Conversion between renin and high-molecular-weight renin in the dog.

Two forms of renin, one of mol.wt. 43,000 and the other 60,000, were found in the dog kidney. Conversion between the two forms of renin was reversible at neutral pH. Though the molecular weight of renin in kidney-cortex homogenate was 43,000, it was completely converted into high-molecular-weight renin in the presence of substances that react with thiol groups. On the contrary, stored renin in the granules was the form of normal size (mol. wt. 43,000) regardless of the absence or presence of such substances. The present experiments indicated that renin is stored in the granules as the form of normal size and might be converted into high-molecular-weight renin when it is released from the granules and attached to some substance in the soluble fraction of renal-cortical tissue.     

High molecular weight renin identified in cytosol fractions of mouse renal cortices

In an attempt to confirm that high molecular weight renin was indeed true renin, we used a specific renin antibody and high performance liquid chromatography to determine characteristics of this protein. In mouse renin granules, renin was stored in a low molecular weight form of 38,000 daltons (LMW renin) and this molecular weight remained unchanged with application 20 mM of sodium tetrathionate. In the cytosol fraction of the renal cortex, LMW renin was partially converted to high molecular weight renin (HMW renin) of 65,000 daltons, as determined using tetrathionate. In both the LMW and HMW renin, enzymatic activity was completely neutralized by application of a specific antiserum of renin and an absolute amount of renin was identified by direct radioimmunoassay. The Km values of HMW and LMW renin were similar. Thus, LMW renin probably binds with renin binding substance and forms HMW renin.     

Investigation of human and rat inactive renin in plasma and kidney.

Under an initial interval of immobilization stress in rats, reciprocal changes of plasma active and inactive renin were observed, suggesting activation of circulating inactive renin. Molecular weight (MW) studies revealed that this activation might proceed via a MW shift from inactive renin with MW of 50,000 to active renin of MW 43,000. In a later interval of stress, under stimulated renin secretion, a lower MW form (38,000) of active renin was released into the circulation. This MW is close to that of active renin (39,000) found in rat kidney renin granules. In renin granules, equilibrated in fractions of 1.6 and 1.7 mol/L sucrose in discontinuous density gradient, trypsin-activatable renin activity formed 36 and 16% of total activity, respectively. In humans, under acute bicycle exercise, a lower MW form (39,000) of active renin was released into the circulation, while the content of inactive renin with MW in the range of 51,000-58,000 and at 47,000 did not substantially change. There was a slight decrease in circulating inactive renin passing through the kidney. The data suggest that, at least in rats, in vivo pathways for activation of inactive renin might exist, other than that proceeding before secretion from renin granules. Under the conditions of increased renin secretion, a lower MW form of active renin is mainly released into the circulation in both rats and humans.     

A novel purification method of human renin

Human renal renin was isolated from a partially purified preparation, Haas' preparation step 5 material, with a remarkably high yield of 28% by a newly developed method. This method consisted of only four steps including affinity chromatography on pepstatin-aminohexyl-agarose, chromatofocusing, gel filtration and DEAE-chromatography after the initial extraction and concentration. This method enabled us to obtain pure human renin without another affinity column used by other investigators. Pure human renin had a molecular weight of 40,000 daltons as estimated by gel filtration and sodiumdodecylsulfate-polyacrylamide gel electrophoresis. The pI of purified renin was 5.6.     

The effects of kallikrein, plasmin, and thrombin on hog kidney renin.

Pretreatment of hog high molecular weight renin for 30 min at 37 degrees C with 0.12 unit of either kallikrein or thrombin significantly increased (p less than 0.001) the amount of angiotensin I formed during subsequent incubations with homologous angiotensinogen. However, the thrombin-treated hog renin had 13 times more activity than the kallikrein-treated enzyme. Aprotinin did not inhibit the kallikrein-mediated activation of renin; the results indicated that aprotinin inhibited renin preferentially. Plasmin (0.25 unit) had little effect on the activity of high molecular weight renin. The molecular weight of hog renin on sodium dodecyl sulfate-polyacrylamide gel electrophoresis was not altered after exposure to either kallikrein, thrombin, or plasmin. These results do not exclude the occurrence of a limited proteolytic event or a conformational change beyond the detection of the current method. The data show that the activation of hog high molecular weight renin by thrombin and kallikrein was not associated with the conversion of renin to Mr = 43,000.     

