Enzyme kinetic studies on human renin and its purified homologous substrate

Hypertension in the transgenic rat TGR(mRen-2)27 may be due to enhanced kinetics of the reaction between mouse renin and rat angiotensinogen

The transgenic rat TGR(mRen-2)27, in which the Ren-2 mouse renin gene is transfected into the genome of the rat, develops severe hypertension with high adrenal renin and low kidney renin. These animals express both mouse and rat renin. To investigate the cause of hypertension in the TGR rat, we compared the kinetics of mouse renin acting on mouse and rat angiotensinogens. The optimum pH of the renin reaction in the Sprague-Dawley rat was 6.5, whereas the optimum pH of the reaction in the TGR rat was approximately 8.5. The optimum pH of the renin reaction in the DBA mouse was 6.0. Purified mouse Ren-2 renin acting on rat angiotensinogen showed a pH profile similar to that for the renin reaction in the TGR rat. The angiotensinogen concentration in pooled plasma from eight DBA mice was 104.5 ng angiotensin I/mL and was clearly lower than that in Sprague-Dawley rats (772.4 +/- 37.3 ng angiotensin I/mL, n = 4). The reaction of purified mouse Ren-2 renin with rat angiotensinogen was 10 times faster than with mouse angiotensinogen. Plasma renin activity in DBA mice increased dramatically on addition of rat angiotensinogen (from 253.4 +/- 66.7 to 225,000 +/- 48,000 ng angiotensin I/mL per hour). Intravenous injection of 2 or 10 microL of DBA mouse plasma into the nephrectomized Sprague-Dawley rat increased the mean arterial pressure of the rat by 27.7 +/- 4.7 and 61.8 +/- 2.7 mmHg, respectively, whereas injection of 200 microL of Sprague-Dawley rat plasma did not change the mean arterial pressure of the rat.(ABSTRACT TRUNCATED AT 250 WORDS)

Reninlike enzymes in human vasculature

The present study was designed to identify angiotensin I (Ang I)-forming angiotensinogenases in human extrarenal vasculature and to examine the theory of Jiménez Días on their stimulation in essential hypertension. Vascular sections obtained intraoperatively from 14 normotensive and 16 hypertensive patients undergoing corrective surgery, 68 umbilical cord blood vessels from parturient women, tissue samples from nine explanted hearts, and serum from anephric and healthy individuals were investigated. Ang I-forming angiotensinogenase activities were determined enzyme-kinetically by using Ang I radioimmunoassay and purified sheep or human angiotensinogens. Three nonrenin Ang I-forming angiotensinogenases (pH optima of 4.0, 5.1, and 6.1) were identified in extrarenal vasculature, in cardiac tissues, and in plasma. Highest specific activities of nonrenin Ang I-forming angiotensinogenase (in nanograms Ang I per gram times hour; mean +/- SD) were found in cardiac tissue (2,821 +/- 497, n = 9), followed by carotid artery intima (1,448 +/- 982, n = 10), arteries (1,307 +/- 736, n = 18), and umbilical cord arteries (135 +/- 55, n = 35). Extrarenal arterial Ang I-forming angiotensinogenases were linearly correlated with those of local angiotensin converting enzyme and plasma renin activity. In essential hypertension, extrarenal arterial Ang I-forming angiotensinogenases were scattered, but not generally stimulated. The data obtained indicate the existence of nonrenin Ang I-forming angiotensinogenases in human extrarenal vasculature, in kidney, and in plasma. The postulate of stimulation of extrarenal arterial Ang I-forming angiotensinogenases in essential hypertension cannot be supported. Similar to the classification of plasma renin activity, a classification of Ang I-forming angiotensinogenase activity is proposed, consisting of patients with essential hypertension divided into subgroups exhibiting high, normal, or low vascular Ang I-forming angiotensinogenase activities.

Inhibition of renin by plasma linoleic acid

The authors have demonstrated previously that human plasma contains an inhibitor(s) of the enzymatic activity of renin. The purpose of this study is to identify the circulating renin inhibitor. Plasma initially was fractionated with preparative sephacryl S-300 chromatography. A fraction with low protein content inhibited the in vitro enzymatic activity of human renin by 76%. The inhibitory activity of this fraction was not altered by boiling and/or acidification. This fraction was applied to an affinity column, using purified mouse submaxillary renin as ligand. The inhibitor was recovered after elution with hypertonic NaCl. Linoleic acid, a previously identified renin inhibitor, was present in this fraction. The authors conclude that circulating linoleic acid inhibits renin. Conceivably, the overall activity of the renin-angiotensin-aldosterone cascade may be modified by alterations of plasma fatty acid concentrations.

