Binding of parathyroid hormone to bovine kidney-cortex plasma membranes

Oxidation of parathyroid hormone

Parathyroid hormone (PTH) is the key hormone regulating calcium homeostasis and, as such, is an important diagnostic and prognostic marker. Although the measurement of PTH has greatly improved over the past few decades, oxidation status thereof is unaccounted for in currently used assays. PTH can be oxidized on methionine residues located at amino acid positions 8 and 18. This is a relevant post-translational modification as, due to refolding of the molecule, it results in the decreased ability to activate the PTH1 receptor. Although this loss of activity after oxidation was observed as early as 1934, only recently a method was developed to measure and distinguish non-oxidized PTH (n-oxPTH) from oxidized PTH. This method creates exciting possibilities for studying more specifically the role of n-oxPTH in physiology and pathology. Therefore, it can now be explored what the clinical implications of measuring n-oxPTH will be. Herein, we review the available evidence of the effect of oxidation on the biological activity of PTH. We also discuss studies examining the mechanism of PTH oxidation in vivo and efforts to stabilize synthetic PTH ex vivo for therapeutic applications. Lastly, the available studies regarding the clinical significance of n-oxPTH are evaluated and future directions discussed.

Hyperphosphatemia Management in Patients with Chronic Kidney Disease

Hyperphosphatemia in chronic kidney disease (CKD) patients is a potentially life altering condition that can lead to cardiovascular calcification, metabolic bone disease (renal osteodystrophy) and the development of secondary hyperparathyroidism (SHPT). It is also associated with increased prevalence of cardiovascular diseases and mortality rates. To effectively manage hyperphosphatemia in CKD patients it is important to not only consider pharmacological and nonpharmacological treatment options but also to understand the underlying physiologic pathways involved in phosphorus homoeostasis. This review will therefore provide both a background into phosphorus homoeostasis and the management of hyperphosphatemia in CKD patients. In addition, it will cover some of the most important reasons for failure to control hyperphosphatemia with emphasis on the effect of the gastric pH on phosphate binders efficiency.

Identification and characterization of parathyroid hormone receptors in rat renal cortical plasma membranes using radioligand binding

Parathyroid hormone (PTH) receptors have been described in renal tissue from several species, but not in the rat. In this study, radioligand binding techniques were used to identify and characterize PTH receptors in rat kidney cortical membranes. The sulfur-free PTH analog [Nle8,18Tyr34]bovine PTH-(1-34)amide was iodinated using the iodogen method. This ligand was suitable for use in identifying PTH receptors in canine renal membranes, but not rat renal membranes. Synthetic, unsubstituted rat PTH-(1-34) was iodinated using the milder, lactoperoxidase technique and was purified by HPLC on a C8 column. [125I]rat PTH-(1-34) bound rapidly to both rat and dog renal membranes. At 22 degrees C reaction reached steady state within 20 minutes, and this level was maintained for at least 3 h. Specific binding was routinely greater than 90% for rat kidney and greater than 95% for dog kidney. Similar results were obtained at 4 degrees C with a longer time required to attain steady state (approximately 45 minutes). Binding was reversible as demonstrated by dissociation of bound ligand after either infinite dilution or displacement with excess nonradioactive PTH. Binding was saturable and of high affinity (rat kidney: Bmax = 2.3 pmol/mg protein, Kd = 3.1 nM, dog kidney: Bmax = 2.1 pmol/mg protein, Kd = 3.7 nM). Rat renal cortical adenylate cyclase activity was stimulated by rat PTH in a dose-dependent manner with an EC50 of 4 nM, a value in good agreement with the binding data. This study demonstrates the feasibility of identifying and characterizing parathyroid hormone receptors in rat renal cortical plasma membranes using radioligand binding techniques.

