Purification and partial characterization of human angiotensinogen

Comparison of angiotensinogen and tetradecapeptide as substrates for human renin. Substrate dependence of the mode of inhibition of renin by a statine-containing hexapeptide

The kinetic properties of two different substrates for human renin, a synthetic tetradecapeptide and the natural substrate human angiotensinogen, have been compared. While the Vmax was similar for the two substrates, the Km values differed by a factor of 10, i.e., 11.7 +/- 0.7 microM (tetradecapeptide) and 1.0 +/- 0.1 microM (angiotensinogen). The mode of inhibition of renin by a statine (Sta)-containing hexapeptide, BW897C, that is a close structural analog of residues 8-13 of human angiotensinogen (Phe-His-Sta-Val-Ile-His-OMe), was determined for the two substrates. Competitive inhibition was observed when tetradecapeptide was the substrate (Ki = 2.0 +/- 0.2 microM), but a more complex mixed inhibition mode (Ki = 1.7 +/- 0.1 microM, Ki' = 3.0 +/- 0.23 microM) was found with angiotensinogen as substrate. This mixed inhibition probably results from the formation of an enzyme-inhibitor-substrate or enzyme-inhibitor-product complex and reflects the more extensive interactions that the protein angiotensinogen, as opposed to the small tetradecapeptide substrate, can make with renin. We conclude that the mixed inhibition observed when angiotensinogen is used as renin substrate could be important in the clinical application of renin inhibitors because it is less readily reversed by increased concentrations of substrate than is simple competitive inhibition.

Renin substrate and the renin-angiotensin system in hog tissues

The individual components of the renin-angiotensin system has been identified in numerous tissues. In this study we have examined whether a functional renin-angiotensin system is operative in several hog tissues including brain, aorta, and liver. The contribution of tissue renin substrate to the rate of local angiotensin generation was also assessed. Electrophoretic differences in plasma and tissue renin substrates, indicating structural differences, were employed as an index of independence of the tissue system from that of the peripheral circulation. Our results indicate that all tissues studied had the potential to locally generate angiotensin and that renin substrate limited to rate of the renin reaction in these tissues. Electrophoretic parameters, polyacrylamide gel electrophoresis, and isoelectric focusing suggest that the tissue renin systems are of local origin. The potential magnitude of local angiotensin production is such that tissue renin-angiotensin systems may significantly contribute to the control and regulation of blood pressure and other regulatory mechanisms influenced by angiotensin.

Angiotensin stimulates beta-endorphin release from anterior pituitary gland cell cultures of rats

The effect of exogenous and locally generated angiotensin II (ANG II) on the release of beta-endorphin (beta-END) from anterior pituitary cell cultures of rats was studied. Angiotensin I (ANG I) and ANG II stimulated the release of beta-END, the ANG I effects being inhibited by addition of the converting enzyme inhibitor captopril. Renin and angiotensinogen had no effect when given separately, but their combination increased beta-END release. Thus ANG II causes the release of beta-END, but the putative pituitary renin system cannot be stimulated by exogenous renin or angiotensinogen; converting enzyme, however, acts locally to produce biologically active ANG II from ANG I.

Characterization of inactive renin from human kidney and plasma. Evidence of a renal source of circulating inactive renin

