Iodinated neurohypophyseal hormones as potential ligands for receptor binding and intermediates in synthesis of tritiated hormones

Vasopressin is a physiological substrate for the insulin-regulated aminopeptidase IRAP

Insulin-regulated aminopeptidase (IRAP) is a membrane aminopeptidase and is homologous to the placental leucine aminopeptidase, P-LAP. IRAP has a wide distribution but has been best characterized in adipocytes and myocytes. In these cells, IRAP colocalizes with the glucose transporter GLUT4 to intracellular vesicles and, like GLUT4, translocates from these vesicles to the cell surface in response to insulin. Earlier studies demonstrated that purified IRAP cleaves several peptide hormones and that, concomitant with the appearance of IRAP at the surface of insulin-stimulated adipocytes, aminopeptidase activity toward extracellular substrates increases. In the present study, to identify in vivo substrates for IRAP, we tested potential substrates for cleavage by IRAP-deficient (IRAP(-/-)) and control mice. We found that vasopressin and oxytocin were not processed from the NH(2) terminus by isolated IRAP(-/-) adipocytes and skeletal muscles. Vasopressin was not cleaved from the NH(2) terminus after injection into IRAP(-/-) mice and exhibited a threefold increased half-life in the circulation of IRAP(-/-) mice. Consistent with this finding, endogenous plasma vasopressin levels were elevated twofold in IRAP(-/-) mice, and vasopressin levels in IRAP(-/-) brains, where plasma vasopressin originates, showed a compensatory decrease. We further established that insulin increased the clearance of vasopressin from control but not from IRAP(-/-) mice. In conclusion, we have identified vasopressin as the first physiological substrate for IRAP. Changes in plasma and brain vasopressin levels in IRAP(-/-) mice suggest a significant role for IRAP in regulating vasopressin. We have also uncovered a novel IRAP-dependent insulin effect: to acutely modify vasopressin.

Targeted tyrosine iodination in a multi-tyrosine vasopressin analog

Iodination of the conserved 2-tyrosine (Tyr(2)) residue in the pressin and tocin rings of arginine- or lysine-vasopressin (AVP or LVP), and oxytocin, respectively, impairs binding to their respective receptors. Synthetic antagonists that have their Tyr(2) either replaced by another amino acid or irreversibly blocked by an O-methyl or O-ethyl ether, but have, instead, an iodinatable phenol moiety outside the pressin/tocin ring, are used for radiolabeling. We explored another approach to avoid iodinating Tyr(2) by capping this residue with a reversible O-acetyl group, incorporated during peptide synthesis. The O-acetyl-Tyr(2) LVP peptide, with a free iodinatable tyrosine attached to the epsilon-amine of 8-lysine, is iodinated at a neutral pH and purified by reverse-phase high-pressure liquid chromatography (HPLC) at an acidic pH, conditions under which the O-acetyl groups are stable. Deacetylation with hydroxylamine is selective, and leaves intact the disulfide bridge. The marked shortening of the HPLC retention time after deblocking produces a chemically homogeneous label, iodinated exclusively on the free tyrosine residue attached to the epsilon-amine of LVP. Hitherto, this (125)I labeled vasopressin agonist could be obtained only in low yield, via conjugation labeling with iodinated N-t-Boc-tyrosine succinimidyl ester. This fully reversible tyrosine protection strategy does not require special equipment, and retains the conserved Tyr(2), typical of vasopressin and oxytocin agonists.

Internalization of V2-vasopressin receptors in LLC-PK1-cells: evidence for receptor-mediated endocytosis

The mechanism of internalization of the vasopressin-receptor (V2-subtype) of LLC-PK1-cells, a pig renal tubular cell line, is unknown. We studied internalization utilizing a novel, highly specific vasopressin analogue ((125I)-[8-p(OH)-phenylpropionyl]-LVP, 2000 Ci/mmol). Scatchard analysis performed with membranes of LLC-PK1-cells revealed a Kd of 0.8 +/- 0.2 nM and a Bmax of 366 +/- 41 fmol/mg of protein. Degradation of the ligand was excluded by RP-HPLC-analysis. Internalization was proven by the acid-wash technique, quantitative light-microscopic autoradiography and electron microscopy. The ligand was internalized in a time- and temperature-dependent manner. At 4 degrees C, no uptake was found; at 22 degrees C, after 30 min of incubation, more than 50% of the radioligand was found inside the cell. Electron microscopy demonstrated that plasma-membrane bound vasopressin receptors are internalized by receptor-mediated endocytosis via coated pits.

