The synthesis of a tetradecapeptide renin substrate

Advances in the renin angiotensin system focus on angiotensin-converting enzyme 2 and angiotensin-(1-7)

The contribution of the renin angiotensin system to physiology and pathology is undergoing a rapid reconsideration of its mechanisms from emerging new concepts implicating angiotensin-converting enzyme 2 and angiotensin-(1-7) as new elements negatively influencing the vasoconstrictor, trophic, and pro-inflammatory actions of angiotensin II. This component of the system acts to oppose the vasoconstrictor and proliferative effects on angiotensin II through signaling mechanisms mediated by the mas receptor. In addition, a reduced expression of the vasodepressor axis composed by angiotensin-converting enzyme 2 and angiotensin-(1-7) may contribute to the expression of essential hypertension, the remodeling of heart and renal function associated with this disease, and even the physiology of pregnancy and the development of eclampsia.

Angiotensin-(1-12) is an alternate substrate for angiotensin peptide production in the heart

Identification of angiotensin-(1-12) as an intermediate precursor derived directly from angiotensinogen led us to explore whether the heart has the capacity to process angiotensin-(1-12) into biologically active angiotensin peptides. The generation of angiotensin I, angiotensin II, and angiotensin-(1-7) from exogenous angiotensin-(1-12) was evaluated in the effluent of isolated perfused hearts mounted on a Langendorff apparatus in three normotensive and two hypertensive strains: Sprague-Dawley, Lewis, congenic mRen2.Lewis, Wistar-Kyoto, and spontaneously hypertensive rats. Hearts were perfused with Krebs solution for 60 min before and after the addition of angiotensin-(1-12) (10 nmol/l). Angiotensin-(1-12) caused the rapid appearance of both angiotensin I and angiotensin II in the perfusate that peaked between 30 and 60 min of recirculation. Production of angiotensin-(1-7) from exogenous angiotensin-(1-12) rose steadily over the course of the 60-min experiment. These data directly demonstrate that angiotensin-(1-12) is a substrate for the formation of angiotensin peptides in cardiac tissue. This finding further suggests that this angiotensinogen-derived product is a previously unrecognized important precursor peptide to the renin-angiotensin system cascade.

Metabolic engineering of Saccharomyces cerevisiae producing nicotianamine: potential for industrial biosynthesis of a novel antihypertensive substrate

Nicotianamine (NA), a metal chelator, is ubiquitous in higher plants. In humans, NA inhibits angiotensin I-converting enzyme (ACE), and consequently reduces high blood pressure. Nicotianamine is synthesized from the trimerization of S-adenosylmethionine (SAM) by NA synthase (NAS). Here, we aimed to produce large amounts of NA fermentatively by introducing the Arabidopsis AtNAS2 gene into Saccharomyces cerevisiae strain SCY4. This strain can accumulate up to 100 times the usual amount of SAM, and this is considered desirable for overproduction of NA. Nicotianamine was produced in the engineered yeast, and the NA level increased with incubation time until the stationary phase. The maximum concentration of intracellular NA obtained was 766+/-33 microg/g wet weight. Successful production of NA in S. cerevisiae should pave the way for industrial production of this novel antihypertensive substrate.

Isolation and identification of a pepsitensin

1. From ox plasma incubated with pepsin a highly purified pepsitensin has been isolated by fractional precipitation, solvent extraction, column chromatography, countercurrent distribution and paper chromatography. 2. Comparison of the properties of this substance with those of synthetic valyl-5 angiotensin I showed: (a) the same specific pressor activity; (b) the same amino acid composition; (c) identical paper-electrophoretic mobilities at various pH values. 3. N- and C-Terminal studies carried out on the intact polypeptide and on the products of chymotrypsin digestion established the following sequence for the amino acids present in the molecule: Asp-Arg-Val-Tyr-Val-His-Pro-Phe-His-Leu. This structure is identical with that of valyl-5 angiotensin I.

The purification and partial characterization of several forms of hog renin substrate

Hog renin substrate has been separated into three major (A, B, and C) and two minor forms (D and E) by DEAE cellulose chromatography. Two of the major forms (B and C) have been further fractionated into two additional types (1 and 2) by countercurrent distribution. The purification of substrates A, C₁, and C₂ has been completed. Analysis shows that all three are glycoproteins with molecular weights of about 57,000, and have similar amino acid compositions. Differences exist in the sialic acid, glucosamine, and neutral hexose content, which may account for different physical properties. All the forms of the substrates are attacked by renin at similar rates, and appear to yield the same angiotensin I.

