Preparation and properties of iodinated angiotensins

[The insulin receptor discovery is 50 years old - A review of achieved progress]

The isolation of insulin from the pancreas and its purification to a degree permitting its safe administration to type 1 diabetic patients were accomplished 100 years ago at the University of Toronto by Banting, Best, Collip and McLeod and constitute undeniably one of the major medical therapeutic revolutions, recognized by the attribution of the 1923 Nobel Prize in Physiology or Medicine to Banting and McLeod. The clinical spin off was immediate as well as the internationalization of insulin's commercial production. The outcomes regarding basic research were much slower, in particular regarding the molecular mechanisms of insulin action on its target cells. It took almost a half-century before the determination of the tri-dimensional structure of insulin in 1969 and the characterization of its cell receptor in 1970-1971. The demonstration that the insulin receptor is in fact an enzyme named tyrosine kinase came in the years 1982-1985, and the crystal structure of the intracellular kinase domain 10 years later. The crystal structure of the first intracellular kinase substrate (IRS-1) in 1991 paved the way for the elucidation of the intracellular signalling pathways but it took 15 more years to obtain the complete crystal structure of the extracellular receptor domain (without insulin) in 2006. Since then, the determination of the structure of the whole insulin-receptor complex in both the inactive and activated states has made considerable progress, not least due to recent improvement in the resolution power of cryo-electron microscopy. I will here review the steps in the development of the concept of hormone receptor, and of our knowledge of the structure and molecular mechanism of activation of the insulin receptor.

A fluorouridine-anti-CEA immunoconjugate is therapeutically effective in a human colonic cancer xenograft model

5-fluorouridine (FUR), an antineoplastic agent, was site-specifically conjugated to the carbohydrate moiety of a anticarcinoembryonic antigen (CEA) monoclonal antibody (MAb) by using amino-dextran as the intermediate carrier. The final immunoconjugate contains approximately 30-35 molecules of FUR per molecule of immunoglobulin, has immunoreactivity retained as examined by flow cytometry, and is cytotoxic to the target cells as examined by 75selenomethionine incorporation studies. In the GW-39/nude mouse model, the conjugate remained efficient in targeting the human colonic tumor and possessed greater inhibitory growth effects on the subcutaneous tumor than free FUR or an irrelevant antibody conjugate. In addition, the reduced host toxicity of the conjugate may permit the use of this agent in a high-dose therapy of this tumor system.

Site-specific linkage of methotrexate to monoclonal antibodies using an intermediate carrier

The site-specific conjugation of methotrexate, 4-amino-N10-methylpteroylglutamic acid, to a monoclonal anti-carcinoembryonic antigen (CEA) antibody, using an intermediate amino-dextran carrier system, resulted in a ratio of 30-50 molecules of MTX per molecule of immunoglobulin. The immunoreactivity of the conjugate was analyzed using flow cytometry or a competitive binding assay, which showed that the conjugate has significant retention of the antigen-binding activity. The pharmacokinetic behavior of the immunoconjugate in BALB/c mice and tumor localization in hamsters indicated that the conjugate remained in the circulation with higher concentration than free antibody, and could recognize the tumor as efficiently as the unconjugated antibody. The high degree of drug incorporation with retained immunoreactivity makes this method preferable to direct antibody conjugation.

Means for the assessment of radioligand quality and its importance in receptor-binding studies. Observations with radiolabelled formylmethionyl-leucyl-phenylalanine

Various methods for testing the quality of radioligands were applied to two different radiolabelled forms of formylmethionyl-leucyl-phenylalanine (fMet-Leu-Phe). The purpose of the study was both to examine the value of these methods for assessing radioligand quality and to determine the suitability of these particular radioligands for studying the chemotactic formylpeptide receptors on the rabbit neutrophil. It is useful in this context to distinguish two different aspects of radioligand quality: these are purity and equivalence to the native ligand. The two methods described for measuring receptor-reactivity (or 'bindability'), by measuring binding to an increasing excess of receptors and by a re-incubation procedure, provide a reliable measure of purity that should readily be applicable to other radioligands. Equivalence to the native ligand is more difficult to establish, and any uncertainty about the specific radioactivity of the radioligand can pose serious problems with this assessment. Commercial preparations of both tritiated and 35S-labelled fMet-Leu-Phe were found to be inadequately pure for detailed receptor studies. Repurification by t.l.c., however, consistently yielded radioligand preparations of high purity and close equivalence to the native ligand. Other radioligands may often also require a suitable repurification step before use for detailed receptor studies; this is especially important whenever a complex receptor-binding pattern is envisaged.