Comparative enzymatic studies of human renin acting on pure natural or synthetic substrates

Some of the essential structural requirements for the enzymatic reaction of pure human renin acting on pure human and rat angiotensinogen and on their synthetic tetradecapeptide substrates were investigated. The five carboxy terminal amino acids of synthetic tetradecapeptides played a significant role in substrate recognition and/or hydrolysis by human renin. Kinetic constants Km, Kcat and kcat/Km of the various human renin assays were different according to the substrate used. The presence of either an asparagine or a threonine residue in the S'4 renin subsite did not affect significantly the kinetic constant values. A tyrosine residue, rather than a histidine residue, in the S'3 renin subsite gave the best synthetic substrate studied. When tyrosine residue was present in the S'2 renin subsite an important decrease in kcat was observed. Human angiotensinogen was hydrolysed by human renin with lower Km and kcat values than those measured with human and porcine synthetic substrates, suggesting that the 3-dimensional structure of human angiotensinogen plays a key role in the hydrolysis. This finding was supported by assays performed with rat angiotensinogen, which was cleared by human renin with the same kcat value as rat tetradecapeptide, but with a 49-fold lower Km. Between human and rat angiotensinogen a kcat/Km value of only 2-fold higher has been found in the renin assay using human substrate.

Investigations of components of the renin-angiotensin system in rat vascular tissue

Investigations were performed on components of the renin-angiotensin system (RAS) in homogenate extracts of vascular tissue and aortic smooth muscle cells cultivated in vitro. Determinations of isoelectric points and pH optima indicated the existence in aortic homogenate extracts of two local angiotensin I (AI)-forming enzymes (AIFE) that were different from those of plasma, renal cortex, veins, and aortic smooth muscle cells. The pH optima for AI-converting enzyme (ACE) from vascular tissues, aortic smooth muscle cells, and plasma were in the same range (pH 8.0-8.5), and in agreement with those measured previously in other tissues. In contrast, in vitro studies with the ACE inhibitors MK-421 and MK-422 and measurement of isoelectric points suggested that aortic ACE was different from the plasma enzyme. AIFE and ACE activities were found to be elevated in spontaneously hypertensive rats (SHR). The biochemical characteristics of the enzymes investigated in the vascular tissue of SHR were not different from those of the normotensive controls. AI- and AII-degrading enzymes were found both in aortic tissue and in aortic smooth muscle cells. One potent AI-degrading enzyme different from ACE was observed in aortic tissue. A high ratio of AI/AII immunoreactivities in arterial walls suggests the availability of renin substrate, and that AI-degrading enzymes are the rate-limiting enzymes for AII formation. The results further support the concept of an intrinsic vascular RAS.

An improved method for determination of active and total renin concentration in human plasma using an excess of sheep substrate

A method for measurement of renin concentration (PRC) in human plasma has been developed and validated experimentally and theoretically. Like most of the previous methods, the present method is based on a radioimmunoassay of angiotensin I generated during incubation of plasma with an excess of renin substrate. In the present study we used, as the substrate, sheep angiotensinogen partially purified from anephric sheep plasma by ammonium sulfate fractionation and pepstatin-aminohexyl-agarose chromatography. The advantage of using sheep substrate is that it has an exceptionally high affinity for human renin. The partially purified substrate, which contained no detectable renin activity, improved the sensitivity, allowing quantification of renin in low-renin plasma. Possible interfering influences of plasma proteins on the radioimmunoassay were eliminated by the introduction of a simple deproteinization step. For determination of total renin concentration (TRC), inactive renin was activated by exposing plasma to low pH or trypsin. Normal values of PRC and TRC after careful selection of assay conditions were 2.9 +/- 0.4 and 33.9 +/- 6.1 ng angiotensin I X ml-1 X h-1, respectively. As an illustrative example of usefulness of the assay, PRC determination of a patient with a ectopic renin-secreting tumor is presented.

