N-Terminal amino acid sequence of rat tonin: homology with serine proteases

A novel leupeptin-sensitive serine endopeptidase present in normal and malignant rat mammary tissues

N-Methyl-N-nitrosourea (MNU)-induced rat mammary adenocarcinomas contain high levels of a novel leupeptin-sensitive serine endopeptidase. Its properties apparently differ from those of other similar endopeptidases reported to be present in various normal and malignant mammalian tissues. The same leupeptinsensitive serine endopeptidase was also detected in normal rat mammary tissues, but at levels approximately 20 times lower than those in MNU-induced mammary tumors. This enzyme, which is a trypsin-like serine endopeptidase, preferentially hydrolyzes various synthetic endopeptidase substrates at the carboxyl side of an arginyl residue. It has an apparent Mr of approximately 160,000 and a Stokes radius of 49 A, as determined by gel filtration. Its isoelectric points range from 4.5 to 4.8, and it has a pH optimum of approximately 7.0. The enzyme is stable from pH 4.0 to 7.0, but is extremely unstable above pH 7.0. Besides leupeptin, its activity is inhibited by antipain, aprotinin, N alpha-p-tosyl-L-lysine chloromethyl ketone and phenylmethylsulfonyl fluoride, but is not inhibited by soybean trypsin inhibitor. Many other potential inhibitors or activators such as 2-mercaptoethanol, p-hydroxymercuribenzoic acid and EDTA have no effect on its activity. The enzyme is adsorbed to p-aminobenzamidine agarose affinity beads at pH 6.5 and elutes at pH 4.0.

Excess antibody immunoassay for the measurement of tonin in rat tissues and plasma

Tonin, a proteolytic enzyme isolated from the rat submandibular gland, can generate angiotensin II directly from angiotensinogen. To date a method for the measurement of tonin in plasma has not been available and the present paper describes a sensitive and specific excess antibody immunoassay for determination of tonin in tissue homogenates and plasma. Interference from immunologically cross-reacting proteins was evaluated and the assay was found to be specific for tonin. Tonin measured in various tissue homogenates was directly proportional to the amount of sample added, giving a linear dose-response curve. The slope of this curve was determined by the recovery of tonin, which was better than 55% for urine and all tissues tested. The highest concentration of tonin was seen in the submandibular and sublingual gland (69 and 0.7 microgram/mg protein, respectively). The parotid gland, the exorbital lacrimal gland, liver, kidney, pancreas, and lung contained only negligible amounts (less than 4 ng/mg protein). Tonin in plasma was bound to one major inhibitor with a molecular weight of about 650,000-750,000. A partial splitting of the tonin-inhibitor complex was obtained by preincubating plasma with guanidine, allowing tonin to be measured with a recovery of 38 +/- 13% (n = 16) and with a linear dose-response curve. The concentration of immunoreactive tonin in normal arterial plasma from adult male rats was 0.90 +/- 0.53 ng/ml (n = 16). The concentration decreased after removal of the submandibular glands and increased after sympathetic stimulation.

Kininogenase activity of tonin

Tonin, known for its specific and direct generation of angiotensin II, was highly purified from rat submaxillary gland and investigated for kininogenase activity. For the substrate, heat-treated plasma from ox blood, and highly purified low-molecular-weight (LMW) and high-molecular-weight (HMW) kininogens, were used. The reaction product formed at pH 8.0 well satisfied the characteristics of kinin, i.e., depressor and oxytocic activities and reactivity with antibradykinin antiserum. Kinin formed by tonin from purified LMW kininogen was identified with bradykinin in high performance liquid chromatography and radioimmunoassay. The results revealed tonin's new capability of forming kinin in addition to the hitherto known pressor angiotensin II, indicating tonin, too, is a member of the "kinin- tensin enzyme system."

Amino acid sequence of rat submaxillary tonin reveals similarities to serine proteases

Tonin, an esteroprotease isolated from rat submaxillary gland, is a serine protease with trypsin- and chymotrypsin-like activity. The substrate specificity of tonin shows that it differs from kallikreins and is definitely not a renin-like enzyme or an angiotensin-converting enzyme. Tonin can produce directly the vasoactive peptide angiotensin II, from angiotensin I, angiotensinogen and the synthetic tetradecapeptide substrate of renin by cleavage of a Phe-His bond. It has also been found to cleave some Phe and Arg bonds in various substrates such as beta-lipotropin (beta-LPH), adrenocorticotropin (ACTH), pro-opiomelanocortin (POMC) and substance P. Here we describe the complete amino acid sequence of rat submaxillary gland, tonin. Comparison of the sequence of 219 amino acids with other serine proteases, particularly kallikreins, gamma-subunit of nerve growth factor (NGF) and the recently described gamma-renin, reveals extensive similarities. More interestingly, it also reveals the substitution of an Asp residue always found in the serine protease active site triad (Asp, His, Ser) by a Leu residue. This unusual substitution does not seem to affect the proteolytic activity of the enzyme.