Rapid purification of radioiodinated peptides with Sep-Pak reversed phase cartridges and HPLC

A simple, rapid method is described for the purification of radioiodinated peptides for use in radioimmuno- and in radioreceptor assays. Iodinated reaction mixtures are applied directly onto Sep-Pak disposable, reversed phase cartridges equilibrated with phosphate buffer. Unreacted 125-iodide and other non-peptide reaction components are eluted with buffer. The peptide fraction is then eluted with 70% buffer: 30% acetonitrile. The peptide fraction is further purified by reversed phase high pressure liquid chromatography to separate the native peptide and the mono- and diiodo-derivatives. In this study the method is used to prepare 125-iodide-labeled monoiodo-leucine enkephalin and monoiodo-angiotensin II, which are free of the parent peptides and diiodo-derivatives and are of maximum obtainable specific radioactivity. The usefulness of these labeled peptides in radioimmuno- and radioreceptor assays is demonstrated by their binding to specific antibodies and receptors, respectively.

Preparation and one-step purification of mono-125I-angiotensin II for radioligand binding assays

A one-step purification of mono-125I-angiotensin II prepared by the chloramine T procedure is described. The purification is effected on a cellulose cation exchange column with isocratic elution by 50 mM sodium acetate, pH 5.0. The purity of the mono-125I-angiotensin II was determined by thin layer chromatography, high pressure liquid chromatography, enzymatic digestion, radioreceptor assay, and radioimmunoassay. Preparation and purification of mono-125I-angiotensin II by this procedure offers significant advantages over existing methods for its preparation in terms of purity, simplicity, efficiency, and cost.

Angiotensin receptors in bovine umbilical artery and their inhibition by nonsteroidal anti-inflammatory drugs

1 The contractile effect of angiotensin II on bovine isolated umbilical arteries was compared to [125I]-angiotensin II binding by a subcellular fraction of that tissue. The ED50 of angiotensin was 3.1 +/- 2.8 x 10(-8) M, while the apparent dissociation constant was 4.9 +/- 1.6 x 10(-9) M. 2 Indomethacin, meclofenamate, and eicosatetraynoic acid inhibited angiotensin-induced contraction of the isolated artery and binding to a particulate fraction at comparable doses. Phenylbutazone inhibited [125I]-angiotensin binding more potently than the response. Inhibition by the first three agents was noncompetitive, whereas phenylbutazone inhibited competitively. 3 Inhibition of angiotensin activity by the nonsteroidal anti-inflammatory agents was not specific. These agents also inhibited 5-hydroxytryptamine-induced contraction, but not the contraction induced by KCl. 4 The data suggest that the angiotensin binding sites studied include receptors that mediate contraction of the isolated umbilical artery. Our data also indicate that indomethacin, meclofenamate, eicosatetraynoic acid and phenylbutazone are capable of direct inhibitory effects on receptors, as well as their well-known synthetase actions. The net effect of these activities will determine the change these agents cause in tissue responses to hormones.

Effect of tyrosine ionization upon biological activities of angiotensin II and two new peptide analogues

The role of the tyrosine side-chain in the smooth muscle contracting activity of angiotensin III was investigated by determining intrinsic activities and ED50 values of [4-(3-chlorotyrosine)]angiotensin II and [4-(3-benzyltyrosine)]angiotensin II in the isolated guinea-pig ileum and rat uterus. [4-(3-chlorotyrosine)]angiotensin II activity was compared with that of angiotensin II at different pH values, in which the ratio of their degrees of phenolic ionization varied. The results indicated that deprotonation of the phenolic group hinders binding to smooth muscle cell receptors, but not triggering of the response by the hormone-receptor complex. Steric hindrance by the benzyl substituent in [4-(3-benzyltyrosine)]angiotensin II reduced both receptor-binding and triggering of the response.