Mechanism of increased renin release in the adrenalectomized rat. Adrenal insufficiency and renin

We have previously suggested that inhibition of renin release by sodium chloride is related to absorptive chloride transport in the loop of Henle. Infusion of sodium chloride fails to inhibit renin release in the adrenalectomized (Adx) rat, and dexamethasone restores renin responsiveness to sodium chloride. The purpose of the present study was to evaluate the relationship between loop function (urinary diluting and concentration capacity) and plasma renin concentration (PRC) in the Adx rat. After hypotonic sodium chloride infusion, free water clearance (CH2O) of Adx rats (0.56 ml/hr/100 g +/- 0.17 SE) was decreased (p less than 0.01) compared to controls (2.86 ml/hr/100 g +/- 0.29 SE); PRC of Adx rats (61.9 units/ml +/- 11.2 SE) was increased (p less than 0.01) above controls (6.0 units/ml +/- 1.7 SE). These differences persisted after administration of d(CH2)5Tyr(Et)VAVP, a potent ADH antagonist. In separate groups of animals, after water deprivation, urine concentration of Adx rat (1,401 mOsm/kg +/- 45 SE) was less (p less than 0.01) than that of controls (2,117 mOsm/kg +/- 169 SE). Dexamethasone normalized both CH2O and urinary concentrating ability and also decreased PRC in Adx rats. Thus, in the glucocorticoid deficient rat, increased renin release is associated with impaired loop function. The loop defect may account for high PRC that is not suppressed by sodium chloride.

Biochemistry and regulation of angiotensinogen

Angiotensin II and angiotensin III, the active peptides of the renin-angiotensin system, are produced by a cascade of enzymatic reactions, whose initial step is the reaction between renin and its substrate, angiotensinogen. In plasma, the concentration of angiotensinogen is a limiting factor: the Km of the enzymatic reaction is between 1 and 2 microM depending on the species. It is therefore of interest to measure its level in plasma and tissues and to examine the main factors which may influence its synthesis and release. The complete purification of angiotensinogen has made possible the preparation of specific antibodies which cross-react with both angiotensinogen and its residue, des-angio I-angiotensinogen, and are currently used in radioimmunoassays and immunohistochemical studies. A small amount of angiotensinogen is stored in hepatic cells, where it can be detected by immunofluorescence and measured by radioimmunoassay. It is also present in proximal tubular cells of the kidney, probably reabsorbed from glomerular filtrate, but it is absent from juxtaglomerular cells. Several hormones are able to increase liver synthesis of angiotensinogen and its release. Thyroxine, angiotensin II, dexamethasone, ethinyl-estradiol and binephrectomy increase both synthesis and release. Adrenalectomy and converting-enzyme inhibition are accompanied by an increased peripheral consumption of plasma angiotensinogen, and by accumulation of des-angio I-angiotensinogen whose metabolism and role are unknown. The major role of angiotensinogen in renal hemodynamics is demonstrated by its effects on the isolated perfused kidney, an experimental observation which parallels the clinical observation of women on estroprogestative therapy, whose renal blood flow is reduced, even in the absence of a detectable increase in their blood pressure. A better knowledge of renin substrate structure in various species is a necessary requirement for the design of inhibitory analogs of angiotensinogen which will have application for the treatment of hypertension and oedema.

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).

Increased enzymatic activity of renin and hyperlipidemia

We have previously reported that the in vitro enzymatic activity of exogenous renin, plasma renin reactivity (PRR), is increased in plasma of patients with chronic renal failure, possibly due to the deficiency of a renin inhibitor. To determine whether increases PRR is related to renal failure per se or to hyperlipidemia, PRR was measured in 10 control subjects, 10 patients with renal failure, and 10 hyperlipidemic patients with normal renal function. Compared to that in control subjects (52.6 ng angiotensin I generated per ml/h +/- 3.8 SE) PRR was increased (P < 0.05) in plasma of uremic patients (65.1 +/- 4.3) and hyperlipidemic patients (71.4 +/- 10.7). Renin substrate concentration did not differ among groups, and after denaturation of endogenous substrate by acidification of plasma, PRR was still increased. A "protein-free" extract of plasma from normal subjects inhibited renin, whereas little or no inhibition occurred with a comparable extract from uremic patients and hyperlipidemic patients. Thus, alterations in lipid metabolism may account for the increased enzymatic activity of renin in uremic plasma. Increased PRR may be related to the deficiency of a normally occurring renin inhibitor.

