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”.     

Immunological evidence that inactive renin is prorenin

Antibody raised to a synthetic dodecapeptide, corresponding to the C-terminal portion of the human renin pro-segment, was tested for its ability to recognize highly purified human inactive or active (mature) renins; immune complexes were detected by precipitation with protein A-Sepharose. Serial antibody dilutions caused identical binding of renal or plasma inactive renins but failed to bind active renin. In contrast, antibody to active renin recognized both active and inactive forms. Reversible acid activation of inactive renin enhanced its binding to the anti-prorenin antibody, whereas irreversible trypsin activation significantly reduced binding. Binding was abolished following prolonged exposure to trypsin or if inactive renin was acidified prior to trypsin treatment. These results indicate that inactive renin shares immunochemical determinants with prorenin; they suggest that acidification alters the conformation of the pro-segment and that trypsin can convert the molecule both to fully mature renin and to intermediate form(s).     

Analysis of inactive renin by renin profragment monoclonal antibodies

Two peptides were synthesized, corresponding to the sequences (-19 to -7) and (-26 to -17) of the prorenin prosegment. Monoclonal antibodies were raised to these sequences and used to characterize human plasma inactive renin. Only anti (-19 to -7) reacted with inactive renin, as measured by direct assay or affinity chromatography. The data were used to evaluate two possible inactive renin stuctures: plasma inactive renin is a truncated prorenin lacking the prosegment N-terminal portion; its spatial conformation masks the N-terminal extremity, preventing interaction of this region with specific antibodies.     

Plasma inactive renin in patients with diabetes mellitus: effects of standing and the relation to serum protease inhibitor

In order to investigate the mechanisms of increased plasma inactive renin in diabetics with microvascular complications, changes in active and inactive renin with the progress of diabetes mellitus were studied, and effects of standing on inactive renin release and the relationship between plasma inactive renin and serum trypsin or protease inhibitors wee also studied. Inactive renin increased with the aggravation of diabetes mellitus, but active renin didn't show significant changes with the aggravation of diabetes mellitus. Active renin was significantly increased both in the healthy subjects and in the diabetic patients when they were in an upright position, but no significant change was observed in inactive renin. Serum trypsin in diabetics with retinopathy and nephropathy was lower than that in those with no clinical sign of microangiopathy, but the correlation between plasma inactive renin and serum trypsin was not significant. There was a significant correlation between plasma inactive renin and serum alpha 2-globulin (r = 0.52, p less than 0.01). Although plasma inactive renin was not significantly correlated with serum alpha 1-antitrypsin, there was a significant correlation between plasma inactive renin and serum alpha 2-macroglobulin (r = 0.61, p less than 0.01). These results show that the increased levels of plasma inactive renin observed with the development of diabetic microangiopathy are probably related to the altered plasma protein metabolism observed in patients with diabetes mellitus. However, it is not clear whether this altered protein metabolism is related to the conversion from inactive to active renin.     

Renin: structural features of active enzyme and inactive precursor

To determine the structural basis for the unique catalytic mechanism of renin and the mechanism of activation of inactive renin, renin and inactive renin were isolated in pure form. The active site of renin consists of two aspartyl residues, two tyrosyl residues, and one arginyl residue, analogous to pepsin and other acid proteases. The complete amino acid sequence of mouse submaxillary gland renin was determined. Of the amino acids, 43% were identical to those in porcine pepsin. Combination of various chromatographic techniques permitted the separation of inactive renin from active renin in human plasma and kidney. Inactive renin of hog kidney was completely purified. Inactive renin consists of a single polypeptide chain and is activated by proteolysis but not by dissociative reagents such as 4 M NaCl or detergent. Thus it was concluded that the inactive renin in these tissues is renin zymogen rather than a renin-inhibitor complex.     

Kinetic comparisons of amniotic fluid inactive renin and renal renin using synthetic and human renin substrates

Inactive renin has been isolated from pooled amniotic fluid and purified approximately 642-fold. Prior to activation the isolates had approximately 4% of the activity found after activation. The observation is similar to that reported for inactive renin from chorionic cell culture and suggests a placental origin of amniotic fluid inactive renin. Using plasma from an estrogen-treated woman, renin substrate was recovered free of renin and inactive renin and a portion was separated into NMW and HMW components. The NMW form constituted approximately 93% and the HMW form approximately 7% of the renin substrate. Amniotic fluid inactive renin was used for determinations of enzyme-substrate kinetics with the pooled, NMW, and HMW plasma substrate and tetradecapeptide synthetic substrate, and the results were compared to similar determinations using standard renal renin. Using synthetic substrate, the kinetics of renal renin and amniotic fluid inactive renin before and after activation were similar. The kinetics of renal renin with pooled, NMW, and HMW plasma substrate were also similar. Amniotic fluid inactive renin had a lower Km with pooled than with NMW substrate, however, which resulted from a significantly smaller Km with HMW component. Although the affinity constants with pooled substrate were not different for renin and inactive renin, the Km of inactive renin was significantly less with the HMW component of plasma renin substrate. The observations are compatible with a role for placental inactive renin in normal pregnancy and suggest the possibility of a further role in hypertensive pregnancy.     