Effect of biological activity of PTH on its peripheral metabolism in the rat

Previous studies from our laboratory have characterized the peripheral metabolism of parathyroid hormone (PTH) in the dog using radioimmunoassay techniques following injection or infusion of biologically active, bovine PTH preparations. Other investigators have used biologically-inactive labelled PTH and interpreted their results as representative of the normal physiological processes. Since oxidized inactive PTH does not bind to PTH receptors and since we have found substantial differences between the tissue uptake of active and inactive PTH preparations, it is possible that results obtained with inactive PTH preparations may not totally represent the normal metabolism of PTH. Therefore, we performed studies in rats to compare the disappearance of immunoreactive PTH (i-PTH) from plasma following injection of active or inactive syn b-PTH 1-34. The maneuvers of bilateral ureteral ligation and bilateral nephrectomy were utilized to characterize the sites of tissue uptake of i-PTH. The results obtained indicate that inactive syn b-PTH 1-34 has a significantly slower disappearance from plasma than biologically active syn b-PTH 1-34. Reduction of glomerular filtration by acute bilateral ureteral ligation decreased the disappearance of oxidized PTH more than active PTH. Thus, the results indicate a major dependence on glomerular filtration for the removal of inactive syn b-PTH 1-34. The demonstration that the peripheral metabolism of active and inactive syn b-PTH 1-34 is not identical suggests that studies of the metabolism of inactive PTH preparations do not accurately reflect that of biologically active PTH.

Preparation and characterization of radioactive monoiodotyrosine and diiodotyrosine derivatives of parathyroid hormone

Highly purified native parathyroid hormone was iodinated by the enzymatic method and separated from unlabeled hormone by isocratic HPLC. The separation system used also resolved iodohistidine, monoiodotyrosine, and diiodotyrosine forms of the hormone from one another. A simplified procedure for direct bioassay of the carrier-free, high specific activity, mono- and diiodinated parathyroid hormone (PTH) by the renal membrane adenylyl cyclase method was also developed. Both labeled forms of the hormone are very potent in this assay, but the iodinated forms appeared to give a lower Vmax than the native hormone. The methods for iodination, separation and biological characterization of this PTH tracer are exceptionally facile, inexpensive, and convenient.

Peripheral metabolism of intact parathyroid hormone. Role of liver and kidney and the effect of chronic renal failure

The plasma disappearance rate (metabolic clearance rate) of administered intact parathyroid hormone (intact PTH) was analyzed in awake dogs with indwelling hepatic and renal vein catheters. The metabolic clearance rate (MCR) of intact PTH was found to be very rapid, 21.6 +/- 3.1 ml/min per kg in 11 normal dogs. The liver accounted for the greatest fraction of the MCR of intact PTH (61 +/- 4%) by virtue of an arterial minus venous (a - v) difference across the liver of 45 +/- 3%. The renal uptake of intact PTH accounted for 31 +/- 3% of the MCR of intact PTH. The renal a - v difference for intact PTH of 29 +/- 2% was significantly greater than the filtration fraction indicating renal uptake of intact PTH at sites independent of glomerular filtration. Together, the hepatic and renal clearances of intact PTH accounted for all but a small fraction of the MCR of intact PTH. The MCR of intact PTH, rendered biologically inactive by oxidation, was markedly decreased to 8.8 +/- 1 ml/min per kg. The a - v difference of oxidized intact PTH was reduced both in the liver and kidney. These data suggested that the high uptake rates of intact PTH are dependent, at least in part, upon sites recognizing only biologically active PTH. Chronic renal failure (CRF) decreased the MCR of intact PTH to 11.3 +/- 1.3 ml/min per kg (n = 10). Both the hepatic and renal a - v differences of intact PTH were reduced in dogs with CRF. This resulted in reductions in the hepatic and renal clearances of intact PTH. These studies identify the liver as a major extrarenal site of PTH metabolism affected by CRF. They suggest that CRF impairs the function of the major uptake sites involved in intact PTH metabolism.

Different handling of parathyrin by basal-lateral and brush-border membranes of the bovine kidney cortex

The two parts of the bovine kidney cortex plasma membrane, the basal-lateral and the brush-border membrane, were simultaneously prepared from the same organ. Both types of membrane bound parathyrin, but only from the basal-lateral fraction was the hormone displaceable by its bioactive N-terminal fragment. In parallel, parathyrin-stimulated adenylate cyclase was predominantly found in basal-lateral membranes. The hormone was fragmented by both membrane types. Basal-lateral membranes generated fragments with a rather uniform size distribution (somewhat smaller than the intact peptide) and apparently preferred the hormone itself as a substrate. In contrast, the fragments produced by brush-border membranes were numberous small peptides.