Chromatographic separation of multiple renin substrates in women: effect of pregnancy and oral contraceptives

Chromatographic separation of multiple renin substrates was carried out to study the effect of pregnancy and oral contraceptives (OCs) in women. Blood plasma proteins were separated on a Sephadex G-200 column and the amount of renin substrate in each fraction was determined. The molecular weight of the renin substrate peaks was estimated by comparison with the elution position of proteins of known molecular weight. Normal nonpregnant women revealed the presence of 2 renin substrates: a major 1 with a molecular weight of 65000 and 1 amounting to 3-5% of the quantity of the major peak with a molecular weight of 450000-500000. In women taking OCs the same 2 peaks were seen but the quantity of each was increased markedly. In pregnant women at term, a 3rd renin substrate of intermediate size was seen (molecular weight 350000 daltons). The quantities of the intermediate and low molecular weight renin substrates were found to be greatly increased in pregnancy.     

Human placental chorionic renin: production, purification and characterization

Native human renin, produced from the culture of human chorionic trophoblasts, has been purified to homogeneity on a milligram scale using a five-step purification scheme. The chorion cells secrete 50-200 milliGoldblatt Units of trypsin-activatable prorenin per ml into the medium. The pro-enzyme is partially purified by ammonium sulfate fractionation and chromatographies on QAE-Sephadex and cibracon blue-agarose. Following conversion of prorenin to the active enzyme by porcine trypsin, the renin is purified to homogeneity by affinity chromatography and gel filtration. Chorionic prorenin has a molecular weight of 43,000; the active enzyme 40,000. Both proteins exist as a single polypeptide chain as determined by SDS-polyacrylamide gel electrophoresis under reducing conditions. The average specific activity of six different preparations was found to be 1072 Goldblatt Units/mg. The amino acid composition and N-terminal sequence of the active enzyme has been determined and is identical to the human kidney enzyme. Microheterogeneity of chorionic renin was demonstrated by isoelectrofocusing analysis. The physical characterization of chorionic renin is compared with that reported for the human kidney enzyme.     

Human renin activation by protease from the renin granule fraction of the dog kidney cortex

To clarify the possible conversion of prorenin in renin granules where conversion reportedly occurred, we investigated whether the renin granule fraction of the kidney could activate prorenin to the active form. Renin granules were isolated from the dog kidney cortex by discontinuous sucrose density gradient centrifugation. Human active renin was quantified by immunoradiometric assay which could detect only the human active renin but not the inactive human renin or dog renin. Inactive renin from human amniotic fluid was incubated with the subcellular fraction of the dog kidney cortex. The renin granule fraction that showed the highest renin activity stimulated the inactive renin to become the active form. The membrane preparation obtained from the renin granule fraction by freezing and thawing the fraction in low osmolarity retained the activity of renin activation. Other subcellular fractions showed less renin activation. The optimal pH for renin activation by the membrane was pH 5.0 to 6.0. The activation depended on the time of incubation and concentration. The activation was inhibited by N-ethylmaleimide but not by EDTA or serine protease inhibitors. These results suggest that renin is processed by a membrane bound protease in renin granules.     

Purification of high molecular weight (HMW) renin from porcine kidney and direct evidence that the HMW renin is a complex of renin with renin binding protein (RnBP)

The high molecular weight (HMW) renin was purified from porcine kidney by a procedure involving extraction with a buffer system containing protease inhibitors, ammonium sulfate fractionation, pepstatin-aminohexyl-Sepharose 4B column chromatography, gel filtration on Ultrogel AcA 44 and aminohexyl-Sepharose 4B column chromatography. The resulting preparation showed a single band on isoelectric focusing, exhibiting an isoelectric point at pH 5.25, and was stable on storage at -80 degrees C for 4 months. The specific activity was 3.97 mg of angiotensin I formed/mg of protein per h at 37 degrees C and at pH 6.5 with porcine angiotensinogen as the substrate. When the HMW renin was exposed to acid, renin activity increased by about 5-fold and the free form of fully active renin was recovered from the acidified HMW renin, leaving an insoluble aggregate of protein. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the HMW renin showed two protein bands, of which one was identified as renin from the electrophoretic mobility and the other was the protein, assigned as renin binding protein (RnBP), that was insolubilized by acidification. The purified HMW renin is a complex of renin with RnBP, and the molecular weights of RnBP and renin in the HMW renin were estimated to be 39,000 and 32,000, respectively, by gel permeation liquid chromatography in 6 M guanidine-HCl. A modified rapid method for purification of renin is also presented.     