Mechanism of activation of inactive renin in human plasma by puff adder venom

Venom of the puff adder (Bitis arietans) contains a potent, basic, Mr 24,000 metalloproteinase activity that can destroy all detectable trypsin and chymotrypsin inhibitory activity, when venom is incubated with human plasma. We have found that during such incubation, concomitant activation of inactive renin occurs. In an examination of the mechanism involved we now report the activation, in addition, of plasma prekallikrein and serine proteinase activity, but not plasminogen, when human plasma is incubated with venom. Furthermore, venom was not able to release active trypsin from its complex with alpha 1-proteinase inhibitor and human renin was not inhibited by alpha 1-proteinase inhibitor. The activities in venom and venom/plasma mixtures were analysed using Sephacryl S-200 gel filtration and the effect of 10 mM EDTA and 5 mM phenylmethanesulphonyl fluoride on activities in column fractions was tested. The inactive-renin-activating, plasma prekallikrein-activating and serine proteinase-activating activities could be accounted for to a large extent by a venom metalloproteinase which was estimated to have a Mr of 24,000 by sodium dodecyl sulphate (SDS)-polyacrylamide gel electrophoresis. This enzyme activity appeared to complex with alpha 2-macroglobulin when venom was mixed with plasma. Since both EDTA and phenylmethanesulphonyl fluoride could inhibit the activation of inactive renin by this metalloproteinase, it is suggested that the enzyme activates serine proteinase(s), which then activate inactive renin. Plasma kallikrein may have a role in this process. Additional peaks of inactive-renin-activating activity eluted from Sephacryl S-200 at Mr 30,000 and 80,000 (minor) and an additional, minor peak of caseinolytic activity eluted at Mr 60,000. The Mr 24,000 metalloproteinase in venom may have considerable utility in activating inactive renin at physiological pH owing to its ability to destroy plasma proteinase inhibitors at the same time.     

Measurement of inactive renin in normal, nephrectomized, and adrenalectomized rats.

In a new method for measurement of inactive rat plasma renin, the trypsin generated angiotensin I immunoreactive material, which was HPLC characterized as similar to tetradecapeptide renin substrate, is removed by a cation exchange resin before the renin incubation step. The method also corrects for trypsin destruction of endogenous angiotensinogen by the addition of exogenous angiotensinogen. When measured with this method inactive renin in rat plasma decreased after nephrectomy and increased after adrenalectomy. This is in accordance with findings in humans. A sexual dimorphism of prorenin (inactive renin) in rat plasma, similar to that reported in humans and mice, was demonstrated. Thus, inactive renin in the rat is no exception among species, and the rat might be a suitable animal model for further studies dealing with the physiology of prorenin in plasma and tissues.     

Does heparin inhibit renin activity?

Although heparin was reported in the 1960s to inhibit renin activity, this has not always been confirmed by other investigators. Hence, we re-examined whether heparin really inhibits renin or not. Renin activities were determined by radioimmunoassay of angiotensin I generated at pH 7.4. (i) No significant difference was found between the two kinds of plasma samples obtained with heparin and with EDTA as anticoagulant, in ARC (renin activity with addition of sheep renin substrate), TRC (ARC after activation of inactive renin by trypsin), or PRA (plasma renin activity without additional substrate). (ii) Even in higher concentrations of heparin up to 500 U/mL, neither PRA, ARC, nor TRC of plasma was affected significantly. (iii) Heparin, in concentrations up to 500 U/mL, exerted no significant effect on TRC of the media of human vascular smooth muscle cell culture. In conclusion, heparin does not exert any significant inhibitory effect on human renin nor does it affect activation of inactive renin by trypsin in the range of concentration of practical use, under the conditions employed in this study.     

Nonproteolytic "activation" of prorenin by active site-directed renin inhibitors as demonstrated by renin-specific monoclonal antibody.

