Purification and partial characterization of rat brain acid proteinase (isorenin)

Angiotensin peptides in brain and pituitary of rat and sheep

Several lines of evidence indicate that angiotensin peptides may be formed in the brain, where angiotensin II (Ang II) and angiotensin-(1-7) (Ang-(1-7)) may function as neurotransmitters. However, there is considerable controversy concerning the identity and levels of angiotensin peptides in the brain. We have used a novel high performance liquid chromatography-based radioimmunoassay to measure Ang-(1-7), Ang II, Ang-(1-9) and Ang I in various brain regions and in the pituitary of the rat and sheep. We also studied the effect of different methods of tissue extraction, and the effect of the converting enzyme inhibitor ramipril, on angiotensin peptide levels in the rat hypothalamus. The levels of Ang-(1-7), Ang II, Ang-(1-9) and Ang I were low (<25 fmol/g) in all brain regions examined, except for the sheep median eminence and cerebellar cortex where Ang II levels were 385±116 and 193±37 fmol/g (mean ± SEM, n = 6), respectively. Pituitary Ang II levels were 103±13 fmol/g in the rat and 63±18 fmol/g in the sheep. The levels of Ang-(1-7), Ang-(1-9) and Ang I were much lower than those of Ang II in brain and pituitary. Ang-(1-7) levels in the rat hypothalamus were low (<6 fmol/g) but methods of extraction which involved freezing and thawing of the tissue resulted in substantially higher levels of this peptide. Ang II levels in the rat hypothalamus (18±3 fmol/g) were reduced to undetectable levels (<6 fmol/g) by ramipril administration. The low levels of angiotensin peptides in the hypothalamus and brainstem indicate that if these peptides function as neurotransmitters in these regions, then they are of particularly low abundance. Moreover, our results indicate that the high levels of Ang-(1-7) reported previously for rat hypothalamus may be artefactual, due to the method of tissue extraction.

Bovine brain cathepsin D: inhibition by pepstatin and binding to concanavalin A

1. Cathepsin D from bovine brain has been purified 1100-fold in 46% recovery. Three isozymes are present with pI (+/- 0.05) = 6.10, 6.30 and 6.40. 2. The isozymes are single polypeptide chains with apparent Mr = 42,000 and are similar with respect to substrate binding and cleavage; the pH-optimum is 3.5 with virtually no activity at neutral pH. 3. Pepstatin inhibits the enzyme and kinetic data are consistent with a "tight binding" mechanism. 4. The dissociation constant for the concanavalin A-enzyme complex is Kd = 19 nM at pH 5.0. 5. Under conditions where 90% of the enzyme is bound to soluble concanavalin A, full enzymatic activity is observed.

Substrate specificity of cerebral cathepsin D and high-Mr aspartic endopeptidase

The specificity of action of bovine brain cortex cathepsin D (EC 3.4.23.5) and high-Mr aspartic endopeptidase (EC 3.4.23.-) was studied with the vasoactive peptides renin substrate tetradecapeptide (RSTP), substance P (SP), and angiotensins I and II, and with model peptides--Lys-Pro-Ala-Glu-Phe-Phe (NO2)-Ala-Leu (I), Gly-Gly-His-Phe (NO2)-Phe-Ala-Leu-NH2 (II), and Abz-Ala-Ala-Phe-Phe-pNA (III). Cerebral aspartic peptidases show identical substrate specificity, cleaving the Leu10-Leu bond in RSTP and Phe-Phe in SP and peptide I-III, and not splitting angiotensins I and II. Because of the higher catalytic efficiency of cathepsin D (Kcat value), the specificity constants (Kcat/Km) for cathepsin D-catalyzed hydrolysis of substrates 1-111 are much higher than those for the high-Mr enzyme. High-Mr aspartic peptidase shares a number of properties with cathepsin D (sensitivity to pepstatin, substrate specificity, pH activity profile) and shows partial immunological identity; however, high-Mr aspartic peptidase has a specific activity 7-10 times lower than that of cathepsin D. The kinetic parameters of proteolysis of model peptides presented indicate that the high-Mr enzyme may be a complex of a single-chain cathepsin D with another polypeptide, although the possibility that it is an independent aspartic peptidase cannot be excluded.