Effects of nucleotides on the interaction of renin with GlcNAc 2-epimerase (renin binding protein, RnBP)

Renin binding protein (RnBP), a cellular renin inhibitor, was identified as an enzyme, GlcNAc 2-epimerase. Recombinant RnBP inhibited porcine renin activity in a dose dependent manner. However, the inhibition was neutralized by nucleotides, such as ATP, dATP, dGTP, dCTP or dTTP. Moreover, ATP inhibited the formation of hetero-complex of renin with RnBP, called high molecular weight (HMW) renin. On the other hand, N-ethylmaleimide (NEM), a SH-alkylating reagent inhibited the GlcNAc 2-epimerase activity concomitant with the decaying of the dimer to the monomer of the enzyme. The inhibition was modulated in the presence of ATP. These results indicate that nucleotides stabilize the dimeric form RnBP (GlcNAc 2-epimerase) and inhibited the formation of the renin-RnBP hetero complex, HMW renin.     

A new form of renin in normal human plasma: “big renin” is a mixture of inactive prorenin and the new active high molecular weight renin

A new form of active renin was separated from inactive prorenin in normal human plasma by a new affinity chromatographic method on a column of Cibacron Blue F3GA-agarose. This active renin has a molecular weight of 54,000, considerably higher than the hitherto recognized active renin of 40,000 dalton in human plasma. The molecular weight of inactive prorenin was 56,000±2,000. Active renin produced from the inactive prorenin by trypsin or pepsin digestion or by acid treatment in $\text{in vitro}$ experiments showed a molecular weight of 54,000±2,000. Active renin with a molecular weight of 40,000 was not found in 6 samples of untreated plasma of normal human subjects nor was it formed by treatment with trypsin, pepsin, or acid pH. It is concluded that a large form of active renin (54,000 dalton) exists in normal human plasma which is distinct from a smaller form and that the activatable “big renin” is a mixture of this active renin and totally inactive prorenin. This explains the absence of molecular weight change during the activation of “big renin”.     

Characteristics and conversion of high molecular weight forms of renin in plasma and their incomplete activation by the current acid treatment.

Two distinctly different high molecular weight forms of renin are present in mouse plasma in addition to the well-recognized active 40 000 dalton form. The biggest form has a molecular weight of about 800 000, and is stable in 4 M urea, but can be converted to the active 40 000 dalton form, by storage, exposure to acid and limited proteolysis. The 70 000 dalton form can be activated by acid and limited proteolysis. However, the 70 000 dalton form does not change molecular weight with activation. By measuring renin, not only by its enzymatic activity, but also by the direct radioimmunoassay for the renin molecule, which measures enzymatically active as well as inactive renin, it was found that both forms were activated but neither of them completely. The validity of the currently used term "total" renin as the enzymatic renin activity after acid activation, is, therefore, questionable. The quantitative significance of this must await methods which can ensure complete conversion or activation of the high molecular weight forms of renin in plasma.     

A study on renin binding protein (RnBP) in the human kidney

We purified, from human kidney, a protein that reacts with rabbit anti-porcine kidney renin binding protein (RnBP) antiserum by trapping with porcine kidney renin. The purified preparation showed a single protein peak on gel filtration by high performance liquid chromatography (HPLC) and two protein bands on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The latter two kinds of protein were identified as the porcine renin and human kidney protein from their electrophoretic mobilities and reactivity toward rabbit anti-porcine kidney renin and RnBP antisera. The molecular weights of the purified preparation and the human kidney protein were estimated to be 56,000 by HPLC and 43,000 by SDS-PAGE, respectively. The specific activity of porcine renin in the purified preparation was 8.6 mg angiotensin I per mg of protein per h at 37 degrees C and pH 6.5. This specific activity was about one-fifth that of free porcine renin. Therefore, it is suggested from the reactivity toward the anti-porcine RnBP antiserum and inhibitory action toward porcine renin that the human kidney protein is RnBP and that the human RnBP is purified as a complex with porcine renin.     