Effect of sulfhydryl reagents on the enzymatic activity of human renin: implications for renin assay

Sulfhydryl (SH) reagents are sometimes used in renin assays, yet their effects on the enzymatic activity of human renin are not clearly understood. We have employed radioimmunoassay of angiotensin I (AI) to assess the effects of dithiothreitol (DTT) and of dimerecaptopropanol (DMP) on the formation of AI at pH 5.5 and at pH 7.4. When ethylenediamine tetra-acetate, phenylmethylsulfonyl fluoride, and 8-hydroxyquinoline were used as angiotensinase inhibitors, both DTT and DMP decreased the rate of AI formation in each of two human plasma pools at both pH values. In contrast, in a system of semi-purified human kidney renin and hog renin substrate, DTT enhanced the formation of AI, increasing Vmax without changing Km at both pH values. The reaction of human kidney renin with synthetic tetradecapeptide renin substrate was stimulated by DTT at pH 7.4 but inhibited by DTT at pH 5.5. In all three systems, SH reagents altered the ratio of reaction velocity at pH 5.5 to reaction velocity at pH 7.4. We conclude that SH reagents affect the assay of human renin in complex ways which depend upon the pH of incubation and upon the subcomplex ways which depend upon the pH in incubation and upon the substrate utilized. Although the mechanism of these effects is not known, such effects probably contribute to the lack of agreement among many of the procedures for renin assay.

Species differences in the kinetics of the renin-substrate reaction in plasma

In view of the substrate-dependence of renin, it was of interest to examine the kinetics of the renin-angiotensinogen reaction in plasma of various species to establish differences in stoichiometry. The results also provide an indication of assay conditions appropriate for accurate measurement of the reaction velocity. Plasma from hogs, dogs, and rats served as the source of renin for incubation with homologous angiotensinogen. The rate of production of radioimmunoassayable angiotensin I increased with increasing concentrations of angiotensinogen. This substrate-dependence of renin conformed to conventional Michaelis-Menten kinetics. The concentration of angiotensinogen was less than that required for half maximum velocity in dog and rat plasma. In contrast, endogenous substrate in hog plasma was sufficient to sustain near maximal rates of generation of angiotensin I. Purification of angiotensinogen altered the catalytic properties of angiotensinogen making it a poor representative substrate for renin. Hog and rat renin were saturable with high concentrations of unextracted plasma angiotensinogen. In contrast, it was not possible to saturate the dog enzyme with unextracted substrate. The interspecies differences in stoichiometry of the reaction indicate that standardization of assay conditions for various species of renin is not justified.

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.

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.

Plasma renin activity, reactivity, concentration and substrate following hypertension during pregnancy. Effect of oral contraceptive agents

Plasma renin activity is suppressed in approximately 25% of patients with essential hypertension, and the rate of in vitro angiotensin I production after addition of exogenous renin (renin reactivity) is increased in plasma of hypertensive patients. We have recently observed that blood pressure (116 +/- 1.5/68 +/- 1.7 mm Hg) of young women who had hypertension during a first pregnancy 3--6 years earlier (n = 63) was higher (p less than 0.005) than blood pressure (109 +/- 1.4/61 +/- 1.7 mm Hg) of women who remained normotensive during pregnancy (n = 52). To determine if alterations of the renin-angiotensin axis observed in patients with established hypertension also occur in young adults with relatively high blood pressure, plasma renin activity (PRA), plasma renin concentration (PRC), plasma renin substrate (PRS) and plasma renin reactivity (PRR) were compared in these two groups of subjects. Overall, PRA and PRC were inversely related to systolic blood pressure (p less than 0.02). Excluding women on oral contraceptive agents, the PRA response to standardized treadmill exercise was suppressed (less than 1.0 ng/ml/hr) in 19% of women with a history of hypertension during pregnancy and in no women who remained normotensive throughout a previous pregnancy; PRR did not differ (p greater than 0.8) in the two groups of young mothers (27.1 ng/ml/30 min +/- 1.2 SE VS 26.2 ng/ml/30 min +/- 0.9 SE). Thus, renin suppression, but not increased PRR, precedes the onset of hypertension. Oral contraceptive usage was associated with higher systolic blood pressures, increased PRS, and low PRC. Highest blood pressures and lowest PRA occurred in women with a history of hypertension during pregnancy who were taking oral contraceptive agents at the time of study.