Mechanisms by which nephrectomy stimulates adrenal renin

Renin has been identified in the adrenal gland by several investigators. Nephrectomy is the most potent stimulator of adrenal renin, and in the present study we investigated the mechanism by which nephrectomy stimulates adrenal renin. The pituitary plays a permissive role since hypophysectomy abolished the response of adrenal renin to nephrectomy (from 117.3 +/- 14.55 to 10.37 +/- 1.63 ng angiotensin I/mg protein/hr) and adrenocorticotropic hormone (ACTH) treatment restored the response to nephrectomy in hypophysectomized rats to 120 +/- 20.62 ng angiotensin I/mg protein/hr. However, large doses of ACTH given to intact rats did not increase adrenal renin to the high level observed after nephrectomy. Potassium also plays an important role, since prevention of hyperkalemia after nephrectomy by treatment with a cation exchange resin, sodium polystyrene sulfonate (Kayexalate), significantly reduced the adrenal renin response to nephrectomy. A third factor involved is the lack of negative feedback by plasma angiotensin II. Infusion of angiotensin II intraperitoneally prevented the rise in adrenal renin after nephrectomy (from 65.25 +/- 7.60 to 9.27 +/- 0.99 ng angiotensin I/mg protein/hr) despite an increase in plasma potassium and corticosterone. In conclusion, three factors influence the response of adrenal renin to nephrectomy: 1) the pituitary through the release of ACTH, 2) a direct stimulation by high plasma potassium levels, 3) the lack of angiotensin II feedback inhibition. Whether the high adrenal renin contributes to the high aldosterone observed in rats after nephrectomy remains to be established.

Role of protein kinase C in inhibition of renin release caused by vasoconstrictors

It was the aim of the present study to get insight into some of the intracellular mechanisms by which the vasoconstrictor hormones angiotensin II (ANG II), arginine vasopressin (AVP), and norepinephrine (NE) inhibit renin release from renal juxtaglomerular cells. To this end a primary cell culture from rat renal cortex was established that consisted of 50% juxtaglomerular cells. The cultured juxtaglomerular cells contained prominent renin granules closely resembling those in the intact kidney and responded to a number of stimuli of renin release. By using these cultures, we found that ANG II (10(-7) M), AVP (10(-6) M), and NE (10(-5) M) inhibited renin release and increased the calcium permeability of the plasma membrane of the cultured cells. Both the effects on renin release and on calcium permeability could be diminished or even be abolished by the calcium channel blocker verapamil (Vp) (10(-5) M). ANG II, AVP, and NE led to an increased formation of diacylglycerol (DAG), a well-known stimulator of protein kinase C (PKC). Moreover, a direct stimulation of PKC by 12-O-tetradecanoylphorbol-13-acetate (TPA) (10(-8)-10(-6) M) also inhibited renin release and increased the calcium permeability of the cell membrane. Similar to ANG II, AVP, and NE, the effects of TPA on calcium permeability and renin release could be diminished by Vp. In conclusion, these results point toward a common mechanism by which vasoconstrictors inhibit renin release from renal juxtaglomerular cells: ANG II, AVP, and NE activate a phospholipase C, which generates DAG.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple forms of immunoreactive renin in human pituitary tissue

Immunoreactive renin was demonstrated in pituitary tissues of postmortem human subjects with different diseases. The specific immunoreactive renin activity comprised the majority of the tissue renin-like activity (mean, 83%), indicating the absence of nonspecific actions of proteases such as cathepsin D. We used three pituitary specimens with high levels of the specific renin activity for further biochemical characterization of the enzyme. Small differences were found in the molecular mass (45 K, 42 K and 37 K), binding to concanavalin A-Sepharose, and isoelectric points (pI) (4.72, 4.78, 4.86, 5.06, 5.28 and 5.44). These results seem to be interpreted as evidence for the presence of specific renin in the human pituitary with microheterogeneity.

Are the renin-containing granules of juxtaglomerular epithelioid cells modified lysosomes?

