Identification of tonin in brain and exocrine tissues and in the cell-free translation products encoded by the mRNA of these tissues

Increased blood pressure and water intake in transgenic mice expressing rat tonin in the brain

Tonin is a serine proteinase of the kallikrein family that can produce angiotensin II directly from angiotensinogen. To clarify the importance of this enzyme for central nervous control of the cardiovascular system, we generated transgenic mice, TGM(rTon), that express rat tonin in astrocytes. These mice present high levels of tonin mRNA and activity specifically in the brain. As a consequence, TGM(rTon) develop increased blood pressure and water intake. Lisinopril, an ACE inhibitor, is less hypotensive for transgenic mice than for control animals. The AT(1) receptor antagonist candesartan equally lowers blood pressure in transgenic and in control mice. Plasma angiotensin II, but not angiotensin I, is increased in TGM(rTon) compared to the wild type, suggesting release of the peptide from the brain into the circulation. However, AT(1) receptors are desensitized in this transgenic model, as demonstrated by a blunted pressor response to intravenous application of angiotensin II. In conclusion, tonin in the brain may represent an alternative pathway for angiotensin II generation with effects on the cardiovascular system.

Distribution of tonin- and kallikrein-like activities in rat brain

Tonin- and kallikrein-like activities were investigated in different regions of the rat brain. The highest values of specific tonin activity, expressed as picomoles of angiotensin II liberated per minute per milligram of protein, were found in the neurohypophysis (359 +/- 190) and in the archicerebellum (200 +/- 68). The highest level of total tonin activity (picomoles of angiotensin II liberated per minute) was observed in the archicerebellum (902 +/- 308) which retained 97% of total tonin activity of whole cerebellum. Tonin activity was not detected in the cortex of cerebellum and in the choroid plexus. Low to intermediate values of specific (1.09 +/- 0.33 to 5.32 +/- 2.37) and total (1.38 +/- 0.55 to 93.00 +/- 49.30) tonin activity were observed in adenohypophysis, cerebellar nuclei, hypothalamus, thalamus, midbrain, pons, medulla and neurohypophysis. The lowest values of specific (0.11 +/- 0.05) and total (0.69 +/- 0.31) activities were observed in the hippocampus. Kallikrein-like activity was expressed as picomoles of p-nitroaniline liberated per minute per milligram of protein. No activity was detected in the neurohypophysis. For other regions, the values of the specific activity ranged between 72 +/- 18 and 282 +/- 14 except for the choroid plexus which was 5 +/- 2. The total kallikrein activity was also homogeneous ranging from 330 +/- 100 to 1870 +/- 112. For the choroid plexus and adenohypophysis the total kallikrein activity was 2.0 +/- 0.8 and 27 +/- 11, respectively.

Activation of angiotensin-generating systems in the developing rat kidney

The present study was designed to determine the developmental changes in intrarenal angiotensin (Ang) peptides in the rat. Kidney Ang I and II levels were threefold and sixfold higher in newborn than adult kidneys, respectively (Ang I, 678 +/- 180 versus 243 +/- 38 fmol/g, P < .01; Ang II, 667 +/- 75 versus 103 +/- 6 fmol/g, P < .001). Intrarenal Ang II levels correlated positively with the temporal changes in renin gene expression (r = .93, P < .001). However, no correlation was found between renal Ang II content and angiotensin-converting enzyme (ACE) expression during development, which prompted us to evaluate whether renal enzymes, other than renin and ACE, contribute to Ang II formation in the developing kidney. Angiotensin peptide levels were measured in newborn and adult kidney homogenates incubated with human angiotensinogen (a poor rat renin substrate) for 30 minutes at 37 degrees C. Inhibitors of aspartyl proteases and metalloproteases were ineffective in preventing the formation of Ang II in either newborn or adult kidneys. However, addition of the serine protease inhibitors soybean trypsin inhibitor and phenylmethylsulfonyl fluoride inhibited Ang II generation in the newborn kidneys only. In contrast, Ang I generation was not affected by inhibition of serine proteases in either newborn or adult kidneys. We conclude that Ang I and II synthesis is activated in the developing rat kidney. In addition to renin and ACE, the newborn rat kidney expresses serine protease activity that is capable of generating Ang II directly from angiotensinogen. This putative enzyme is induced in the newborn kidney and may cooperate with renin in the activation of Ang II synthesis during early development.