Human renal renin. Complete purification and characterization

Complete purification of human renin from noncancerous, autopsied kidneys is reported. A 480,000-fold purification was achieved to yield renin with a specific activity of 950 Goldblatt units/mg. This preparation satisfied multiple criteria of purity as tested by polyacrylamide gel electrophoresis, isoelectric focusing, specific activity, analytical ultracentrifugation, and immunodouble diffusion. The molecular weight of the pure enzyme determined by sedimentation equilibrium is 40,000. The apparent molecular weight estimated by gel filtration is 41,000. The enzyme has an isoelectric point of pH 5.7. Human renin shows an affinity for concanavalin A, suggesting the presence of carbohydrates. These properties and the amino acid composition of human renin are different from those of renin obtained from other mammalian species. Human renin antibodies prepared with the pure enzyme preparation showed negligible cross-reactivity with renin from other mammalian species. The activity with homologous human renin substrate has a pH optimum of 6, whereas with substrates from other mammalian species the optima were in higher or lower pH ranges.     

Quantitative changes in rat renin secretory granules after acute and chronic stimulation of the renin system

In order to study the cellular mode of renin secretion, stereological methods were used to estimate number and volume of rat renin secretory granules during stimulation of the renin system. An acute decrease in renal perfusion pressure to 40 mmHg for 5 min increased plasma renin concentration (PRC) twofold, but did not significantly change the number of renin granules per arteriole or the renin-containing volume of the arteriole. Chronic stimulation was achieved by a combination of low-salt diet and inhibition of angiotensin-converting enzyme (ACE) for 14 days, and resulted in a 36-fold increase in PRC, a 20-fold increase in the number of granules per arteriole, and a 17-fold increase in the arteriolar volume that contained renin. An acute decrease in renal perfusion pressure to 40 mmHg for 5 min in the chronically stimulated rats increased PRC further (1.6-fold), and significantly reduced the number of granules per arteriole by 4000 (45% reduction), but did not change the renin-containing arteriolar volume significantly. The average renin granule size was 0.35 μm³ with no significant differences among the groups. We conclude that recruited granular cells contribute significantly to renin release, and that all granular cells along the arteriole participate in secretory responses. The reduced number of renin granules after acute stimulation is compatible with exocytosis as the dominating mechanism of renin release.     

Different isotonic density gradients in separation of renin granules from rat kidney cortex

Crude renin granule preparations isolated from the rat renal cortex were further purified in isotonic conditions (300 mOsm/kg) using various density gradient materials. It was not possible to separate renin granules from other subcellular organelles using dextran, 40,000-sucrose or metrizamide-sucrose gradients at about 300 mOsm/kg. When osmolality of dextran-sucrose gradients was increased, some separation was found but both renin granules and mitochondria gained density. During a short centrifugation (4640 X g, 30 min) renin granules remained intact and appeared in two populations in Percoll-sucrose gradients. The apparently heavier (larger) particles (at 1.12-1.13 kg/l) were greatly purified from mitochondria (80 X purification vs. the whole homogenate), protein (120 X) and lysosomes (24 X). Electron micrographs demonstrated many dense core granules. The fraction containing apparently lighter (small) granules (at 1.08-1.09 kg/l) was heavily contaminated with mitochondria and lysosomes. During longer centrifugation (4640 X g, 60 min), only one major peak showing renin activity was observed at 1.12-1.13 kg/l, and other cell organelles were lighter. Hence the two renin populations evidently do not differ in density but rather in size. In the animals kept on a low-sodium diet, both types of renin granules were increased.     