Mature secretory granules of epithelioid cells--the so-called renin granules--exhibit certain properties, which in this particular combination are expressed only by lysosomes: Renin granules have autophagic capabilities; they react to the application of lipidosis-inducing, lysosomotropic substances by the gradual accumulation of polar lipids; all secretory granules of epithelioid cells contain acid phosphatase until maturity; and exogenous tracers reach renin granules without labeling the Golgi complex. Several functional implications can therefore be considered. Hydrolytic enzymes, constitutive elements of the granule matrix, might either cleave inactive prorenin to yield active renin within the granules or, by unspecific hydrolysis of renin, participate in the regulation of the overall quantity of secretory product. Autophagic phenomena, the involvement of renin granules in the traffic of exogenous tracers, and the build-up of polar lipids following experimental interference with lipid catabolism indicate a large turnover of membrane material in renin granules. They also suggest that cytoplasmic and extracellular fluid gains access to the granule content and may thus be involved there in the regulation of biochemical reactions by changing the intragranular milieu or via signal molecules. In addition to the lysosome-like properties of epithelioid cell secretory granules, the secretory product, renin, as a carboxyl protease, is structurally related to other acidic proteases. In the case of cathepsin D, even functional similarities exist.

True renin in human pituitary tissue

Readily detectable levels of renin activity were demonstrated in human pituitary tissues. This activity was inhibited by specific antibody raised against human renal renin, indicating that it was not due to the nonspecific action of proteases. It shared some biochemical features with well-known kidney renin, such as molecular weight, optimum pH, and the presence of trypsin-activatable inactive renin. These results suggest that true renin exists in human pituitary tissue.

Distribution of cathepsin D immunoreactivity in the central nervous system of rat and selected brain regions of man

The regional distribution and cellular localization of cathepsin D immunoreactivity was demonstrated at the light microscopic level in the CNS of rat and man by use of unlabelled immunoenzyme technique. A wide but uneven distribution was substantiated for the rat brain. Furthermore, we present evidence that antiserum produced against rat liver enzyme is capable of recognizing cathepsin D in human brain.

Angiotensin I-generating acid endopeptidase activity in neurosecretory vesicles isolated from bovine pituitary

Secretory vesicles purified from the neural and intermediate lobes of the bovine pituitary contain acidic endopeptidases which are capable of converting renin tetradecapeptide (RTD) substrate to Angiotensin I (AI). Preliminary characterization of the neurosecretory vesicle (NSV) endopeptidase showed that it had a pH optimum of 4.0, and unlike renin was inactive at pHs greater than 6.0. It is inhibited by 10(-6) M pepstatin A, but not by PMSF, leupeptin, PMBS, or the specific renin inhibitor H-142. This NSV endopeptidase differed from cathepsin D in that it was unable to degrade alpha-casein, but was quite active in generating AI from RTD (Vmax = 5 moles/g protein/hour). No enzyme activity that could convert AI to Angiotensin II could be detected in the NSVs suggesting that the acidic endopeptidase is involved in processing neurosecretory vesicle proteins other than those associated with the renin angiotensin system in the brain.

Immunoreactive renin in mouse adrenal gland. Localization in the inner cortical region

The existence of renin in the adrenal gland of the mouse was determined by its enzymatic activity and by immunohistochemical techniques using monospecific antibodies to mouse submandibular gland renin. The adrenal gland of mouse was found to contain a very high level of renin significantly greater than other mouse tissues except for the kidney and submandibular gland. Also, the renin level in mouse adrenal was significantly higher than that in adrenals of other species. This renin activity was distinct from the nonspecific renin-like activity of acid proteases in that its activity was optimal at neutral pH and specifically inhibited by antirenin antibody. Adrenal renin increased upon nephrectomy indicating that it is not derived from the kidney. Immunohistochemical studies localized the renin-immunoreactive substance to cells in the inner region of the cortex. The intensity of staining was highest in the innermost region and decreased in cells in outer layers.

Absence of renin-like activity in rat aorta and microvessels

Vascular renin-like activity was studied in the aortas and the cerebral microvessels of Sprague-Dawley rats and in the aortas of spontaneously hypertensive rats. Methods were employed to maximize detection of tissue renin and to simultaneously minimize contamination of that activity by either plasma renin or nonspecific proteases capable of angiotensin I generation. To this end, renin activity was measured near its pH optimum; plasma renin was eliminated by nephrectomy; and nonspecific proteases such as cathepsin D were either inhibited by proteolytic blockers or removed by chromatography over immobilized bovine hemoglobin. Aortic vascular renin-like activity was detected in rats not subjected to nephrectomy and could be inhibited by preincubation of samples with antimouse renin antibody shown to cross-react and inhibit rat plasma renin activity. Furthermore, vascular renin-like activity disappeared after nephrectomy in parallel with the disappearance of plasma renin activity. In the absence of contaminating enzymatic activities, no tissue renin-like activity could be demonstrated in either aortas or cerebral microvessels of Sprague-Dawley rats or in aortas of spontaneously hypertensive rats.