Confirmation of direct angiotensin formation by kallikrein

This study was undertaken to confirm our previous preliminary observation that hog pancreas kallikrein (EC 3.4.21.35) directly liberated an angiotensin-like substance from human plasma protein Cohn fraction IV-4 at an acidic pH of 4.0-5.0. First, the possibility of proangiotensin or des-Asp1-angiotensin being the pressor substance was ruled out by t.l.c. Secondly, the pressor substance was purified by Sephadex G-25 and Bio-Gel P-2 gel filtration, and finally by high-performance liquid chromatography. The amino acid composition of the isolated pressor substance (residues/mol) was: Asp, 1.03; Val, 1.03; Ile, 1.00; Tyr, 0.69; Phe, 1.04; His, 0.91; Arg, 0.86; Pro, 0.86. This composition was identical with that of angiotensin. Since the reaction mixture was not contaminated with common proteolytic enzymes, such as trypsin, chymotrypsin, renin, cathepsin D and proangiotensin-converting enzyme, and other enzymes activated by kallikrein, it is clear that hog kallikrein directly produces angiotensin in vitro.

Effects of tonin on the adrenal secretion in the rat

Tonin infused intravenously at a rate of 12 micron g/kg/min for 2 hours stimulated aldosterone and corticosterone secretion in conscious rats. In pentobarbital-anesthetized rats treated with dexamethasone and morphine, tonin stimulated aldosterone secretion, but corticosterone was unaffected. Tonin did not stimulate steroidogenesis in isolated rat adrenal glomerulosa cells unless substrate, in the form of "stripped" rat plasma, was added, in which case large amounts of immunoreactive angiotensin II were generated. These results suggest that tonin may generate an angiotensin peptide in vivo, which in turn stimulates aldosterone secretion. ACTH is secreted in response to tonin infusion, and contributes to the adrenal response.

Tonin, an esteroprotease from rat submaxillary glands

Tonin is an enzyme found in the rat submaxillary glands which liberates angiotensin II from angiotensinogen, the Skeggs tetradecapeptide renin substrate, and angiotensin I. Tonin hydrolyzes benzoyl-arginine ethyl ester, benzoyl-arginine methyl ester, tosyl-arginine methyl ester, benzoyl-arginine p-nitroanilide and other small synthetic substrates at an optimum ph of 9.0. Tonin shows, however, a great specificity with respect to angiotensin I. Tonin is inhibited by diisopropyl fluorophosphate and phenylmethylsulfonyl fluoride at high concentrations (greater than 10(-2) M) and by soybean trypsin inhibitor and aprotinin. Tonin is thus an esteroprotease of the class of the serine protease with trypsin- and chymotrypsin-like activity. Tonin belongs to the same family of enzyme as glandular kallikrein and the gamma subunit of the nerve growth factor.

Chemistry and biosynthesis of pro-opiomelanocortin. ACTH, MSH's, endorphins and their related peptides

Studies of lipotropins, melanotropins and endorphins on one hand, and of adrenocorticotropin on the other, has given rise to the concept of a multipotent precursor molecule recently renamed proopiomelanocortin. The preferential sites of cleavage of the precursor to produce its biologically active components are made of pairs of basic amino acid residues as described for the biosynthesis of beta-MSH and pro-insulin. Such structural feature is also found in other pro-hormone molecules. Pulse chase experiments and secretory studies carried out in both anterior and intermediate lobes of rat pituitary glands revealed the transformation of different forms of the precursor into different end-products, the anterior lobe producing preferentially ACTH and beta-LPH while the intermediate produces mainly the alpha-MSH and beta-endorphin. The multiple forms of precursors seem to differ in their carbohydrate content although at least two different gene products are still possible. The presence of similar peptides in the hypothalamus makes it highly probable that neuropeptides are biosynthesized with similar process. Thus the model of beta-LPH precursor, proposed as early as in 1967, is now applicable to the biosynthesis of all other neuropeptides. Major advances in this field are expected in the 1980s.

Renin in mouse but not in rat submandibular glands

Renin was found in the submandibular glands of male Quackenbush mice in concentrations higher than has been reported for any tissue of any strain or species. However, no renin-like activity could e detected in glands from male and female Wistar rats using either pH 5.8 or 7.4 for assay and a radioimmunoassay specific for renin's reaction product, angiotensin I. Rabbit submandibular glands contained renin.