Kidney membrane binding of native parathyroid hormone compared to binding of its synthetic 1 - 34 fragment

Kidney membrane binding of tritiated native parathyroid hormone (PTH) was compared to that of its active fragment 1 - 34 PTH. Native hormone specific binding is transient and disappears rapidly (integral of 30 minutes) at 37 degrees C but is stable up to 2 hours at 0 degrees C. The rate of binding loss appears relatively independent of the amount of hormone-receptor complex or of the amount of cold PTH in the medium. Loss of specific binding is also seen with iodinated PTH. Loss of specific binding of native PTH does not appear to be the result of enzymatic degradation of the hormone since significant amounts of intact hormone are present, both bound to membranes and in the medium after incubation. Biologically active tritiated 1 - 34 PTH binds specifically to isolated membrane, and both total and specific binding is stable for at least one hour at room temperature. The pH dependence for binding of 1 - 84 PTH and its activation of adenylyl cyclase are very similar, but differ from that reported for specific binding of 1 - 34 PTH. these results suggest that the interaction of native PTH with kidney cells may be more complex than that of its 1 - 34 fragment.

Parathormone receptor binding and the influence of membrane degradation of the hormone

Concentration of receptor bound parathormone was determined using a labelled-antibody-membrane-assay (LAMA). 4.2-31.2 fmol PTH per mg membrane protein (0.42-3.12 fmol/tube) were bound at apparent PTH concentrations of 15.6-500 fmol/tube. Only 0.65%-3.2% of PTH could be detected at the receptor sites. The amount of free hormone was corrected for inactivation of intact immunologically reactive PTH and of intact biologically reactive PTH. Only after the latter correction were values for binding affinity (Kd = 1.1 x 10(-12) mol/mg) and number of binding sites (Bmax = 5.8 x 10(-14) mol/mg) obtainable. These results indicate that allowance must be made for loss of biological activity of hormone at membrane receptor sites if binding parameters are to be defined free of artifacts. Binding data may otherwise be misinterpreted as indicating negative cooperativity, heterogeneity of binding sites or multi-step reactions.

Insulin inhibition of hormone-stimulated protein kinase systems of rat renal cortex

Parathyroid hormone (PTH) and glucagon increase the urinary fractional excretion of phosphate, but insulin administration is associated with a decreased fractional excretion of phosphate. It was the purpose of this study to determine whether insulin will antagonize the effects of PTH and glucagon on cAMP levels and protein kinase activation of rat renal cortex. In situ incubation studies were performed on rat renal cortical slices exposed to insulin, PTH, and glucagon. Insulin alone did not affect the tissue cAMP and cGMP levels or the state of protein kinase activation. Preincubation of slices with insulin, however, did significantly inhibit increases in protein kinase activation induced by both PTH and glucagon. Insulin also significantly inhibited PTH-stimulated increases in tissue cAMP levels, but did not blunt the elevations of cAMP levels induced by glucagon. Insulin (10(-9) M) had no effect on either the in vitro activity of adenylate cyclase, basal or PTH-stimulated, or on the activities of low Km cytosolic or membrane-bound cAMP phosphodiesterase. The data show that insulin antagonizes activation of protein kinase by both PTH and glucagon in renal cortex. Separate mechanisms are probably involved for PTH and glucagon interaction. The antiphosphaturic effect of insulin in vivo may result in part from this antagonism at the cellular level.

The regulation of adenylate cyclase of the adrenal cortex

This short review summarizes some of the data concerning the regulation of adrenocortical adenylate cyclase by ACTH and other putative effectors, such as guanosine and nucleotides, divalent cations and adenosine. The available information on ACTH-sensitive adenylate cyclase of the adrenal cortex is discussed in comparison to other cyclase systems and the possible biochemical mechanisms of action of ACTH on the adrenal cortex.

The cleavage and adsorption of parathyroid hormone at high dilution: implications for receptor binding studies

Like other polypeptide hormones, purified intact parathyroid hormone (1-84)parathyroid hormone is notoriously unstable and is subject to large adsorptive losses in routine laboratory manipulation. The present studies were undertaken with 125I-labeled hormone to quantitate the problem and to develop preventative measures, particularly with concentrations of physiological interest, 1 . 10(-10) M. It was found that spontaneous cleavage of the hormone takes place upon its incubation in air or oxygen. This can be prevented by the presence of mercaptoethanol or by plasma levels of cysteine and ascorbate. Under non-cleavage conditions, adsorption was found to be extensive on all materials tested. This adsorption increased with time up to 2 h, was independent of ionic strength, increased with increasing temperature and was presumed to involve hydrophobic interactions. Under given conditions, adsorption was proportional to concentration (constant percentage). However, at very high concentrations, 1 . 10(-6) M, adsorption was markedly reduced. Adsorption was minimized at low pH (2). Bovine serum albumin reduced adsorption under all conditions when present at concentrations of 2 mg/ml or more. Coating laboratory ware with cetyl alcohol also was helpful. Using optimal conditions, cleavage is prevented and losses are less than 5% at neutral pH, and under 2% at pH 2.