Differential regulation of angiotensin peptides in plasma and kidney: effects of adrenalectomy and estrogen treatment

Eight angiotensin peptides [angiotensin-(1-7), angiotensin II, angiotensin-(1-9), angiotensin I, angiotensin-(2-7), angiotensin-(2-8), angiotensin-(2-9), and angiotensin-(2-10)] were measured in plasma and kidney of adrenalectomized rats and estrogen-treated rats. In comparison with sham-operated rats, adrenalectomy increased plasma renin levels by 50-fold and reduced plasma angiotensinogen levels by 67%. Adrenalectomy increased plasma angiotensin peptide levels by 9- to 30-fold, but the increases in renal angiotensin peptide levels were much less than those seen for plasma. In comparison with vehicle-treated rats, estrogen treatment increased plasma angiotensinogen levels by 3-fold and reduced plasma renin levels by 41%. Estrogen treatment decreased plasma angiotensin peptide levels, whereas renal angiotensin peptide levels increased by as much as 2- to 3-fold. These results confirm the differential regulation of angiotensin peptide levels in plasma and kidney, and provide further support for the essential role of angiotensinogen in modulating plasma and tissue angiotensin peptide levels.

THE PRESENCE OF RENIN ACTIVITY IN BLOOD VESSEL WALLS

The preparation, of an extract of hog blood vessels and organs containing renin activity has been described. The extract hydrolyzes natural or synthetic renin substrate to form a vasopressor material. This reaction could not be distinguished from the hydrolysis of renin substrates by kidney renin. The activation energy, effect of substrate concentration on velocity, inhibition by antirenin, and pH optimum of the extract and of kidney renin are the same. Renin activity is found in the adventitia and media of the aorta, the liver and, to a lesser extent, in other vascular organs.

Differential regulation of angiotensin peptide levels in plasma and kidney of the rat

We compared the effects of the converting enzyme inhibitor perindopril on components of the renin-angiotensin system in plasma and kidney of male Sprague-Dawley rats administered perindopril in their drinking water at two doses (1.4 and 4.2 mg/kg) over 7 days. Eight angiotensin peptides were measured in plasma and kidney: angiotensin-(1-7), angiotensin II, angiotensin-(1-9), angiotensin I, angiotensin-(2-7), angiotensin III, angiotensin-(2-9), and angiotensin-(2-10). In addition, angiotensin converting enzyme activity, renin, and angiotensinogen were measured in plasma, and renin, angiotensinogen, and their respective messenger RNAs were measured in kidney; angiotensinogen messenger RNA was also measured in liver. In plasma, the highest dose of perindopril reduced angiotensin converting enzyme activity to 11% of control, increased renin 200-fold, reduced angiotensinogen to 11% of control, increased angiotensin-(1-7), angiotensin I, angiotensin-(2-7), and angiotensin-(2-10) levels 25-, 9-, 10-, and 13-fold, respectively; angiotensin II levels were not significantly different from control. By contrast, for the kidney, angiotensin-(1-7), angiotensin I, angiotensin-(2-7), and angiotensin-(2-10) levels did not increase; angiotensin II levels fell to 14% of control, and angiotensinogen fell to 12% of control. Kidney renin messenger RNA levels increased 12-fold, but renal renin content and angiotensinogen messenger RNA levels in kidney and liver were not influenced by perindopril treatment. These results demonstrate a differential regulation of angiotensin peptides in plasma and kidney and provide direct support for the proposal that the cardiovascular effects of converting enzyme inhibitors depend on modulation of tissue angiotensin systems. Moreover, the failure of kidney angiotensin I levels to increase with perindopril treatment, taken together with the fall in kidney angiotensinogen levels, suggests that angiotensinogen may be a major rate-limiting determinant of angiotensin peptide levels in the kidney.