Placental transfer of a large angiotensin fragment in the guinea pig

Angiotensin II (AII) levels are usually higher in the fetal than in the maternal circulations. Radioiodinated AII has been used to establish whether or not this hormone crosses the placenta from the fetus to the mother in the pregnant guinea pig. No intact radioiodinated AII was found to have crossed in any of 13 experiments. A single radioiodinated AII fragment was found in 10 of these experiments. This ran as (des-(Asp1 Arg2 Val3)) in one solvent, but this could not be confirmed when a different solvent was used. The results support the hypothesis, previously based on indirect evidence, that AII does not cross the placenta.

Angiotensin receptors in vascular tissue

The biologic effect of angiotensin II is triggered by its interaction with components of target organs, which specifically recognize the hormone. These receptors have been studied with the use of radioactive angiotensin and, as for other peptidic hormones, have been localized in the plasma membrane of target cells. Such angiotensin receptors have been characterized in three target organs: vascular tissue, uterus and adrenal cortex. The binding characteristics differ in contractile tissue and in adrenal glands, the N and C terminal ends of angiotensin being involved in the former, whereas the N terminus does not appear to have the same importance in the latter. Numerous factors, including ionic composition, seem to be able to modify angiotensin-receptor interaction in vascular smooth muscle. However, the molecular mechanisms responsible for angiotensin binding and for the transmission of the signal determined by receptor-angiotensin interaction are not yet understood. As observed with other peptidic hormones, the number of angiotensin receptors seems to be susceptible to variation under certain conditions. In uterine smooth muscle, it was shown that the number of receptors increased after nephrectomy, a phenomenon which was prevented by the prolonged infusion of angiotensin. The significance of such a variation remains unknown, but it may be partially responsible for the inverse relationship that exists between the endogenous angiotensin level and the pressor effect of exogenous angiotensin. In the near future, investigation of the angiotensin-receptor mechanism will probably answer whether the variation in angiotensin receptors is similar in all target tissues and whether an angiotensin-receptor mechanism is involved in the pathogenesis of certain varieties of hypertension. In addition, a precise understanding of the angiotensin-receptor interaction with help the development of new angiotensin antagonists.

Iodoglucagon. Preparation and characterization

Iodinated derivatives of glucagon containing an average of 1 to 5 g-atoms of 127I per mol have been prepared by reacting the hormone with increasing amounts of iodine monochloride. Their iodoamino acid composition has been determined by ion-exchange chromatography and electrophoresis, following hydrolysis by pronase. Iodination of the two tyrosyl residues occurs first and is nearly complete after addition of a 4-fold molar excess of ICl. Iodination of the single histidyl residue is a later event and does not exceed an average of one atom per residue. Hydrolysis of iodoglucagon by trypsin and subsequent separation of the iodotyrosyl peptides shows that iodine is equally distributed between tyrosyl residues 10 and 13. Crude iodoglucagon containing an average of 1 g-atom of iodine per mol has been resolved into several components of differing iodine content and iodoamino acid composition by chromatography on DEAE-cellulose. Monoiodoglucagon isolated by this procedure shows a single band when analyzed by polyacrylamide gel electrophoresis. Iodoglucagons containing an average of 1 to 4 g-atoms of iodine per mol are more potent than native glucagon in their ability to stimulate adenylate cyclase activity and to bind to glucagon receptors of liver cell membranes of the rat. The maximal increase in biological potency occurring upon iodination is about 5-fold with respect to adenylate cyclase activity, and 2-fold with respect to binding to receptors; tetra and triiodinated derivatives show, respectively, the highest potency. Similar effects occur whether inactivation by liver membranes is inhibited or not, indicating an enhancement in the intrinsic affinity of iodoglucagon for the receptors. Iodination beyong 4 g-atoms per mol slightly decreases the affinity of the hormone for adenylate cyclase and for the receptors. Iodination causes a 2-20 fold decrease in the ability of liver plasma membranes and of blood plasma to inactivate glucagon in vitro; these effects correlate with the degree of iodination. With liver microsomal membranes, a decrease in glucagon inactivation occurs only at iodine contents exceeding 4 g-atoms per mol, and lower degrees of iodination result in opposite effects. Monoiodination causes a 4-6-fold increase in the plasma concentration of glucagon within the first 18 min following a single intrvenous injection of the hormone to rats. More extensive iodination results, in addition, in a marked decrease in the rate of dissappearance of glucagon from the blood. The immunological reactivity of glucagon is little affected by monoidination, but strongly depressed by higher degrees of iodination...