Incubation of human plasma prorenin (PR), the enzymatically inactive precursor of renin (EC 3.4.23.15), with a number of nonpeptide high-affinity active site-directed renin inhibitors induces a conformational change in PR, which was detected by a monoclonal antibody that reacts with active renin but not with native inactive PR. This conformational change also occurred when inactive PR was activated during exposure to low pH. Nonproteolytically acid-activated PR, and inhibitor-"activated" PR, as well as native PR, were retained on a blue Sepharose column, in contrast to proteolytically activated PR. Kinetic analysis of the activation of plasma prorenin by renin inhibitor (INH) indicated that native plasma contains an open intermediary form of prorenin, PRoi, in which the active site is exposed and which is in rapid equilibrium with the inactive closed form, PRc. PRoi reacts with inhibitor to form a reversible complex, PRoi.INH, which undergoes a conformational change resulting in a tight complex of a modified open form of prorenin, PRo, and the inhibitor, PRoi.INH-->PRo.INH. The PRoi-to-PRo conversion leads to the expression of an epitope on the renin part of the molecule that is recognized by a renin-specific monoclonal antibody. Presumably, PRo corresponds to the enzymatically active form of PR that is formed during exposure to low pH. Thus, it seems that the propeptide of PR interacts with the renin part of the molecule not only at or near the enzyme's active site but also at some distance from the active site. Interference with the first interaction by renin inhibitor leads to destabilization of the propeptide, by which the second interaction is disrupted and the enzyme assumes its active conformation. The results of this study may provide a model for substrate-mediated prorenin activation and increase the likelihood that enzymatically active prorenin is formed in vivo.     

Mouse submaxillary renin has a protease activity and converts human plasma inactive prorenin to an active form

The activation of inactive prorenin by active renin was investigated. Inactive prorenin extensively purified from human plasma was activated by active renin which had been purified from mouse submaxillary glands by multiple chromatographic steps. The apparent lack of protease activity in renin was puzzling in view of the close similarity of its active site structure with that of acid proteases. After a series of affinity chromatographic steps designed to eliminate minute contaminants, renin was found to contain a very low but finite level of a neutral protease activity which was equivalent to 1/40,000 of that of cathepsin D tested by hemoglobinolytic activity. The protease activity was considered as intrinsic to renin since it co-purified with renin persistently at a constant ratio to the renin activity, was precipitated by a monoclonal antibody specific for renin, showed a neutral pH optimum of the enzyme activity in the same pH range as that of renin, and was inhibited by pepstatin. The neutral protease activity is likely to mediate the activation of inactive prorenin.     

Purification of human plasma inactive renin by immunoaffinity chromatography on profragment-specific IgG

Inactive renin was purified to apparent homogeneity from human plasma by ion exchange, gel filtration, Affi-Gel blue, immunoaffinity chromatography on profragment-specific IgG coupled to Sepharose, and preparative HPLC. By this method, a 460000-fold purification was obtained. The purified renin was totally inactive and was activated by trypsin.     

Trypsin activation of inactive renin: plasma blanks and angiotensin I radioimmunoassay.

Divergent conclusions exist as to whether inactive renin is present in nephrectomized rat plasma. A major factor contributing to this conflict may be related to significant changes in the "plasma blank" when trypsin-treated plasma is subjected to angiotensin I (AI) radioimmunoassay (RIA). In normal, but not nephrectomized rat plasma, AI-like substances are present in direct proportion to active renin. These substances are destroyed by trypsin. However, trypsin generates additional AI-like material, in both normal and nephrectomized rat plasma. This material, which is present in proportion to the renin substrate concentration, does not appear to be tetradecapeptide (TDP). In normal plasma, however, exogenous TDP is converted to AI in proportion to the active renin concentration and AI generation from TDP is increased by activation of inactive renin. However, in nephrectomized rat plasma, no AI generation from TDP was evident either before or after trypsin treatment. The coincident tryptic generation of a substance that quenches the levels of AI detected by RIA, combined with significant changes in the levels of endogenous and trypsin generated AI-like substances, may have significant bearing on the measured levels of inactive renin.     