Processing of rat and human angiotensinogen precursors by microsomal membranes

We have studied the processing of rat and human angiotensinogen precursors by microsomal membranes as a means of determining the number of asparagine-linked oligosaccharide units per angiotensinogen molecule, and thus the utilization of potential sites of N-glycosylation. Glycosylated, processed forms of angiotensinogen were isolated by chromatography on lentil lectin-Sepharose 4B. 35S-Methionine-labeled precursor and processed forms of angiotensinogen were compared with glycosylated and nonglycosylated 35S-methionine-labeled mature forms of angiotensinogen secreted by hepatoma cells, using immunoprecipitation, sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. N-Glycosylation of secreted angiotensinogen was inhibited using tunicamycin. For rat angiotensinogen, only 2 of 3 potential sites of N-glycosylation were utilized; in contrast, all 4 potential sites of N-glycosylation of human angiotensinogen were utilized. For neither rat or human angiotensinogen precursor was there any evidence for a prosequence.

Characterization of precursor and secreted forms of human angiotensinogen

To define the basis of the heterogeneity of angiotensinogen, we have characterized the immunoreactivity of high molecular weight (HMW) and low molecular weight (LMW) plasma angiotensinogen, the angiotensinogen precursor synthesized by cell-free translation, and angiotensinogen secreted by human hepatoma (Hep G2) cells. Angiotensinogen precursor synthesized by rabbit reticulocyte lysate primed with RNA prepared from liver or Hep G2 cells was compared with angiotensinogen secreted by Hep G2 cells by using immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). So as to assess the contribution of N-glycosylation of angiotensinogen, Hep G2 cells were incubated in the presence of tunicamycin. Glycosylation of secreted angiotensinogen was further characterized by using chromatography on concanavalin A-Sepharose, digestion with neuraminidase, and treatment with trifluoromethane sulfonic acid. In Sephadex G-200 column chromatography, HMW plasma angiotensinogen eluted just after the column void volume and was clearly separated from LMW angiotensinogen which eluted just before bovine serum albumin. Both HMW and LMW plasma angiotensinogen were shown to bind to monoclonal and polyclonal antibodies raised against pure LMW angiotensinogen. Only one angiotensinogen precursor (mol wt 50,000) was identified by cell-free translation which, after cleavage by renin, was reduced to mol wt 45,600. Angiotensinogen secreted by Hep G2 cells showed electrophoretic heterogeneity (mol wt 53,100-65,400). Tunicamycin-treated Hep G2 cells secreted five discrete forms of angiotensinogen, a predominant form of mol wt 46,200, with other forms (mol wt 46,800, 48,100, 49,200, and 49,600) representing 10% of secreted angiotensinogen. All five forms showed a similar reduction in molecular weight after cleavage by renin. The predominant 46,200-mol wt protein represented nonglycosylated angiotensinogen in that, after cleavage by renin, it had an electrophoretic mobility (mol wt 45,600) identical to the desangiotensin I-angiotensinogen resulting from renin cleavage of the angiotensinogen precursor. The other higher molecular weight forms of angiotensinogen secreted by tunicamycin-treated Hep G2 cells were shown to represent O-glycosylated angiotensinogen in that they were reduced to 46,200 mol wt by treatment with trifluoromethane sulfonic acid. Dexamethasone (10(-7) and 10(-6)M) stimulated angiotensinogen secretion by Hep G2 cells two- to fourfold, both in the absence and presence of tunicamycin. However, a small stimulatory effect of mestranol (10(-7) M) was evident only in the presence of tunicamycin. Neither dexamethasone nor mestranol influenced the electrophoretic pattern (SDS-PAGE) of angiotensinogen secreted by Hep G2 cells. However, when incubation media were chromatographed on Sephadex G-200 with subsequent immunoprecipitation of the column fractions, both dexamethasone and mestranol were shown to stimulate the secretion of HMW angiotensinogen (eluting just after the column void volume) which, on SDS-PAGE, migrated in a position identical to LMW angiotensinogen. From these studies, we conclude that all forms of human angiotensinogen are derived from a single precursor. The heterogeneity of secreted angiotensinogen represents differences in posttranslational processing of angiotensinogen. This processing includes both N- and O-glycosylation, and also the formation of HMW complexes (HMW angiotensinogen) through association either with other angiotensinogen molecules or with some other protein(s) whose secretion by hepatocytes is stimulated by glucocorticoids and estrogens.