Potent V2 vasopressin antagonists with structural changes at their C-terminals

A variety of structural changes were made in the C-terminals of four potent antidiuretic (V2) antagonists. The parent analogs were all derivatives of [1-(beta-mercapto-beta,beta-cyclopentamethylenepropionic acid)]arginine-vasopressin, d(CH2)5AVP, namely d(CH2)5[D-Phe2,Ile4]AVP, d(CH2)5[D-Ile2,Ile4]AVP, d(CH2)5[D-Tyr(Et)2, Val4]AVP and d(CH2)5[D-Tyr(Et)2,Ile4]AVP. A number of amino acid amides were substituted for the C-terminal 9-glycinamide without reducing their V2-antagonistic potencies in rats. Many non-amino acid structures were also tolerated at the C-terminals of these antagonists and this end of these peptides can be prolonged without interfering with antagonistic potencies. Such altered V2-antagonists may be useful for the development of radioactive ligands, affinity labels and in affinity columns for studies on antidiuretic receptors. These C-terminal modifications also provide useful information for the further development of potent and specific V2-antagonists which can be valuable pharmacological tools and also promise to become useful clinically for the treatment of excessive water retention.

Synthesis and characterization of iodinated vasopressin antagonists which retain high affinity for the vasopressin receptor

The mono- and di-iodinated analogs of lysine and arginine vasopressin have been prepared previously but not well characterized chemically. Their biological activities are greatly reduced with respect to LVP or AVP. In this paper we report a convenient synthesis of iodinated AVP agonists and antagonists, their purification by high performance liquid chromatography, and their characterization by fast atom bombardment mass spectrometry. In contrast to the results obtained with agonists, the mono-iodinated vasopressin antagonist retains virtually the full receptor activity of its non-iodinated parent. This should allow the preparation of labeled vasopressin antagonists of high specific activity for receptor characterization and isolation.

Light microscopic autoradiographic localization of [3H]arginine-vasopressin binding sites in the rat brain and kidney

Specific binding sites for arginine-vasopressin (VP) were demonstrated in various major target areas of VP in the rat brain and kidney by light microscopic autoradiography. In the kidney moderate and intense labelling was found in the cortical and medullar areas, respectively. Within the brain intense labelling was shown in the lateral septum, which lends further support to the hypothesis that VP acts as a neurotransmitter. In the hypophysis moderate and heavy labelling was found in the anterior and neural lobes, respectively, which is in agreement with the idea that VP influences hypophyseal functioning.

Vasopressin and angiotensin II receptors in rat aortic smooth muscle cells in culture

Rat aortic smooth muscle cells were isolated and maintained in primary culture. After 2-3 days, cells recovered their contractile phenotype and could be induced to contract in response to vasopressin and angiotensin II. Vasopressin- and angiotensin-specific binding sites were detected on these cells, using tritiated Lys8-vasopressin, Asn1-Val5-angiotensin II, and Sarc1-Ile8-angiotensin II. Vasopressin binding sites had Kd values of 30 and 12 nM for Lys8-and Arg8-vasopressin, respectively, and a maximal binding capacity of 25,000 sites/cell. They displayed several of the expected characteristics of vasopressin receptors involved in the vasopressor response in vivo. A highly significant correlation was found between the relative agonistic or antagonistic vasopressor potencies of a series of vasopressin structural analogues and their relative abilities to inhibit [3H]vasopressin binding to aortic smooth muscle cells. Specific binding sites for Asn1-Val5-angiotensin II and Sarc1-Ile8-angiotensin II had the following characteristics: Kd = 2.3 and 1.3 nM, respectively; maximal capacity: 50,000 sites/cell. Vasopressin and angiotensin did not modify the intracellular cyclic AMP content of aortic smooth muscle cells.

Synthesis of p-amino-L-phenylalanine derivatives with protected p-amino group for preparation of p-azido-L-phenylalanine peptides

For the synthesis of p-azidophenylalanine peptides, the p-amino group of p-amino-L-phenylalanine is protected with the Z- or Boc residue via the copper complex or by specific acylation at pH 4.6. The alpha-amino or alpha-carboxy group is blocked by a protecting group (Boc, Ddz, OMe respectively Z, Nps, Ddz), which can be removed selectively. The synthesis of nine derivatives of p-amino-L-phenylalanine for incorporation into the peptide chain is described. The p-amino-phenylalanine is converted to p-azidophenylalanine without affecting disulfide bridges.