An inactive form of renin has been isolated from human plasma. It has been suggested that this may represent renin precursor secreted from the kidney. However, early studies failed to isolate inactive renin from human renal tissue. In this investigation, rapid processing of human kidney cortex at temperatures below 4 degrees C in the presence of protease inhibitors followed by cibacron-blue affinity chromatography allowed us to extract a totally inactive form of renal renin. Furthermore, we found that in kidney inactive renin constituted from 10 to as much as 50% of the total renin concentration. Biochemical characterization of the inactive renin from plasma and from kidney indicates that they are structural homologues and, when activated, have enzymatic properties that resemble active renal renin. Renal and plasma inactive renin were found to have the following properties in common: (a) a pH optimum of activation of 3.3; (b) reversible activation by acid dialysis on return to pH 7.4, 37 degrees C; (c) pH optima of enzyme activity of 7.8 with sheep angiotensinogen and 5.5 and 6.7 (biphasic) with human angiotensinogen; (d) Michaelis-Menten constants, Km, of 0.29-0.34 microM with sheep angiotensinogen, and 0.99-1.25 microM with human angiotensinogen; (e) an antibody to human renal renin mean inhibitory titer of 1:30,000 with 1 X 10(-4) Goldblatt units of activated renal or plasma inactive renin; (f) gel filtration profiles consisting of two peaks with apparent molecular weights of 56,000 +/- 1,500 and 49,200 +/- 1,000. Activation of plasma and kidney inactive renin by acid plus renal kallikrein was not accompanied by a change in gel filtration elution patterns. To determine whether inactive renin is released by the kidney, we measured inactive renin in samples obtained simultaneously from both the renal veins and inferior vena cava below the origin of the renal veins. In eight consecutive patients, inactive renin concentration was significantly higher in renal venous blood than in inferior vena caval blood. These data indicate that human kidney contains and secretes significant quantities of inactive renin. Thus, the kidney appears to be a major source of inactive renin in human plasma.

Hormonal and hemodynamic effects of short- and long-term clonidine therapy in patients with mild-to-moderate hypertension

Two studies of the responses to clonidine as the sole antihypertensive drug in the treatment of mild-to-moderate hypertension are reported. In the first, 11 patients with mild hypertension were treated with 0.1 mg clonidine twice daily for eight weeks. Those patients with "low renin" hypertension (n = 7) were noted to show an increase in plasma renin activity; the patients with "normal renin" hypertension (n = 4) tended to show a decrease. Both groups had a similar decrease in blood pressure. The changes in renin activity correlated significantly (p less than 0.01) with the small changes in endogenous creatinine clearance (r = 0.84). In the second study, 16 patients with mild-to-moderate essential hypertension were treated for three months with 0.2 mg clonidine three times daily. Blood pressure decreased from 167 +/- 4/105 +/- 2 to 140 +/- 3/90 +/- 2 mm Hg (p less than 0.01). Blood pressure changes correlated with decreases in plasma catecholamines (r = 0.61, p less than 0.001) and heart rate (r = 0.78, p less than 0.001). No significant changes in cardiac output, blood volume, renal blood flow, or glomerular filtration rate were noted. Clonidine is an effective and safe therapy when used as the sole medication in treating mild-to-moderate hypertension.

An increase in high-molecular weight renin substrate associated with estrogenic hypertension

We have previously reported that estrogens have the potential to induce new forms of renin substrate in addition to elevating the major circulating form of this protein. One of these estrogen-induced forms had a molecular weight in excess of 150,000. In this study we have compared the plasma concentration of the high-molecular-weight renin substrate in normotensive women receiving estrogen therapy and women with estrogenic hypertension. A statistically significant elevation of this protein was associated with estrogenic hypertension and normotensive pregnant women at term. This form of renin substrate differed from the major form with respect to electrophoretic mobility, isoelectric point, and immunologic cross-reactivity. In addition, kinetic analysis indicated that this high-molecular-weight substrate has a significantly higher affinity for the enzyme renin than the major circulating form (Km = 1800 +/- 290 versus 3520 +/- 260 ng angiotensin I equivalents/ml). These results suggest that in addition to renin substrate concentration, substrate composition may play an important role in blood pressure regulation.

Separation and characterization of two different species of rat angiotensinogen

Two distinct species of rat angiotensinogen (A-1 and A-2) were purified from plasma of nephrectomized rats by combining ammonium sulfate fractionation, chromatography on Blue Sepharose and SP-Sephadex, gel filtration and preparative polyacrylamide gel electrophoresis. Separation of the two species was accomplished in the SP-Sephadex chromatography step, A-1 eluting before A-2. The two angiotensinogen species had identical electrophoretic mobilities on analytical polyacrylamide gel electrophoresis, but differed in their apparent molecular weights as obtained by SDS-gel electrophoresis (A-1, Mr 60 000; A-2, Mr 56 400). In analytical isoelectric focusing each species displayed a characteristic double band with isoelectric points of 4.54 and 4.60 for A-1, and 4.69 and 4.76 for A-2. These physicochemical differences can be accounted for by the difference in carbohydrate content: A-1, when compared to A-2, had a higher content of sialic acid (5.0 and 2.1 mol/mol), neutral hexoses (10.2 and 5.9 mol/mol) and aminohexoses (10.5 and 7.0 mol/mol, respectively). Antiserum raised against rat angiotensinogen crossreacted completely with both angiotensinogens. Both species could also be isolated from plasma of non-nephrectomized rats, which indicates that they may be present under physiological conditions. The physiological significance of the occurrence of these species of angiotensinogen is still unknown.