Pure human inactive renin. Evidence that native inactive renin is prorenin

To clarify contradicting observations on the identity of inactive renin and prorenin, inactive renin was completely purified from native human chorion laeve and the culture medium of human chorion cells. A 720,000-fold purification with 14% recovery was achieved from chorion laeve in 6 steps, including immunoaffinity chromatography on a monoclonal antibody to human renin coupled to Protein A-Sepharose CL-4B. A 3,100-fold purification with 40% recovery was achieved from chorion culture medium in 4 steps, including immunoaffinity chromatography. Inactive renin purified from the two different sources migrated as a single protein band with the same molecular weight of 47,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and consisted of multiple components that could be resolved by isoelectric focusing. Both had the same pI values which shifted downward upon activation by trypsin; however, relative peak heights were different between the two preparations. The purified inactive renin from chorion laeve was completely inactive and did not bind to pepstatin-aminohexyl-Sepharose; however, that from chorion culture medium was partially active and completely bound to the pepstatin gel, indicating that each molecule is partially activated. Trypsin-activated inactive renins from both sources were identical with human renal renin in terms of pH optimum and Km. Specific activities of trypsin-activated inactive renin from chorion laeve and chorion culture medium were 529 Goldblatt units/mg of protein and 449 Goldblatt units/mg of protein, respectively. Amino acid sequence analysis of both of the purified inactive renin preparations demonstrated a leucine residue at the amino terminus. The sequence of 11 additional amino acids was identical in both and agreed with that predicted from the base sequence of the renin gene. These findings indicate that preprorenin is converted to prorenin following removal of a 23-amino acid signal peptide and that the native inactive renin, whose amino acid sequence commences with Leu-Pro-Thr..., is prorenin.     

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.     

Evidence from peptide mapping for a human renin zymogen

Data obtained from peptide mapping of the active and inactive forms of human renin show that there are extensive regions of common sequence in the two forms of the enzyme, and are consistent with the hypothesis that inactive renin is a renin zymogen.     

Active and inactive renin after exercise

The effects of graded exercise on plasma concentrations of active and inactive renin were studied in seven healthy men. Exercise was performed on a cycle ergometer at four different exercise intensities (corresponding to 30%, 50%, 80% and 87% of VO2max) for 10 min each. Concentrations of active renin and total renin after activation by trypsin were measured by direct immunoradiometric assay. Non-trypsin-activated renin concentration (inactive) was obtained by subtraction. Active renin concentrations at 30%, 50%, 80% and 87% of VO2max were 1.2, 1.9, 3.1 and 4.6 times higher than the control concentration, respectively. Similar increases in plasma renin concentration, determined by conventional enzymatic assay, were observed at every stage. In contrast, changes in inactive renin concentration were not significant at any stage. Significant increases in noradrenaline concentration were found at every exercise stage, but adrenaline, aldosterone and lactate concentrations were significantly elevated only after exercise at 50%, 80% and 87% of VO2max. The similarity between the changes in concentration of active renin and noradrenaline would suggest that sympathetic nerve activity may have been responsible either for the release of active renin or for the conversion of inactive renin to its active form in the kidney.     

Prorenin-renin axis in synovial fluid in patients with rheumatoid arthritis and osteoarthritis.

This study was undertaken 1) to determine whether or not renin is present in synovial fluid in patients with rheumatoid arthritis and osteoarthritis, and, if present, 2) to investigate whether it is synthesized in synovial fluid, or it is only transported from the circulation into the synovial cavity. The active renin concentration (indirect) was measured with angiotensin I radioimmunoassay kits. Inactive renin was converted into active renin with Sepharose-bound trypsin. Both active and inactive forms of renin were found in synovial fluid. They were significantly higher in patients with rheumatoid arthritis (n = 9) than in those with osteoarthritis (n = 16). In plasma, the concentration of inactive renin was significantly higher (P less than 0.001) in the former. Albumin, transferrin, alpha 2-macroglobulin, ceruloplasmin and immunoglobulins G and M were also found in synovial fluid. In each disease, a plot of the log ratio of synovial fluid to the serum concentration against the log molecular weight of each protein gave an approximately straight line curve, suggesting that these proteins are derived from the circulation and are transported into the synovial cavity. In contrast, the ratio of synovial fluid to plasma concentrations of active renin was significantly higher than that predicted on the basis of the above-mentioned interrelationships in both diseases, whereas the ratio of inactive renin was significantly lower. These findings suggest that 1) inactive and active renin are filtered into the synovial fluid from the circulation, and that 2) inactive renin is converted into the active form in the fluid.     

Dipeptide glycols: a new class of renin inhibitors

The discovery of a new class of novel renin inhibitors consisting of protected dipeptide amides derived from aminoglycols (Formula I) prompted a study of structure-activity in vitro and efficacy in vivo. Thus, Boc-L-Phe-N-[(1S,2R)-1-benzyl-(2,3-dihydroxy)propyl]-L-leucinamide (1) and the corresponding histidinamide (2) inhibit human renin in vitro (IC50: 8.7 X 10-6 M and 2.6 X 10-6 M, respectively). Compound 1 has a slight inhibitory effect on pepsin and compound 2 does not inhibit pepsin at all (at 10-4M); these compounds are inactive against rat renin. Compound 1 is efficacious in lowering plasma renin activity in the Rhesus monkey (i.v.). Results indicate that this new class of low molecular weight inhibitors is specific for human renin and thus constitutes a new source of drug candidates.