The characterization of actin associated with postsynaptic membranes from Torpedocalifornica

SDS-polyacrylamide gel electrophoresis of acetylcholine receptor from $\text{Torpedo}\text{californica}$ electroplax membrane fragments shows, in addition to the four receptor subunits of 40,000, 50,000, 60,000 and 65,000 daltons, other components of apparent molecular weights 43,000, 47,000 and 90,000 daltons. In this study deoxyribonuclease I inhibitory activity has been used to identify actin in $\text{Torpedo}\text{californica}$ receptor-enriched membranes and affinity chromatography on a deoxyribonuclease I agarose column has been used to purify this protein from the membrane preparations. In addition the membrane protein components have been analyzed by electrophoresis on a series of SDS-polyacrylamide gels of varying acrylamide concentrations. Evidence is presented that actin is a component of most preparations of receptor-enriched membrane fragments, having an apparent molecular weight of 47,000 daltons, and is distinct from the 43,000 dalton protein.     

Biosynthesis of a renin binding protein

The biosynthesis of a porcine renin binding protein (RnBP), which specifically binds to renin and forms an inactive high molecular weight renin, was investigated. mRNAs from various porcine tissues were used to investigate in vitro protein synthesis. The kidney mRNA directed the synthesis of a high level of RnBP, whereas the liver, adrenal and pituitary gland mRNAs gave as low but significant level of it. The in vitro synthesized RnBP as well as the immunologically detected RnBP synthesized in vivo had the same molecular weight, 42,000, as that of the purified protein. Moreover, both the human and rat kidney mRNAs directed the synthesis of this protein identified with an anti-porcine RnBP antibody. These results strongly indicate that RnBP, present in various mammalian species, is synthesized in renin-producing tissues as the mature size and undergoes binding with renin without proteolytic processing.     

Purification and characterization of human kidney renin

By means of a new rapid and small scale purification method, human kidney renin has been purified from a single kidney in a homogeneous state, as judged on SDS-PAGE. The kidney which showed unusually high renin activity was from a patient with cardiomyopathy. 8,000-fold purification was attained by means of only pepstatin-aminohexyl-Sepharose chromatography and FPLC on a Mono Q column, and the yield was 34%. The specific activity was 5.63 mg angiotensin I per mg protein per h at 37 degrees C and pH 6.5 with porcine angiotensinogen as the substrate. The molecular weight was estimated to be 37,000 by SDS-PAGE and 38,000 by HPLC on a TSK G-3000 SW column. The preparation showed three bands on isoelectric focusing. The molecular weight and the profile on isoelectric focusing of the purified renin agreed with those found for the extracts of both the patient's kidney and a kidney with the usual low renin activity.     

Purification and characterization of renin binding protein (RnBP) from porcine kidney

Renin binding protein (RnBP) was purified from porcine kidney using pepstatin affinity column chromatography, DEAE-Sepharose column chromatography, gel filtration on Ultrogel-AcA 34, aminohexyl-Sepharose 4B column chromatography, and high performance liquid chromatography (HPLC) on TSK-gel G-3000 SW. The purified preparation was homogeneous as judged by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, polyacrylamide disc gel electrophoresis and isoelectric focusing on polyacrylamide gel. The isoelectric point was at pH 4.85, and the apparent molecular weight of RnBP was estimated to be 42,000 by SDS-polyacrylamide gel electrophoresis. The preparation did not show any renin activity and was stable for 30 min at 37 degrees C between pH 5.0 and 9.0 or on storage for 4 weeks at 4 degrees C or -80 degrees C. The activity of renin was greatly inhibited by RnBP. From the kinetic analysis of the inhibition we roughly estimated the dissociation constant between renin and RnBP to be about 0.2 nM, assuming that the stoichiometry in the complex, i.e., high molecular weight (HMW) renin, is one to one, and that the complex is inactive. The inhibitory activity of RnBP was lost by acidification at pH 3.0 and the activity of renin was restored. The purified RnBP formed a single precipitin line with the antiserum prepared with the purified HMW renin as antigen, which is RnBP-renin complex (Takahashi, S., et al. (1983) J. Biochem. 93, 265-274), and this line fused with one of the two precipitin lines formed between HMW renin and anti-HMW renin antiserum. The other of the two lines was between renin and anti-HMW renin antiserum. The purified preparation was thus identified as RnBP. The HMW renin was reconstituted with the purified RnBP and renin, and the apparent molecular weight of the reconstituted specimen was estimated to be 60,000 by gel filtration on Ultrogel AcA 44.