamster ovary cells. Cu(II) and Mn(II) (10(-4) or 10(-5) M) enhanced both actions. Fe(II) and Fe(III) (10(-4) or 10(-5) M) reduced or abolished the mitosis-inhibiting action of ascorbate. At 10(-4) M, Fe(II) and Fe(III) strongly enhanced the chromosome-damaging capacity of ascorbate. Up to 100% of all examined metaphase plates had multiple chromosome exchanges or breaks. Since the cytostatic and clastogenic effect of ascorbate of H2O2 to induce chromosome aberrations was examined. H2O2 and a H2O2: Fe(II) mixture (Fenton reagent) induced chromosome breaks and exchanges but to a lesser degree than did ascorbate: Cu(II), Mn(II), Fe(II), or Fe(III) mixtures. Whether the strong chromosome damaging capacity of ascorbate plus transition metals as seen in the in vitro test system poses a health hazard only properly designed in vivo studies can reveal.

The need for a mammalian test system for mutagens: action of some reducing agents

Reducing agents and cysteine, cysteamine, glutathione, ascorbic acid and H2O2 with and without the addition of Cu2+ did not increase significantly the frequency of mutations in the Salmonella test at non-toxic concentrations but triggered a marked DNA repair synthesis and induced a relatively high frequency of chromosome aberrations in cultured mammalian cells. Both latter effects were reduced by the addition of catalase to solutions of the reducing agents plus Cu2+. To avoid 'False Negatives' in mutagenicity screening the use of several test subjects including mammalian cells seems to be required.