Renin cleavage of a human kidney renin substrate analogous to human angiotensinogen, H-Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu-Val-Ile-His-Ser-OH, that is human renin specific and is resistant to cathepsin D

A synthetic tetradecapeptide, H-Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu-Val-Ile-His-Ser-OH, which corresponds to the 13 amino terminal residues of human angiotensinogen plus a carboxy terminal serine to replace a suggested site of carbohydrate attachment, has been shown to be a good substrate for human kidney renin. At pH 7.2 and 37 degrees C the KM or Michaelis constant was 8.4 +/- 2.9 microM, and the VM or velocity at infinite tetradecapeptide concentration was 11.3 +/- 2.4 mumol angiotensin I made per hour per milligram renin. The tetradecapeptide was highly resistant to cleavage by mouse submaxillary renin. The tetradecapeptide was also slowly cleaved by human liver cathepsin D, by rabbit lung angiotensin-converting enzyme, and by reconstituted human serum, but did not yield angiotensin I. Thus, this synthetic renin substrate should permit more specific measurement of human kidney renin activity.

Mechanism of the pressor response to tetradecapeptide renin substrate in the rat

The synthetic tetradecapeptide renin substrate (TDP; Asp-arg-val-tyr-ile-his-pro-phe-his-leu-leu-val-tyr-ser) has been employed frequently to elucidate the enzymatic action of renin in vitro and, to a lesser extent, in vivo. We assessed the utility of TDP as a renin substrate in vivo using conscious spontaneously hypertensive rats. Intravenous injection of TDP (1 and 3 micrograms/kg) increased diastolic pressure by 45 +r2 and 67 +/- 2 mmHg, respectively. The pressor response to TDP was significantly inhibited by captopril (3 mg/kg, po), indicating its dependence on conversion by ACE to some active molecule. Pressor responses to TDP also were less in animals subjected to bilateral nephrectomy 18-24 hr before study. However, responses to angiotensin I and II also were reduced, implying a non-specific effect of nephrectomy. Intravenous infusion of the renin inhibitor pepstatin (200 micrograms/min) inhibited pressor responses to hog renin by approximately 60%, but did not affect those to TDP. Intravenous infusion of the water soluble renin inhibitor, pepstatinyl-arginine-o-methyl ester (500 micrograms/min), also inhibited pressor responses to renin (approx. 80%) and did not affect those of TDP. Incubation to TDP (5 microM) with rabbit lung ACE resulted in generation of AI that was blocked by captopril (1 microM). These data suggest that TDP is a substrate for ACE and that the increase in blood pressure produced by TDP is due to its sequential cleavage by ACE to AII and can be independent of renin.

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.

Concept of internal structural controls for evaluation of inactive synthetic peptide analogs: synthesis of [Orn13,14]apamin and its guanidination to an apamin derivative with full neurotoxic activity

The importance of arginine residues 13 and 14 in the bee venom neurotoxin, apamin, was teste by the synthesis of replacement analogs. [13,14-di-Ndelta-trifluoroacetylornithine]Apamin was synthesized by the solid phase method on a benzhydrylamine resin. It was deprotected to [13,14-diornithine]apamin, which was then guanidinated to produce the 4-homoarginine-13,14-diarginine analog, [Har4]apamin. Neither the trifluoroacetylornithine analog nor the ornithine analog produced any detectable symptoms when injected intravenously into mice. However, the synthetic [Har4]apamin exhibited the full neurotoxic activity of native apamin and of [Har4]apamin derived from the natural toxin. This provided an internal structural control for the correctness of the primary structure of the inactive synthetic analogs and strengthened the conclusion that one, or both, of the arginine residues plays an important role in the action of apamin.

Conformations of synthetic tetradecapeptide renin substrate and of angiotensin I in aqueous solution

The properties of aqueous solutions of synthetic renin substrate tetradecapeptide (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu-Leu-Val-Tyr-Ser) were examined through electrometric titrations, infrared and circular dichroism spectroscopy, and spectrofluorometry. Titration studies of angiotensin I (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His-Leu) were also made, whose results indicated a flexible folded conformation similar to that previously proposed for the octapeptide angiotensin II, with a possible additional beta turn at the C terminus. The experimental results of the tetradecapeptide study, associated with Chou and Fasman calculations and with an analysis of structure-activity relationships in renin substrates and competitive inhibitors, led to the proposal of a beta turn involving the His-Pro-Phe-His sequence of the tetradecapeptide. This beta turn would be stabilized by beta-antiparallel interaction between residues 3-4 and 10-12 and by electrostatic attraction between the N-terminal ammonium and C-terminal carboxylate groups and would be destabilized below pH 5 by electrostatic repulsion between His6 and His9. The capacity to assume this conformation is related to structural requirements for renin substrates and competitive inhibitors.