Rodent kinin-forming enzyme systems--II. Purification and characterization of an acid protease from Murphy-Sturm lymphosarcoma

An acid protease from the rat Murphy-Sturm lymphosarcoma (MSLS) tumor was purified 640-fold by extraction of the tumor tissue, acid precipitation with glacial acetic acid, ammonium sulfate precipitation, DEAE-Sephadex A-50 batch adsorption, QAE-Sephadex A-50 column chromatography, Sephadex G-200 gel filtration, and CM-32 cellulose chromatography. The protease hydrolyzed bovine hemoglobin and formed vasopeptide kinins when incubated with purified rat plasma kininogen. Two protease fractions obtained by Sephadex G-200 gel filtration had identical molecular weights of 39,500-41,000 and were similar in other physico-chemical and kinetic characteristics. The purified enzyme showed three major isozymic forms (alpha, beta and gamma) with isoelectric points (pI) of 5.2, 5.5 and 5.8, respectively, and nearly identical amino acid compositions. The enzyme had a high moles percent of both aspartic and glutamic acids. The carbohydrate moiety of the enzyme contained 2 moles of N-acetylglucosamine and 8 moles of mannose per mole of enzyme. The pH optimum for the digestion of bovine hemoglobin was approximately 3.0 with a sharp decline of activity on either side of the pH optimum. The protease activity was very stable above pH 3.4. The Km values for the purified enzyme fractions A and B were 31.17 and 31.19 microM, respectively, and the corresponding Vmax values were 6.17 and 5.5 microM tyrosine per mg per min at 37 degrees and pH 3.0. The enzyme was inhibited strongly by pepstatin (Ki = 31 X 10(-9)M and alpha = 0.1). The acid protease released kinin from purified rat plasma kininogen at an initial rapid rate which plateaued at 460 ng bradykinin equivalents/mg substrate after a 2-hr incubation at 37 degrees.

The distribution of cathepsin D in rat tissues determined by immunocytochemistry

The distribution of cathepsin D (CD) was surveyed in rat tissues by light microscopic immunocytochemistry. Although immunoreactive CD was detected in all tissues examined, there was a marked difference in the amount in the cytoplasm of different cell types of the same organ. In the retina large amounts of CD were present in the pigment epithelium, ganglion cells, and Müller cells. Moderate to large amounts of CD were also found in neuronal perikarya of the gastrointestinal tract and adrenal medulla; in macrophages in the lung, liver, and spleen; in some secretory cells of the submandibular and lacrimal glands; in parts of renal distal convoluted and collecting tubules; and in the surface transitional epithelium of the calyx, ureter, and urinary bladder. Other cells adjacent to cells containing large amounts of the enzyme had little or no detectable CD themselves. These included hepatocytes, the proximal tubular cells of the kidney, selected cells of the submandibular gland, cells of the zona glomerulosa of the adrenal cortex, and lymphocytes in lymphoid organs. The localization of CD indicates that its degradative effect is exerted preferentially in certain cell types and suggests that physiological influences on CD may have a variety of effects in different organs.

Subcellular localization in rat brain of angiotensin I-generating endopeptidase activity distinct from cathepsin D

The generation of angiotensin I from the artificial renin substrate tetradecapeptide by proteolytic enzymes in rat brain tissue was studied. The involvement of endopeptidase activity in the enzymatical cleavage of the renin substrate was inferred from the simultaneous accumulation of both angiotensin I and the complementary tetrapeptide Leu-Val-Tyr-Ser on incubation of tetradecapeptide with rat brain tissue. This endopeptidase activity was active over a pH range of 3.5--7.5. In contrast, cathepsin D released angiotensin I from tetradecapeptide only at acidic pH. The angiotensin I accumulation on incubation of tetradecapeptide with brain endopeptidase activity was only partly inhibited in the presence of an excess of the carboxyl protease inhibitor N-acetyl pepstatin. Further, the brain endopeptidase activity displayed a subcellular localization different from that of acid protease activity. It is concluded that angiotensin I can be generated in the brain by soluble endopeptidases, which are distinct from cathepsin D.