Partial purification of dog angiotensinogen

Dog angiotensinogen was purified 450-fold from the plasma of nephrectomized dogs by a simple four-step procedure involving precipitation between 1.5 and 2.3 M ammonium sulfate, gel filtration on Sephadex G-150, ion-exchange chromatography on DE-52 cellulose, and affinity chromatography on Concanavalin A-Sepharose. The purity of the final preparation was over 50%. The preparation of dog angiotensinogen had an apparent molecular weight of 80,000 determined by gel filtration on Sephadex G-100. Kinetic studies indicated that the Km of the reaction of dog renin with partially purified dog angiotensinogen (1,840 pmol/ml) was similar to that for the reaction with angiotensinogen in diluted dog plasma (1,820 pmol/ml). Thus the purification procedures employed did not alter the affinity of dog renin for the Leu10-Leu11 bond of dog angiotensinogen. Because the concentration of angiotensinogen in dog plasma is about 700 pmol/ml, a first order reaction with respect to substrate is indicated in vivo.

Renin-like (angiotensinogenase) activity in human eccrine sweat

The presence of renin or renin-like activity (RLA) was demonstrated in human eccrine sweat incubated with purified sheep angiotensinogen, using rat bioassay and angiotensin I radioimmunoassay. Following cholinergic stimulation, sweat RLA was found to range between 0 (unmeasurable) and 266 ng/ml.h, i.e. RLA-values of sweat can be about 10 times higher than those of plasma. Therefore, renin synthesis in sweat glands could be assumed. RLA following activation of beta-adrenergic receptors by the administration of isoprenaline (Aludrin) did not exceed the mean values obtained by cholinergic activation. After beta-adrenergic receptor blockade by propranolol (Dociton), RLA became unmeasurably low. Higher RLA-values were found after local injection of dibutyryl-c-AMP (90--210 ng/ml.h). The results indicate a beta-adrenergic regulation of RLA-release in human sweat glands. Human sweat glands appear to be useful for studying extrarenal renin release.

Measurement of plasma renin activity as a valid estimation of plasma angiotensin status

Measurement of plasma renin activity is widely used as an indirect assessment of plasma angiotensin II concentration. There has been some controversy over the validity of this assay as an estimate of circulating angiotensin II levels because, during the in vitro generation of angiotensin I by renin, over a period of time, substrate concentration may diminish to such an extent that it becomes rate-limiting, giving an artificially low reflection of angiotensin II levels. In this paper the initial angiotensin I concentration, that is the concentration before in vitro angiotensin I generation, has been compared with the corresponding plasma renin activity for 2752 individual plasma samples. A linear relationship was found between the initial angiotensin I concentration and the plasma renin activity below 60 ng ml-1 h-1. This indicates that, under the conditions of this assay, substrate does not appear to become rate-limiting except at exceedingly high levels of plasma renin activity. These results appear to provide further validation for the use of plasma renin activity measurement as a reflection of the concentration of circulating angiotensin II levels.

In vitro and in vivo inhibition of renin by fatty acids

Circulating neutral lipids inhibit the in vitro renin reaction. To identify the inhibitor(s), free fatty acids were added to human renin and homologous substrate. Capric, lauric, palmitoleic, linoleic, and arachidonic acids each inhibited the rate of angiotensin I production in vitro (P less than 0.01). Inhibition by polysaturated fatty acids (linoleic and arachidonic) was less (P less than 0.01) after catalytic hydrogenation of the double bonds. To evaluate an in vivo effect of renin inhibition intra-arterial blood pressure responses to infusions of renin and angiotensin II (5.0 microgram) were measured in anephric rats (n = 6) before and after infusion of linoleic acid (10 mg iv). Mean increase of blood pressure to angiotensin II before (75 mmHg +/- 9) and after (90 +/- 12) linoleic acid did not differ (P greater than 0.05). However, the pressor response to renin after linoleic acid (18 +/- 3) was less (P less than 0.00)) than that before (102 +/- 13). In summary, several fatty acids inhibit the in vitro renin reaction, and in part inhibition is dependent on unsaturation. Linoleic acid also inhibits the in vivo pressor response to renin. These results suggest that fatty acids may modify the measurement of plasma renin activity and may also affect angiotensin production in vivo.