The renal handling of parathyroid hormone. Role of peritubular uptake and glomerular filtration

The mechanisms of uptake of parathyroid hormone (PTH) by the kidney was studied in anesthetized dogs before and after ureteral ligation. During constant infusion of bovine PTH (b-PTH 1-84), the renal arteriovenous (A-V) difference for immunoreactive PTH (i-PTH) was 22+/-2%. After ureteral ligation and no change in renal plasma flow, A-V i-PTH fell to 15+/-1% (P < 0.01), indicating continued and significant uptake of i-PTH at peritubular sites and a lesser role of glomerular filtration (GF) in the renal uptake of i-PTH. Since, under normal conditions, minimal i-PTH appears in the final urine, the contribution of GF and subsequent tubular reabsorption was further examined in isolated perfused dog kidneys before and after inhibition of tubular reabsorption by potassium cyanide. Urinary i-PTH per 100 ml GF rose from 8+/-4 ng/min (control) to 170+/-45 ng/min after potassium cyanide. Thus, i-PTH is normally filtered and reabsorbed by the tubular cells. The physiological role of these two mechanisms of renal PTH uptake was examined by giving single injections of b-PTH 1-84 or synthetic b-PTH 1-34 in the presence of established ureteral ligation. After injection of b-PTH 1-84, renal A-V i-PTH was 20% only while biologically active intact PTH was present (15-20 min). No peritubular uptake of carboxyl terminal PTH fragments was demonstrable. In contrast, after injection of synthetic b-PTH 1-34, renal extraction of N-terminal i-PTH after ureteral ligation (which was 13.4+/-0.6% vs. 19.6+/-0.9% in controls) continued for as long as i-PTH persisted in the circulation. These studies indicate that both GF and peritubular uptake are important mechanisms for renal PTH uptake. Renal uptake of carboxyl terminal fragments of PTH is dependent exclusively upon GF and tubular reabsorption, whereas peritubular uptake can only be demonstrated for biologically active b-PTH 1-84 and synthetic b-PTH 1-34.

Effect of parathyroid hormone and cyclic adenosine 3',5'-monophosphate on isotonic fluid reabsorption: polarity of proximal tubular cells

Isotomic fluid reabsorption (JV) of rat renal proximal tubules was examined by the shrinking droplet method in combination with simulatneous perfusion of blood capillaries. Sensitivity of JV measurement was improved by using each punctured tubule for control measurements: 1) Parathyroid hormoen (PTH) on the contraluminal cell side reduced JV in a dose-response behavior. The maximal inhibition was achieved at a PTH concentration of 10(-5) M, the half maximal inhibition at a concentration of 3 X 10(-9) M. PTH on the luminal cell side had a small inhibitory effect. 2) Cyclic AMP inhibited JV preferentially when applied to the luminal cell side. On the luminal cell side, both cyclic AMP and dibutyryl cyclic AMP inhibited JV in a similar dose-dependent behavior. Concentrations of both nucleotides as low as 10(-10) M had a definite inhibitory effect. Tested at a high concentration, N6-butyryl cyclic AMP was almost as effective as cyclic AMP. Deoxy cyclic AMP, 5' AMP, cyclic guanosine monophosphate (cyclic GMP), dibutyryl cyclic GMP had no effect. ATP inhibited JV to a very small extent. 3) The reduction of JV after administration of PTH and dibutyryl cyclic AMP was not additive. The similar inhibitory effect of PTH at the contraluminal cell face and of cyclic AMP at the luminal cell face suggests the following sequence of events in the mediation of the action of PTH: 1) activation of adenylate cyclase by PTH in the contraluminal cell membrane, and 2) action of the generated cyclic AMP on the luminal cell membrane. The interaction of cyclic AMP and the luminal cell membrane is initiated at the luminal cell surface.