Purification and characterization of two forms of rat plasma proangiotensin

Two forms of rat plasma proangiotensin were purified by (NH4)2SO4 fractionation, chromatography on DEAE-cellulose at pH 6.5, DEAE-Sepharose at pH 8.9, Sephadex G-150, hydroxyapatite and hexyl-agarose. Both forms were finally separated by affinity chromatography on concanavalin-A--Sepharose. Presence or absence of carbohydrate side chains seems to be the only difference between these forms of proangiotensin. Both proteins consist of single polypeptide chains having apparent molecular weights of 52000 and 55000 and isoelectric points around 4.7 and 4.4, respectively. No significant difference between the proteins could be observed with respect to the amino-terminal amino acid sequence which was found to be the same (H2N-Asp-Arg-Val) as for angiotensin I and II. Furthermore, extensive digestion with renin, releasing the decapeptide angiotensin I, did not significantly reduce the molecular weights of both polypeptides. It can therefore be concluded that the angiotensin I peptide is located at the amino terminus of the prohormone. Kinetic constants measured for the release of angiotensin I by renin were found to be Km = 5.0 microM proangiotensin and V = 270 nmol of angiotensin I h-1 unit renin-1 for the concanavalin-A-binding form and Km = 5.6 microM proangiotensin and V = 250 nmol angiotensin I h-1 unit renin-1 for the prohormone which did not bind to concanavalin-A--Sepharose. The form of proangiotensin not bound to concanavalin-A--Sepharose was found to be more thermally labile (tm of 59.0 degrees C) than the form binding to concanavalin A (tm of 61.5 degrees C, where tm = temperature at which 50% reactivity is lost).

Clonidine, a centrally acting sympathetic inhibitor, as monotherapy for mild to moderate hypertension

Sixteen patients with uncomplicated essential hypertension were treated with 0.2 mg of clonidine three times daily as the sole antihypertensive drug. Blood pressure decreased from 167 +/- 4/105 +/- 2 to 139 +/- 3/89 +/- 2 mm Hg (mean +/- standard error of the mean) after 1 week (p less than 0.001) and remained at 140 +/- 3/90 +/- 2 mm Hg after 3 months of therapy. There were no significant changes in cardiac output, blood volume, renal blood flow or glomerular filtration rate during clonidine therapy. Clonidine significantly decreased plasma catecholamines and there was a linear correlation between the change in blood pressure and decreases in plasma catecholamine concentration (r = 0.61, p less than 0.001). There was also a significant correlation between the decreases in heart rate and blood pressure (r = 0.78, p less than 0.001). It is concluded that clonidine can be used effectively and safely as the sole agent in the treatment of mild to moderate hypertension.

In vivo activity of purified mouse brain renin

Mouse brain renin and kidney renin were purified by a 3-step procedure: acetone powder extraction. Sephadex G-100 chromatography, and blue agarose affinity chromatography. The latter efficiently separated from cathepsin D-like acid protease activity. Mouse brain renin had an optimum of enzyme activity of pH 7.0. This differed from mouse kidney renin, which had an optimum at pH 8.5. In vitro, brain renin formed angiotensin I from rat plasma angiotensinogen and had no angiotensinase activity. Mouse brain renin was inhibited by monospecific antibodies raised against pure mouse submandibular gland renin. In vivo activity of the enzyme was tested by injection of brain renin into the lateral brain ventricle of rats. This resulted in the formation of angiotensin I from endogenous brain angiotensinogen, in the stimulation of water uptake, and in a long-lasting increase of arterial blood pressure. The latter could be blocked by the competitive angiotensin II receptor antagonist, saralasin. The results showed that brain renin is active under physiological conditions.