Tonin-like activity present in the human submandibular gland

An enzyme which is able to liberate angiotensin II from angiotensin I, angiotensinogen(1-14) fragment and angiotensinogen was purified from human submandibular gland. Its molecular weight is 110,000; is inhibited by PMSF but not by EDTA or enalaprilat. The pH optima for angiotensin II liberation were 4.0 for angiotensin I, 7.0 for angiotensinogen(1-14) fragment and 8.0 for angiotensinogen. The total amount of angiotensin II generating activity in the human submandibular gland is 5,000-times smaller than that in the rat gland.

Identification of serine proteinases with tonin-like activity in the rat submandibular and prostate glands

Two enzymes with tonin-like activity, designated rSMT3 and rSMT4, were purified from rat submandibular glands and another, rPT1, was obtained from the prostate. The three enzyme fractions hydrolysed angiotensin I, angiotensinogen (AG) and synthetic AG(1-14) to form angiotensin II. With angiotensin I as substrate, pH optima were 6.5 for rSMT3, 6.8 for rSMT4 and 7.5 for rPT1. With AG(1-14), the three enzymes had optimal activity at pH 7.5. The three enzymes had negligible activity upon a kallikrein substrate, Ac-Phe-Arg-Nan. The enzymes were inhibited by aprotinin, soybean trypsin inhibitor and phenylmethanesulfonyl fluoride but not by two angiotensin converting enzyme inhibitors, ethylenediaminetetracetic acid or enalaprilat. N-tosyl-L-phenylalanine chloromethyl ketone (1 mM) inhibited rPT1 and rSMT4 but not rSMT3. Molecular weights (SDS-PAGE) were 31,700 for rSMT3, 29,800 for rSMT4 and 28,100 for rPT1. Total activity in the prostate is 150-times lower than in the submandibular gland, where 92% of the tonin activity is related to rSMT4. Physical and chemical properties suggest that rSMT4 is tonin, whereas rSMT3 and rPT1 are tonin-like enzymes which can generate angiotensin II from different substrates.

Molecular cloning and characterization of two rat renal kallikrein genes

Kallikreins compose a multigene family coding for a subgroup of serine proteases, which are involved in the processing of bioactive peptides. Two rat kallikrein-related genes, RSKG-7 (rat submandibular gland kallikrein gene 7) and RSKG-3, have been cloned and their sequences analyzed. RSKG-7 is approximately 4200 bases in length and consists of five exons and four introns. The 5' end region contains the variant CATAT box and TTTAAA box; the 3' end region contains the polyadenylation signal AATAAA. This gene encodes a putative 28,935-dalton preproenzyme of 261 amino acids (aa). The active enzyme consists of 237 aa and is preceded by a deduced signal peptide of 18 aa and a profragment of 6 aa. RSKG-3 is highly homologous to RSKG-7 in terms of its sequence and structure; it encodes a 28,730-dalton prepropeptide consisting of a signal peptide of 18 aa, a profragment of 6 aa, and an active peptide of 235 aa. Sequence comparisons of RSKG-7, RSKG-3, and other kallikrein-related enzymes reveal the key amino acid residues needed for both serine protease activity (His/Asp/Ser) and kallikrein-like cleavage specificity at basic amino acids. Northern blot analyses using specific oligonucleotide probes demonstrate that, among the 12 tissues studied, RSKG-7 and RSKG-3 are expressed in the rat kidney and submandibular gland. Castration of male rats results in a decrease in submandibular gland RSKG-7 mRNA, which can be restored to the normal level by treatment with thyroxine or testosterone. On the other hand, neither castration nor hormonal manipulation affects RSKG-7 mRNA levels in the kidney.