The biochemistry of the renin-angiotensin system and its role in hypertension

The renin-angiotensin system has an important role in maintaining elevated blood pressure levels in certain forms of experimental and human hypertension. Renin, an enzyme produced by the juxtaglomerular cells of the kidney, acts on a protein substrate found in the alpha 2-globulin fraction of the plasma to produce a decapeptide, angiotensin I. This decapeptide is not directly pressor, but on passage through the pulmonary circulation is converted to an octapeptide, angiotensin II, a very potent pressor substance which acts by causing constriction of arteriolar smooth muscle. In addition to its direct action which increases blood pressure, angiotensin II acts on the adrenal cortex to cause the release of the sodium-retaining hormone aldosterone. Recent evidence suggests that this action may be mediated by the heptapeptide, angiotensin III. Both renin and its protein substrate exist in multiple forms and renin may also exist as a high molecular-weight "pro-hormone," although the physiologic significance of these forms is not clear. The elucidation of the biochemistry of the renin-angiotensin system has provided us with inhibitors which allow the system to be blocked effectively in vivo. Thus, angiotensin antagonists such as Sar 1, IIe 8-angiotensin II and converting enzyme inhibitors such as BPP 9a (SQ 20881) have proved useful in the study of experimental and human hypertension.

Purification and partial characterization of human angiotensinogen

Renin substrate was initially extracted from human plasma by (NH4)2SO4 followed by chromatography on Sephadex G-150, DEAE cellulose, calcium phosphate gel, isoelectric focusing and preparative polyacrylamide gel electrophoresis. On the basis of one mol of angiotensin per mol of substrate, the purity of the preparation is in excess of 95%. On analytical polyacrylamide gel electrophoresis in the presence of 1% sodium dodecyl sulfate or 8 M urea, the protein appears homogenous. In addition, the purified protein shows only one preciptin line against anti-normal human serum on either Ouchterlony immunodiffision or immunoelectrophoresis. The biological activity appears similar to "native" renin substrate since the Km is the same as that reported for the renin reaction in whole plasma. The molecular weight was determined as 110 000 by gel filtration and polyacrylamide gel electrophoresis; amino acid analysis of the human substrate differs from that reported for hog, especially in the Asp, Glu and Gly composition.

Circular-dichroism spectra of truncated and other analogs of angiotensin II

Circular dichroism spectra on angiotensin II and analogs, and its truncated N-terminal and C-terminal peptides were determined in fluroinated alcohols under several conditions in the peptide or aromatic spectral regions. The following conclusions were suggested: (a) evidence for a beta structure for angiotensin II; (b) evidence for a folding at the N-terminal and C-terminal part of the molecule; (c) an interaction involving the C-terminal residue which decreases progressively when phenylalanine is replaced by isoleucine and then by alanine; (d) the N-terminal amino acid seems to play an important role in the overall conformation of the molecule possibly by interacting with the C-terminus, its absence in the 2 -- 8 heptapeptide giving rise to a more pronounced signal than angiotensin II; (e) in trifluoroethanol the conformation of these peptides is well defined and fits well with observed structure-activity relationships and observed binding data. There is a loss of this relationship when these solvents are diluted with water.

The effect on drinking of peptide precursors and of shorter chain peptide fragments of angiotensin II injected into the rat's diencephalon

1. Recently it has been shown that injection of angiotensin II into the anterior diencephalon causes the rat to drink water. In the present experiments the dipsogenic action of a number of other substances including substances related to angiotensin was tested.2. Injection of 0.001 Goldblatt u. renin into the angiotensin-sensitive region causes the water-replete rat to drink. Drinking is slower in onset and continues for longer than after injection of angiotensin II.3. Synthetic tetradecapeptide renin substrate and angiotensin I were as effective as angiotensin II at causing water-replete rats to drink.4. beta-aspartic acid(1)-valine(5)-angiotensin II was also fully effective; but the D-arginine substituted octapeptide was much less effective.5. The (2-8) heptapeptide retained about 50% of the dipsogenic activity of the octapeptide, whereas the absence of phenylalanine at the other end of the peptide chain in the (1-7) heptapeptide results in an inactive compound.6. The (3-8) hexapeptide and the (4-8) pentapeptide, both of which have phenylalanine at the end of the chain, and the (1-4) and (5-8) tetrapeptide fragments of angiotensin II showed only a slight action on intake of water.7. Kallikrein, bradykinin, adenosine-3'5-cyclic phosphate, vasopressin and oxytocin caused no drinking when injected into the angiotensin-sensitive region.8. It is concluded that the requirements for the dipsogenic activity of angiotensin are the same as those for its other biological actions with the qualification that the precursor peptides are also active, presumably because they give rise to angiotensin II locally.