A human neutrophil-dependent pathway for generation of angiotensin II: purification and physicochemical characterization of the plasma protein substrate

Human neutrophils contain a neutral protease, previously designated "neutral peptide-generating protease," which generates a smooth muscle contractile activity from a plasma protein substrate. The plasma protein substrate has been purified to homogeneity from fresh citrated human plasma by 45% (wt/vol) ammonium sulfate precipitation of contaminating proteins, Affi-Gel Blue affinity chromatography, hydroxylapatite chromatography, phenyl-Sepharose hydrophobic chromatography, and Sephacryl S-200 gel filtration. The purified product produced a single stained protein on alkaline disc gel electrophoresis and elicited a monospecific goat antiserum. Purification was approximately 330- to 350-fold, and overall recovery was 6-11% of substrate protein in starting plasma as determined by quantitative radial immunodiffusion assay. The substrate has an isoelectric point of pH 4.6-5.1 and is a single polypeptide chain glycoprotein of Mr 62,000-67,000 by sodium dodecyl sulfate electrophoresis. The mean(+/- SD) concentration of this plasma protein substrate in normal serum is 120 +/- 22 microgram/ml. The plasma protein substrate of the neutrophil neutral protease may be identical to human angiotensinogen (renin substrate) because the physicochemical characteristics are similar and the peptide product is recognized by antibody to angiotensin II.

Studies on angiotensinogen of plasma and cerebrospinal fluid in normal and hypertensive human subjects

The presence of a renin-angiotensin system in the central nervous system (CNS) has been demonstrated by several investigators, but little is known regarding the origin of its components. In this study we have compared the immunological and physical-chemical nature of angiotensinogen in plasma and cerebrospinal fluid (CSF) of human subjects and explored whether differences are present in CSF angiotensinogen concentrations of normal and hypertensive subjects. No significant differences in the nature of plasma and CSF angiotensinogen was observed with respect to molecular weight (65-70,000) electrophoretic mobility (RFalb = 0.67 plus or minus 0.003) or angiotensin I (AI) generated (pI = 6.6). Following isoelectric focusing, the plasma angiotensinogen was shown to consist of a single component with an isoionic point of 4.40 plus or minus 0.04. CSF angiotensinogen, on the other hand, resolved into three components (pI = 4.76 plus or minus 0.02; 5.16 plus or minus 0.04; 5.76 plus or minus 0.04). Although no correlations were observed between angiotensinogen levels in the CSF or plasma with blood pressure (BP), a statistically significant difference in angiotensinogen concentration of both plasma and CSF was observed between normotensive and hypertensive subjects. The differences in the chemical and immunological nature of human plasma and CSF angiotensinogens suggest that the angiotensinogen of CSF is not of peripheral origin.

A microassay for active and total renin concentration in human plasma based on antibody trapping

We have developed and validated a new enzyme-kinetic method for measurement of renin concentration (PRC) in human plasma, based on radioimmunoassay of angiotensin I generated during incubation of plasma and excess sheep or ox renin substrate. Angiotensin I breakdown during incubation is prevented by the presence of anti-angiotensin I serum. The assya does not require prior extraction of renin, is technically simple, and is sufficiently sensitive to measure subnormal renin levels. With minor modifications both "active and "total" renin may be measured. Assay results have been calibrated with the International Standard Renin. PRC measured by this technique correlates significantly with angiotensin I and II, plasma renin activity, and with the PRC method previously used by us.

Human plasma angiotensinogen: a review of purification procedures

The current status of the purification and characterization of human angiotensinogen is reviewed. One problem encountered in the past has been the copurification of a protein with similar porperties. This protein has tentatively been designated alanine-protein. An efficient separation of angiotensinogen and alanine-protein was obtained on a zinc chelate column. Alanine-protein has been purified and its amino acid and carbohydrate composition determined. The COOH-terminal amino acid and the NH2-terminal amino acid were determined to be serine and alanine, respectively. Alanine-protein exhibited multiple forms on isoelectric focusing.