Isolation of the basal and lateral plasma membranes of rat kidney tubule cells

A method was developed to isolate renal basolateral membranes from cortical kidney tubule cells of single rats. The isolated membrane fraction was characterized by the measurement of marker enzyme activities and by electron microscopy. 1. After centrifugation of crude plasma membranes on a discontinuous sucrose density gradient the basolateral membranes accumulated at a sucrose density of p= 1.14-1.15 g/ml. The yield was 147 mug membrane protein/g kidney wet weight. Protein recovery was 0.1%. 2. (Na+ + K+)-ATPase was enriched 22-fold from the homogenate. The recovery was 2.6%. The (Na+ + K+)/Mg2+-ATPase ratio was 4.1. 3. The contamination by brush borders was small. Alkaline phosphatase was 1.6-fold enriched and 0.2% was recovered. Aminopeptidase was 1-fold enriched with a recovery of 0.1%. The contamination by mitochondria, lysosomes and endoplasmic reticulum was negligible. 4. In electron micrographs the basolateral membranes showed a typical triple layered profile and were characterized by the presence of junctional complexes, gap junctions or tight junctions.

Characterization of the parathyrin receptor in renal plasma membranes by labelled hormone and labelled antibody binding techniques

The parathyrin receptor in renal cortex has been investigated by studying the binding of 125I-labelled parathyrin, or of unlabelled parathyrin detected with 125I-labelled antibodies, to a partially purified plasma membrane fraction. The kinetics of hormone uptake demonstrated a biphasic response in both systems at 22 degrees C but this phenomenon was not detectable at 37 degrees C. Specific displacement of lactoperoxidase labelled 125I-labelled parathyrin occurred with 8 ng unlabelled bovine parathyrin. The apparent affinity constant was 2.3-10(8) M(-1) and the apparent binding capacity of the membranes 1.25 pmol/mg protein. Using the labelled antibody technique the receptor showed maximal binding at pH 7.0-7.5. As little as 80 pg bovine parathyrin produced a significant increase in binding of labelled anti-bovine parathyrin antibody and saturation of binding sites was demonstrated at 2.5 pmol/mg protein. Oxidized hormone showed undetectable binding. Treatment of membranes with phospholipases A or D, or Trypsin greatly reduced subsequent hormone binding. Prior incubation of membranes with 1-34 synthetic parathyrin decreased the binding of intact hormone whereas gastrin, insulin and glucagon had no effect. Growth hormone and calcitonin slightly increased parathyrin binding.

The interrelationships between antidiuretic hormone, adenyl cyclase, tissue cyclic AMP and diffusional water permeability

1. Physiological concentrations of antidiuretic hormone increase diffusional water permeability but not measurable cyclic AMP content in the isolated papilla of the rat's kidney. 2. Theophylline (6 mM) increases diffusional water permeability and cyclic AMP content in the isolated papilla of the rat's kidney. 3. The increase in water permeability is detected with 5 muunits.ml-1 of ADH and is maximal with 50 muunits.ml-1. The same maximum was achieved with 6 mM theophylline. 4. Cyclic AMP and dibutyryl cyclic AMP both increase water permeability, but to a lesser extent than theophylline or ADH. 5. In the presence of theophylline, ADH causes a dose related generation of tissue cyclic AMP up to a dose of 2,000,000 muunits.ml-1. 6. Adenyl cyclase is increasingly activated by ADH up to doses of 2,000,000 muunits.ml-1. 7. These results suggest that while ADH activates the adenyl cyclase system and changes water permeability there are sufficient disparities to cast doubt on an exclusive role for cyclic AMP as the second messenger.

Metabolism and biological activity of parathyroid hormone in renal cortical membranes

Recent studies from several laboratories have documented the presence of fragments of parathyroid hormone in blood or peripheral tissues or in both. Inasmuch as amino-terminal fragments are known to be biologically active, it has been suggested that fragments, rather than the intact polypeptide of 84 amino acids, might be the active molecular species in tissue fluids. Accordingly, the metabolism of native bovine parathyroid hormone, bPTH-(1-84), was studied in purified renal cortical membranes from several species and correlated with hormonal stimulation of adenylyl cyclase in these membranes in vitro. Analysis of whole incubation mixtures or membrane-bound hormone by gel electrophoresis and gel chromatography after incubation of [3H]bPTH-(1-84) or 125-I-labeled bPTH-(1-84) or unlabeled biologically active bPTH-(1-84) with purified canine renal cortical membranes revealed no evidence of proteolysis, and yet the uncleaved hormone readily stimulated adenylyl cyclase. Kinetic studies of hormone-stimulated adenylyl cyclase activity revealed no difference in rate of onset of activity between bPTH-(1-84) And the active synthetic amino-terminal tetratriacontapeptide bPTH-(1-34), and hence there was no evidence of precursor-product relationship between the native hormone and an active amino-terminal fragment. The results suggest, insofar as the activity detected in these membranes reflects the biological response of the hormone in vivo, that the native hormone is indeed biologically active at the receptor level directly without the requirement for cleavage into active fragments.