Oxytocin receptors coupled to uterine contraction in estrogen-dominated rabbits

The present study investigated whether specific [3H]oxytocin binding sites previously demonstrated in estrogen-dominated rabbit uterus have properties expected of physiologic receptors coupled to uterine contraction. Microsomal membranes from estrogen-dominated rabbit uterus were found to contain high-affinity specific oxytocin binding sites with Kd = 2-3 nM. These sites were predominantly myometrial in locus. Specific oxytocin binding exhibited a pH optimum between 7.5 and 8.0. Mg2+ or Mn2+ was necessary for maximal specific [3H]oxytocin binding; in contrast, Ca2+ at submillimolar concentrations inhibited specific binding. Oxytocin binding sites were not detectable in microsomal membranes isolated from progesterone-dominated rabbit uterus. Relative binding and uterotonic activities of 10 synthetic neurohypophyseal hormone analogues were determined in estrogen-dominated rabbit uterus. A qualitative correlation was observed between binding and uterotonic responses. Angiotensin II and insulin did not compete with [3H]oxytocin for uterine binding sites. It is concluded that the specific high affinity [3H]oxytocin binding sites demonstrated in estrogen-dominated rabbit uterus have the selectivity for neurohypophyseal hormone analogues expected for physiologic receptors coupled to uterine contraction.

Synthesis of oxytocin using iodine for oxidative cyclization and silica gel adsorption chromatography for purification

Oxytocin (OT) was synthesized employing the solid phase method. Resins made of copolymers of polystyrene-1%-crosslinked with divinylbenzene gave better yields (73-95%) of Z-Cys(Bzl)-Tyr(Bzl)-Ile-Gln-Asn-Cys(Bzl)-Pro-Leu-Gly-NH2 (I) than 2%-crosslinked resins (10--56%). Reduction of I with Na-liq.NH3 and oxidation with I2-MeOH at -40 degrees minimized dimer and polymer formation, and resulted in good yields (49--54%) of OT. The large volumes of MeOH required when several grams of I are reduced and then oxidized were rapidly evaporated in vacuo, and the residue was desalted by dissolving the peptide in a small volume of glacial acetic acid and filtering to remove the salt. OT was purified by adsorption chromatography on a silica gel column with combinations of MeOH-CHCl3 of graded polarity. Oxytocin elutes with 33% MeOH-CHCl3. After two purification steps by adsorption chromatography, the resulting OT was found to be homogeneous. The hormone was characterized chemically and found to be active biologically.

Arginine vasopressin metabolism in dogs. II. Modeling and system analysis

System modeling and analysis methods were applied to interpret data regarding arginine vasopressin (AVP) metabolism in dogs. Based on this analysis a new nonlinear 3-pool model of AVP distribution and disposal was proposed and quantified; the model pools included the plasma, a receptor pool, and an extravascular nonreceptor pool. The receptor pool mediated a portion of the rapid flux of hormone between the plasma and the extravascular pool. Mathematical analysis indicated that the plasma AVP impulse response (bolus) data would be insufficient to uniquely estimate all the model constants, but additional plasma impulse data using 125I-labeled AVP, which does not bind to physiologic hormone receptors, would allow unique model quantification. Other required measurements were the urinary excretion of intact hormone, and plasma AVP degradation. The model was successfully fitted to the data from 10 dogs. The results suggest that, in the normal dogs studied, plasma contained 25% of the total AVP, 19% was bound to receptors, and the remaining 56% was in the extravascular pool. Eighty percent of the flux of AVP from the vascular compartment was mediated by the receptor pool; 98% of AVP degradation occurred in the extravascular pool; and urine excretion and plasma degradation made up the remainder.

Arginine vasopressin metabolism in dogs. I. Evidence for a receptor-mediated mechanism

The plasma clearance rates (PCR) of arginine vasopressin (AVP), and iodinated AVP (125I-AVP) were determined after pulse injection in conscious water-loaded dogs. Both the PCR and the apparent initial volume of distribution were significantly greater for AVP than for the biologically inactive iodinated AVP 37.4 +/- 4.8 ml/kg per min vs. 6.7 +/- 0.8 ml/kg per min (P less than 0.001) and 12.7 +/- 0.9% body wt vs. 7.1 +/- 0.4% body wt (P less than 0.001). AVP clearance was then determined by the constant-infusion technique at doses that produced equilibrium AVP concentrations within and above the physiological range. AVP-PCR was 37.4 +/- 7.1 ml/kg per min at 34 microU/kg per min, which was comparable to that after pulse injection (P less than 0.9). AVP clearance fell progressively, and urine osmolality progressively increased with increasing AVP infusion rates to plateau values at 136 microU/kg per min; a strong negative correlation was observed between mean AVP-PCR and urine osmolality (r = -0.993). The data suggest a relationship between the biological activity of AVP and its clearance. It is proposed that plasma membrane receptors may mediate a portion of the metabolic clearance of AVP.