Evidence for renin in rat brain: differentiation from other reninlike enzymes

We observed that unfractionated rat brain extract incubated with substrate at pH 6.0 yielded 12 times the quantity of angiotensin I as incubations at pH 7.4, but the enzyme activity measured at pH 6 was not primarily due to renin. To examine the existence of renin in brain, we used three methods of affinity chromatography (pepstatin-, renin-specific antibody-, and alpha-casein-Sepharose) to fractionate the angiotensin I-generating enzymes in the brain. 1) Brain extract applied to renin-specific column eluted a peak of angiotensin-releasing activity (ARA) that had a pH optimum of 6.0. This ARA was inhibited by antirenin antibody. Another peak of ARA with a pH optimum of 4 appeared in the nonbound fraction. This peak was not affected by antirenin antibody and had acid protease activity. 2) Pepstatin affinity column elution with lithium bromide yielded an early ARA peak (pH optimum 6.5), inhibited by antirenin antibody and a later peak (pH optimum 4.0) not inhibited by antirenin antibody. The latter contained acid protease activity. 3) alpha-Casein-Sepharose column also separated neutral proteases and immunoreactive renin from acid protease capable of generating angiotensin. In summary, rat brain contains a host of angiotensin I-generating enzymes that can be detected and separated as neutral and acid proteases and immunoreactive renin depending on the pH of the assay and conditions of purification. These findings indicate the presence of an enzyme with immunoidentity to renin in rat brain but do not imply local biosynthesis.

Markedly elevated specific renin levels in the adrenal in genetically hypertensive rats

The specific renin (EC 3.4.99.19) activity in the adrenal of spontaneously hypertensive rats was determined by a method that is capable of distinguishing renin from nonspecific renin-like activity of proteases by using specific antibody to renin. The renin level in the adrenals of adult spontaneously hypertensive rats with established hypertension was found to be 6-8 times as high as that of the normotensive control Wistar-Kyoto strain. The large difference in the adrenal renin level was observed even in 3-wk-old rats in which hypertension has not yet developed. The adrenal renin level was increased by bilateral nephrectomy in both the hypertensive and normotensive strains. A larger quantity of renin was found in the adrenal cortex than in the medulla, and the difference between the hypertensive strain and the normotensive strain was more prominent in the cortex than in the medulla. These results suggest possible involvement of adrenal renin in the development and in the early maintenance phase of hypertension in this animal mode of human essential hypertension by affecting the adrenocortical or adrenomedullary activity, or both.

Brain renin

Although the brain contains cathepsins at high concentrations which exhibit a non-specific renin-like activity at acidic pH, the presence of specific renin in the brain has been demonstrated by characterizing its specific properties. Renin was separated from cathepsin by affinity chromatography on casein-Sepharose. Brain renin showed neutral pH optima for the reaction to generate angiotensin I. The presence of inactive prorenin was also found. The isoelectric points of brain renin were significantly lower differences from that of renal or plasma renin. Immunohistochemical studies demonstrated a wide-spread localization of renin in many different regions. Angiotensin II, the final product of the prohormone-to-hormone conversion reaction mediated by renin and angiotensin converting enzyme, was found to exist in the same cell as renin by immunohistochemical studies of brain sections and with cloned and cultured neuroblastoma cells. This is the first demonstration of the mechanism of peptide hormone formation in neuronal cells. Similar intracellular formation was demonstrated in gonadotrophs of adenohypophysis. Coexistence of renin and angiotensin II was demonstrated in some cells. Electrophysiological studies have shown that angiotensin II functions to disinhibit the inhibition of neuronal response to electrical stimuli in the hippocampus.

Angiotensin--forming enzyme active at the physiological Ph in the brain of normal and nephrectomized rats

Enzymatic activity generating angiotensin at pH 5.5 and 7.2 has been detected in different areas of the central nervous system (CNS) of the rat. Control animals and those subjected to bilateral nephrectomy 48 h before the experiment (Nx) were analyzed. The different areas of the CNS were studied by the incubation of tissue homogenates in the presence (enzyme concentration) or not (enzyme activity) of an excess of added angiotensinogen. Concentration was determined by incubation at pH 7.2 and 5.5 while activity was evaluated only at pH 7.2. The enzymatic renin-like concentration at both pHs did not change after Nx thus showing they do not depend on plasma and vascular renin. On the other hand the activity of the enzyme showed a significant increase in the cerebral cortex and cerebellum after Nx suggesting an increased concentration of renin substrate and/or different concentrations of inhibitors or activators of the enzymatic system in those areas.