Studies on renin activation in normal human plasma

The phenomenon of plasma renin activattion by acid dialysis and preincubation with trypsin was studied in normal human plasma. Activation of plasma renin by exposure to pH 3.3 was shown to require at least one dialysis step and could be inhibited by the presence of Trasylol, indicating the involvement of a protease in acid activation. Amniotic fluid exposed to pH 1.5 to destroy renin and renin substrate was also found to contain an enzyme capable of activating plasma renin. The Michaelis-Menten constant Km and the molecular weight of activated "renin" were found to be similar to those of normal plasma renin. Inactive renins or renin-like enzymes were partially purified from plasma by affinity chromatography on concanavalin A, precipitation with (NH4)2SO4 and isoelectric focusing. Trypsin and acid exposure gave similar results with regard to the activation of this zymogen, suggesting that trypsin and acid dialysis may increase plasma renin activity by the same mechanism.

Purification and characterization of rat angiotensinogen

1. Angiotensinogen (renin substrate) was purified from plasma of nephrectomized rats by a four step procedure using ammonium sulfate fractionation, chromatography on Blue Sepharose CL-6B and SP-Sephadex C-50, and gel filtration on Sephadex G-150. 2. The final preparation had a specific concentration of 9.3 microgram angiotensin I/mg (mean of six separate runs). The best preparation so far obtained contains 14.6 microgram angiotensin I/mg protein, which represents a purity of 62%. 3. By sodium dodecyl sulfate disc electrophoresis an apparent molecular weight of 56,400, and by isoelectric focusing an isoelectric point of 4.85 has been determined. These properties of rat angiotensinogen are similar to those reported for human angiotensinogen.

Relation between blood pressure and renin, renin substrate, angiotensin II, aldosterone and urinary sodium and potassium in 574 ambulatory subjects

Five hundred and seventy-four ambulatory subjects with blood pressures ranging from 94/58 to 250/145 mm Hg were studied on their usual dietary and sodium intake. Renin, renin substrate, angiotensin II, aldosterone and urinary sodium and potassium were compared with blood pressure to access the contribution of these variables to the blood pressure variance. Our analyses revealed that renin substrate was highly and positively correlated with diastolic blood pressure (r = +0.39; p < 0.00001) but all other components of the renin-aldosterone system exhibited a significant negative correlation with blood pressure. A highly significant relationship between potassium, the renin-aldosterone system and blood pressure was found but no such relationship could be demonstrated for sodium. Subjects with higher blood pressures had lower urinary potassium concentrations and lower potassium/creatine ratios. These findings raised the possibility of a significant pathogenetic relationship between potassium and high blood pressure. Multiple linear regression reveals that influences of the renin-angiotensin-aldosterone system can only account for less than 20% of the variance exhibited by the blood pressure in these subjects.

Multiple forms of human plasma renin substrate

The objective of this investigation was to determine whether heterogeneity of plasma renin substrate could be observed in states of steroid excess and various forms of hypertensive disease. In states of stimulated renin substrate production by estrogens or glucocorticoids, multiple forms of renin substrate were apparent when stimulation was excessive. Stimulation of substrate production caused by uremia associated with hypertension showed similar results. None, or only trace quantities of the additional forms of renin substrate were evident in subjects with normal or suppressed levels of plasma renin substrate. The additional forms of renin substrate could be distinguished from the normal form on the basis of cross-reactivity with a specific antiserum to the normal form, electrophoretic mobility, and kinetic rate constants. Differences in rate constants of the various forms of plasma renin substrate may account for the altered rate of the renin reaction associated with several states of hypertension. In plasma of patients with renovascular hypertension, significant quantities of a protein which cross-reacted with the antiserum but could not generate angiotensin I were observed.

Separation of human renin substrate from renin and a major contaminating albumin using a concanavalin A-sepharose column

Human plasma renin substrate was purified and separated from renin by a concanavalin A-Sepharose affinity column. Human renin substrate as well as renin were bound to concanavalin A. Renin substrate was eluted with 0.1 M D-glucose in 20 mM Tris/HCl buffer, pH 8.0. The specific activity increased from 38.5 to 653 ng of angiotensin-I equivalents per mg of protein (17-fold) and the recovery was 85%. Renin was eluted completely with 0.2 M alpha-D-methylglucoside and 0.2 M alpha-D-methylmannoside.