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.

Kidney and plasma renin in human renovascular hypertension

Renin activities were determined in plasma and in single, microdissected juxtaglomerular apparatus in 19 patients with unilateral renal artery stenosis. The mean juxtaglomerular apparatus renin concentration in the stenosed kidneys was 5.5 +/- 1.2 (SEM) mug.l-1.h-1 which is about ten times that of the suppressed renin concentration in the contralateral kidneys (0.6 +/- 0.05 mug.l-1.h-1). On the affected side a positive correlation was found between intrarenal and renal venous renin concentration (r = 0.93; p less than 0.001). Both intrarenal and renal venous renin concentrations of the stenosed kindeys were positively correlated to renin secretion rates, as calculated from renin analysis in plasma from the vena cava and renal veins. No relationship could be demonstrated between intrarenal or renal venous renin concentration and the degree of blood pressure elevation or transstenotic pressure gradient. However, a positive correlation was evident between peripheral plasma renin activity and diastolic blood pressure (r = 0.88; p less than 0.001). Comparative enzyme kinetic analyses of renin from the juxtaglomerular apparatus and renal venous plasma were performed using sheep substrate. The lowest apparent Km-values of renin were found in renal venous plasma from the stenosed kidneys (198 +/- 13 mug/l) compared with the contralateral side (301 +/- 20 mug/l; p less than 0.001). Mean apparent Km-values of juxtaglomerular apparatus renin in the stenosed (270 +/- 36 mug/l) and contralateral (292 +/- 37 mug/l) kidneys did not differ. No significant differences were found between mean apparent Km-values for renin in peripheral plasma of renovascular hypertensive patients and control subjects using either homologous human or heterologous sheep renin substrate. The results suggest that, in addition to the renin concentration other factors are relevant to chronic high blood pressure in renovascular hypertension.

Partial purification of human renin substrate

A two-step purification method is described for the preparation of renin substrate from human plasma. Pooled plasma from women on oral contraceptives is used for the purification. The overall yield of renin substrate is 57%, with a twenty-fold purification. The specific renin substrate content of the preparation, as determined by enzymatic degradation with an excess of human renin, is 2 mug angiotensin I per mg protein. The product has a very low endogenous renin activity and is free from angiotensinase activity. An additional purification step involving affinity chromatography is described. In pilot studies a renin substrate yield of 37% has been achieved, with a hundred-fold purification. The final product has a specific renin substrate content of 10 mug angiotensin I per mg protein. The preparation contains up to 12 different plasma proteins, nine of which have been identified and quantitated.

Reaction constants of renin in juxtaglomerular apparatus and plasma renin activity after renal ischemia and hemorrhage

Reaction constants of renin in juxtaglomerular apparatus and plasma renin activity after renal ischemia and hemorrhage. During and after total renal ischemia and acute hemorrhage, renin activity in plasma (PRA) and microdissected juxtaglomerular apparatus (JGA) of rabbits were investigated. In controls, the apparent Michaelis-Mentoen constant (MMC) of semipurified standard renin of rabbits was 1025 plus or minus 223 SD ng/ml. Plasma renin of normal rabbits showed similar values: 1062 plus or minus 138 SD ng/ml. Intrarenal JGA renin, however, showed a great scatter of MMC (920 to 4760 ng/ml) and a significantly higher mean value of 2572 plus or minus 1156 SD ng/ml (pis less than 0.001). After complete renal ischemia by clamping both renal arteries for a 90-min period, the following results wereobtained: 1) Sixty min after the beginning of ischemia, PRA decreased from 20.9 plus or minus 9.8 SD to 7.6 plus or minus 5.2 SD ng/ml-hr (P is less than 0.05) and increased to 103, 68 and 42 ng/ml-hr 10, 30 and 90 min after removal of the clamps, respectively (P is less than 0.05).