A radiochemical renin assay

1. A synthetic 3-([(14)C]valine)-labelled tetradecapeptide renin substrate was used to measure renin concentration. Renin liberated (14)C-labelled angiotensin I, which was separated from the labelled substrate by paper chromatography. The conversion of substrate into angiotensin I was quantitated by liquid-scintillation counting of radioactivity. 2. The rate of conversion of the substrate into angiotensin I was shown to be linearly related to renin concentration and time under suitable conditions. Angiotensin generation measured in this system agrees well with that measured by bioassay. 3. It is suggested that the use of a pure substrate offers advantages that include the standardization of current units of renin measurement.

Pseudorenin. A new angiotensin-forming enzyme

A new enzyme, pseudorenin, has been discovered which resembles renin in its ability to form angiotensin I from the synthetic tetradecapeptide renin substrate and from purified hog renin substrate A. Its maximal activity occurs at a much lower pH value than does that of renin. The two enzymes may be easily separated by DEAE-cellulose chromatography. Unlike renin, pseudorenin does not attack substrate A in the presence of serum, nor does it produce angiotensin I from renin substrate as it exists in serum. In contrast to renin, which occurs primarily in the kidney, pseudorenin has been found in every one of the 13 different tissues which have been tested, and also in the plasma. The natural substrate for the new enzyme as well as its physiological function are not known.

Kinetics of the reaction of renin with nine synthetic peptide substrates

A number of peptides have been synthesized which represent portions of the tetradecapeptide renin substrate molecule, and which contain the hydrolyzable leu-leu bond. An automatic chemical method for determination of the velocity of the reaction of renin with these compounds was developed. Application of the method at several levels of substrate concentration permitted construction of Lineweaver-Burk plots, and calculation of Michaelis constants (K(m)) and maximal velocities (V(max)). The results show that the maximum affinity of the enzyme (lowest K(m)) for substrate is achieved only with the full tetradecapeptide molecule (asp(1)-arg(2)-val(3)-tyr(4)-ileu(5)-his(6)-pro(7)-phe(8)-his(9)-leu(10)-leu(11)-val(12)-tyr(13)-ser(14)). Removal of asp(1) and arg(2) from the N-terminal increases the K(m) eight-fold. Further, moderate increase in K(m) occurs when the next amino acids, val(3), tyr(4) and ileu(5), are removed. The further removal of his(6) results in a marked reduction in the V(max). Removal of ser(14) from the C-terminal of the nonapeptide his(6)-pro(7)-phe(8)-his(9)-leu(10)-leu(11)-val(12)-tyr(13)-ser(14) does not greatly affect the K(m) nor the V(max). Further removal of tyr(13) from this compound results in complete loss of substrate activity. It is suggested that the compounds his(6)-pro(7)-phe(8)-his(9)-leu(10)-leu(11)-val(12)-tyr(13)-ser(14) or his(6)-pro(7)-phe(8)-his(9)-leu(10)-leu(11)-val(12)-tyr(13) might be used as substrates for the chemical assay and standardization of renin.

Chemistry of Synthetic Angiotensin

A short review is given of the development of synthetic angiotensin. Also discussed are enzymic experiments, showing that production of the synthetic substance leads to valyl-5-angiotensin-II, in which all amino acids are completely in the l -configuration. Small amounts of diastereoisomers in intermediates, like l -valyl- d -tyrosyl- l -valyl- l -histidyl- l -prolyl- l -phenylalanine which was isolated and identified, are completely eliminated during the synthetic process. Valyl-5-angiotensin II-aspartyl-β-amides are very slowly hydrolyzed in aqueous solution to valyl-5-angriotensin II and a small amount of the heptapeptide, H·Arg-Val-Tyr-Val-His-Pro-Phe·OH, with negligible decrease in pressure activity of the solution.

The histologic distribution of renin in the kidneys of the rat and rabbit

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