Insulin-dependent regulation of insulin receptor concentrations: a direct demonstration in cell culture

Chronic (5-16 hr) exposure of cultured human lymphocytes to 10(-8) M insulin at 37 degrees in vitro produced a decrease in insulin receptor concentrations unaccounted for by simple occupancy of sites; acute exposure (0-2 hr) was without effect. These results reproduced observations in vivo where chronic hyperinsulinemia (e.g., 10(-8) M insulin in the circulation of obese insulinresistant hyperglycemic mice) is associated with a substantial reduction in the concentration of insulin receptors per cell, while acute hyperinsulinemia in vivo has no effect on receptor concentration. These data suggest a reciprocal relationship between insulin in the extracellular fluid and the concentration of insulin receptors per cell, which is mediated at the target cell itself by intracellular insulin-sensitive regulatory processes and directly affects target-cell sensitivity to hormone.

Substrate requirements for angiotensin I conversion in vivo and in vitro

The substrate requirements for angiotensin I-converting enzyme were studied in vivo in the dog lung and in vitro in plasma, using angiotensin I (AI), 5- D -Ile-AI, 7- D -Pro-AI, and 8-D-Phe-AI which had been synthesized by the solid-phase technique. All peptides were inactive in the rabbit aortic strip preparation. None of the D-amino acid-substituted peptides gave a pressor response in the anesthetized dog; none showed significant immunologic cross-reactivity with anti-AI serum or antiangiotensin II serum. Each peptide was labeled with 125 I, and the monoiodinated species was isolated. The iodinated peptides were incubated with diluted dog plasma or injected into the right ventricle of intact anesthetized dogs. In vitro, 125 I-AI, 125 I-5-D-IIe-AI, and 125 I-7-D-Pro-AI were converted to angiotensin II (AII). 8-D-Phe-AI was not converted. In vivo, 15 seconds after injecting 125 I-AI, 125 I-5-D-IIe-AI, or 125 I-7-D-Pro-AI into the right ventricle, 125 I-AJI accounted for 70, 60, and 45%, respectively, of the radioactive material in aortic blood. These results and our previous observations on the importance of the C-terminal sequence of AI for conversion indicate that the enzymatic binding sites for AI-converting enzyme in vivo and in vitro extend from position 10 to position 8 but not to position 7. D-amino acid substitution at positions 7 and 5 abolishes biologic and immunologic activity without interfering with conversion.

Preparation and immunoreactive properties of monoiodinated angiotensin labelled at high specific activity

Monoiodinated angiotensin II at a specific activity in the order of 1 C/mg can be prepared with high yields by controlling the pH of iodination and can be purified on G-25 Sephadex. The fractions from the head to the centre of the single peak of radioactivity obtained by gel filtration of the iodination mixture contain pure monoiodinated angiotensin. The monoiodinated derivative shows an immunoreactivity very close to that of the native hormone; in contrast, the diiodinated derivative has a low immunoreactivity and its use as a tracer results in a loss of sensitivity in the radioimmunoassay of angiotensin II. It is suggested that the use of the monoiodinated derivative in pure form should be recommended whenever a small hormonal polypeptide labelled at high specific activity is to be prepared for radioimmunoassay purposes.