Immunohistochemical localization of renin in luteinizing hormone-producing cells of rat pituitary

The location of renin (EC 3.4.99.19) in rat pituitary was determined by the peroxidase-antiperoxidase immunohistochemical technique. By using antisera prepared with purified rat renal renin, an immunoreactive substance was localized within ovoid cells scattered throughout the anterior pituitary. These cells were shown to be luteinizing hormone-producing cells by staining with anti-luteinizing hormone antisera in adjacent sections. By using the double staining method, the renin-containing cells were differentiated from cells containing corticotropin, thyrotropin, growth hormone (somatotropin), and prolactin (mammotropin). These results suggest a possible local role for renin in the anterior pituitary.

Angiotensin generation in the brain and drinking: indications for the involvement of endopeptidase activity distinct from cathepsin D

The dipsogenic activity of two artificial renin substrates, tetradecapeptide and tridecapeptide, was studied. The dose-response curves obtained with these peptides, following intracerebroventricular administration, were similar to that of angiotensin I. The angiotensin II antagonist, Sar1, Ala8-angiotensin II, inhibited the dipsogenic effect of tetradecapeptide, indicating the conversion of the latter peptide into angiotensin II. The lower dipsogenic activity of tridecapeptide points to a conversion of this renin substrate into angiotensin III. Specific inhibition of tetradecapeptide induced drinking by the endopeptidase inhibitor N-acetyl-pepstatin suggests the involvement of an endopeptidase in the conversion of the renin substrates in the brain. Two endopeptidases present in the brain (cathepsin D and renin), were compared with respect to their capacity to generate angiotensin I from artificial renin substrate in vitro. Cathepsin D was active under only acidic pH conditions, whereas renin showed a wider pH range with maximal activity in the non-acidic region. Moreover, cathepsin D did not generate angiotensin I from natural, cerebrospinal fluid-angiotensinogen in vitro, and lacked dipsogenic activity following central administration. Small amounts of renin, however, were able to release angiotensin I from cerebrospinal fluid in vitro. In addition, this enzyme induced high dipsogenic activity upon intracerebroventricular injection. These results support the existence of a functionally active central renin-angiotensin system and provide an argument against the involvement of cathepsin D in the formation of angiotensin I in the brain.

Renin, angiotensins, and angiotensin-converting enzyme in neuroblastoma cells: evidence for intracellular formation of angiotensins

The mechanism of formation of various peptide hormones in neuronal cells in the brain is not clear. The question of whether brain angiotensin II is formed by an extracellular mechanism as in the peripheral system or by an intracellular mechanism can be answered by using cloned cells in culture. We have screened several neuroblastoma cell lines of rat and mouse origin and found at least three cell lines that contain renin (EC 3.4.99.19), angiotensin-converting enzyme (dipeptidyl carboxypeptidase; peptidyldipeptide hydrolase, EC 3.4.15.1), and angiotensins I and II. This finding was interpreted to indicate that in these cells angiotensin formation takes place by an intracellular mechanism, in contrast to the extracellular mechanism well known to occur in plasma. This study also demonstrates the existence of viable and cloned cell lines that produce renin.

Renin-like activity in the rat brain during the development of DOC-salt hypertension

Levels and distribution of an angiotensin-forming enzyme, active at the physiological pH (isorenin), were determined in the central nervous system of 24 rats treated with 25 mg/kg of deoxycorticosterone (DOC) subcutaneously, twice a week, plus saline to drink during 30 days and in 14 control animals. Different areas of the brain were excised and homogenized. Renin activity and concentration were determined by incubation of the supernatant of each homogenate at pH 7.2 alone and in the presence of an excess of renin substrate. The angiotensin generated was measured by radioimmunoassay. Concentration of the renin-like enzyme was significantly higher in the posterior hypophysis and in the brain stem of the experimental group; isorenin activity was higher in the hypothalamus, cerebral cortex, cerebellum, and brain stem of the DOC-salt-treated rats than in the control rats. Changes in the angiotensin-forming enzyme in the central nervous system of experimental animals, active at physiological pH, suggest that this isorenin system may play a role in the physiological response to DOC-salt in the rat. The significance of the brain isorenin system in the regulation of blood pressure requires further analysis.

In vivo activity of purified mouse brain renin

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

Immunocytochemical localization of cathepsin D in rat neural tissue

The localization of cathepsin D (CD) in normal adult rat neural tissue was determined with an indirect immunoperoxidase technique utilizing rabbit anti rat brain CD followed by a horseradish peroxidase conjugate of the Fab portion of goat antirabbit IgG. The immunoreactive enzyme protein was distributed predominantly in a granular pattern, presumably lysosomal, in neurons and choroid plexus epithelium. Smaller amounts were detected in oligodendrocytes and ependymal cells. The neuronal localization included the perikaryon and its processes, was widely distributed, and displayed a range of staining intensities in different anatomical areas. Immunoreactive CD was heavily concentrated in brain stem and spinal cord motoneurons, large neurons of the caudate nucleus, neurons of several nuclear groups, especially the paraventricular and supraoptic, in the hypothalamus, and neurons of superior cervical and dorsal root ganglia. CD was also readily detected in brain stem sensory neurons, pyramidal cells of the hippocampus, inferior olive and Purkinje cells, but was absent or present in very small quantities in the granule cells of the cerebellar cortex, the more superficial layers of the neocortex, and smaller neurons of the caudate nucleus. This distribution suggests that CD may have a major role in specific chemical events in neural functions and peptidergic pathways and could be involved in the alterations of certain neural structures in disease states.

Protein degradation in the mouse visual system. I. Degradation of axonally transported and retinal proteins

The analysis of proteolysis in the nervous system is complicated by the heterogeneity of cell types, extensive reutilization of liberated amino acids, and artifacts that may arise when the integrity of the tissue is disrupted during experimentation. For these reasons, changes in proteolytic activity that are observed during brain development and in neuropathological states may often be difficult to interpret. To minimize these problems, we have developed a technique that permits protein degradation to be investigated specifically within axons of the mouse retinal ganglion cells (RGC). In the present study, the method has been used to examine the degradation of proteins conveyed in the slow phases of axoplasmic transport. When adult C57Bl/6J mice were injected intravitreally with L-[3H]proline, labeled proteins within the primary optic pathway (optic nerve and tract) after 5 days were almost exclusively the slow phase axonal proteins. The rate of degradation of these proteins was then determined within the excised, but otherwise intact, optic pathway by measuring the release of acid soluble radioactivity at 37 degrees C in vitro. At physiological pH, the amino acids released by proteolysis were extensively reutilized. Unless amino acid reutilization was prevented, protein degradative rates were artifactually lowered 3-fold. At least two proteolytic systems within RGC axons actively degraded the slowly transported axonal proteins. A 'neutral' system, stimulated by exogenous calcium ions, was optimally active within the physiological pH range (pH 7.0--7.8). The rate of protein degradation at pH 7.4 was uniform along the RGC axon. An 'acidic' system was optimally active with the incubation was carried out at pH 3.8. This proteolytic activity was calcium-independent and exhibited a proximodistal gradient within the RCG axon with higher activity proximally. Similar proteolytic activities were present in isolated intact retinas but in different proportions. The half-lives of axonal and retinal proteins were comparable to CNS protein half-lives estimated in vivo by methods that take amino acid reutilization into account. These and other recent findings demonstrate the utility of this neuron-specific approach in characterizing proteolytic processes within one cell type that may otherwise be obscured by proteolytic events in other cells when brain tissue is analyzed by conventional methods.

Isolation and characterization of multiple forms of renin from bull kidney

Different forms of renin have been purified from bull kidney by combined gel filtration, affinity chromatography and ion-exchange chromatography. The specific activity of the enzyme was determined by a biochemical method of synthetic substrate and by radioimmunoassay on both synthetic and natural substrates; molecular characterization was carried out by molecular weight determinations, polyacrylamide gel electrophoresis, isoelectric focusing, amino acid analysis and optical rotatory dispersion. Three forms (renin C, D, E) are distinct on the basis of amino acid composition and chromatographic behavior, while possessing the same molecular weight, and displaying only minor differences in specific activity, alpha-helix content and isoelectric point; the occurrence of a group of renin isoenzymes may be suggested. Another form (A) has a lower specific activity and a higher molecular weight (57 000) compared with C, D and E and further differs markedly in chromatographic behavior, amino acid composition, alpha-helix content and isoelectric point, as well as in substrate specificity; it may be regarded as a pseudorenin. The fifth form (B) possesses the highest specific activity and does not correspond to a single molecular form; the presence of two components of different molecular weight (27 000 and 46 000 respectively) has been established both by polyacrylamide gel electrophoresis and isoelectric focusing.

Renin and prorenin in hog brain: ubiquitous distribution and high concentration in the pituitary and pineal

With the objective of clarifying the nature of renin-like activity in the brain, we have devised methods to distinguish true renin from acid protease. These methods were used to determine the regional distribution of true renin in hog brain. The pineal was found to be the richest source of renin followed by the adenohypophysis and choroid plexus. The hypothalamus, cerebellum and amygdala contained moderately high concentrations of renin. Renin concentration in the neurohypophysis was negligible. Many regions contained activatable prorenin. The molecular weight and the pH-dependence of the brain renin were identical to these same properties of renal and plasma renins. Based upon its specific affinity to concanavalin A, brain renin was judged to be a glycoprotein. The electrofocusing pattern of renin from different regions of the brain differed from that of plasma and kidney renins, a discrepancy which could be interpreted as evidence for the endogenous synthesis of renin in the brain.

Immunohistochemical localization of renin in mouse brain

Immunohistochemical studies of mouse brain by the peroxidase-antiperoxidase method using monospecific antimouse renin antibodies has revealed the intracellular localization of renin in stellate and small ovoid cells. Renin-containing ovoid cells were observed in the spinal cord, medulla oblongata, pons, granular layers of the cerebellum, deep cerebellar region, and the lamina terminalis; whereas immunostainable stellate cells were found in the cerebral cortex, hippocampus, dentate gyrus and septum. Intracellular localization of renin rather than intravascular localization supports an endogenous origin of this enzyme in the brain. Wide distribution in different types of cells suggests different types of regulatory mechanisms. Double immunostaining with antigalactocerebroside and antirenin antibodies indicates the presence of renin in oligodendrocytes.

Isorenin, pseudorenin, cathepsin D and renin. A comparative enzymatic study of angiotensin-forming enzymes

1. Renin was purified 30 000-fold from rat kidneys by chromatography on DEAE-cellulose and SP-Sephadex, and by affinity chromatography on pepstatinyl-Sepharose. 2. The enzymatic properties of isorenin from rat brain, pseudorenin from hog spleen, cathepsin D from bovine spleen, and renin from rat kidneys were compared: Isorenin, pseudorenin and cathepsin D generate angiotensin from tetradecapeptide renin substrate with pH optima around 4.9, renin at 6.0. With sheep angiotensinogen as substrate, isorenin, pseudorenin and cathepsin D have similar pH profiles (pH optima at 3.9 and 5.5), in contrast to renin (pH optimum at 6.8). 3. The angiotensin-formation from tetradecapeptide by isorenin, pseudorenin and cathepsin D was inhibited by albumin, alpha-and beta-globulins. These 3 enzymes have acid protease activity at pH 3.2 with hemoglobin as the substrate. Renin is not inhibited by proteins and has no acid protease activity. 4. Renin generates angiotensin I from various angiotensinogens at least 100 000 times faster than isorenin, pseudorenin or cathepsin D, and 3000 000 times faster than isorenin when compared at pH 7.2 with rat angiotensinogen as substrate. 5. The 3 'non-renin' enzymes exhibit a high sensitivity to inhibition by pepstatin (Ki less than 5.10(-10) M), in contrast to renin (Ki approximately 6-10(-7) M), at pH 5.5. 6. It is concluded from the data that isorenin from rat brain and pseudorenin from hog spleen are closely related to, or identical with cathepsin D.

In vivo enzyme activity of purified human brain renin

Cerebrospinal fluid (CSF) of rats contains high angiotensinogen concentrations. When 3500-fold purified renin from human brain was injected into the brain ventricles of rats, angiotensin I concentrations increased from undetectable levels to 147.9 +/- 18.8 fMol per ml CSF. In parallel, mean arterial blood pressure increased from 93 +/- 2.4 mm Hg to 107 +/- 3.7 mm Hg. The increase in blood pressure could be abolished by intraventricular administration of saralasin, a blocker of angiotensin II receptors. Intraventricular injection of cathepsin D had no effect on arterial blood pressure and the agiotensin I concentration in CSF remained below detection limits of the radioimmunoassay. We conclude that brain renin acts on endogenous brain angiotensinogen under physioloical in vivo conditions to form angiotensin I. The latter is converted to angiotensin II and leads to biological effects, i.e. increase of blood pressure.