Distribution of parathyroid hormone-stimulated adenylate cyclase in plasma membranes of cells of the kidney cortex

Free flow electrophoresis was employed to separate renal cortical plasma membranes into luminal (brush border microvilli) and contraluminal (basal-lateral membrane) fractions. During the separation adenylate cyclase activity was found to parallel the activity of Na+-K+-activated ATPase, an enzyme which is present in contraluminal but not in luminal membranes. In the basal-lateral membrane fraction the specific activities of adenylate cyclase and Na+-K+-activated ATPase were 4.4 and 4.6 times greater, respectively, than in the brush border fraction. The adenylate cyclase of the basal-lateral membrane fraction was specifically stimulated by parathyroid hormone which maximally increased enzyme activity eightfold. The biologically active (1-34) peptide fragment of paratyhroid hormone produced a 350% increase in adenylate cyclase activity. In contrast, calcitonin, epinephrine and vasopressin maximally stimulated the enzyme by only 55, 35 and 30%, respectively. These results indicate that adenylate cyclase, specifically stimulated by parathyroid hormone, is distributed preferentially in the contraluminal region of the plasma membrane of renal cortical epithelial cells.

Hormone metabolism and response of adenylate cyclase to parathyroid hormone in kidney

1. Incubation of parathyroid hormone with plasma membranes from rat kidney cortex resulted in rapid loss of all hormal activity. 2. Chick kidney membranes showed no ability to inactivate parathyroid hormone even with prolonged incubation. 3. Biologically active, labelled parathyroid hormone was degraded to fragments by rat kidney membranes, but not by chick kidney. 4. Hormone-responsive adenylate cyclase activity in a mixture of rat and chick kidney membranes was additive. 5. Parathyroid hormone bound specifically to chick kidney palsma membranes. 6. It is concluded that hormone in activation during incubation has little relevance to the effectiveness of parathyroid hormone in stimulating adenylate cyclase activity in kidney, and furthermore that failure of chick kidney to metabolize the hormone is not the explanation for the greater sensitivity of this species to the hormone.

The metabolism of labeled parathyroid hormone. V. Collected biological studies

Biologically active 125I-labeled parathyroid hormone (125I-PTH) was used in a series of studies in dogs and chickens designed to confirm and augment earlier studies in rats. As in rats, a three exponential equation was required to describe disappearance of 125I-PTH from the blood in the dog. The first two "half-lives" (1.8 and 7 min) accounted for the bulk of the dose. Also as in rats, deposition of apparently intact hormone took place rapidly in kidney, liver and bone in both the dog and the chicken. Degradation occurred very rapidly in all three target organs. Three labeled hormones of different biological activities were compared in the rat. Inactive, oxidized hormone was rejected by the liver but showed markedly increased deposition in kidney and the higher the purity of the hormone the higher was its uptake by liver. Exploration of a wide range of dosages revealed few effects on distribution (smaller deposition in liver and kidney at highest dosages, 65 mug/rat). Fresh sera did not degrade hormone rapidly or extensively. There was no deposition of hormone in intestinal mucosa, marrow, and red cells. Nephrectomy increased deposition in liver and bone. Finally, the perfused liver was capable of extensive degradation of the hormone.

Selective proteolysis of the receptor for parathyroid hormone in skeletal tissue

Parathyroid hormone, calcitonin, and prostaglandin E2 activate the adenylate cyclase-cyclic AMP system in fetal-rat calvaria. These agents presumably interact with the tissue at separate receptor sites. When calvaria were preincubated with trypsin, 500 mug/ml for 45 min, the subsequent increase in 3',5'-AMP in response to parathyroid hormone was markedly diminished, whereas the response to calcitonin and prostaglandin E2 were not altered significantly. The effect was attributable to an action of the enzyme on the tissue and not to hydrolysis of the hormone. Similarily, preincubation of calvaria with trypsin prior to homogenization and preparation of a crude plasma membrane fraction decreased PTH-sensitive adenylate-cyclase activity by 58% but did not alter the degree of stimulation of the enzyme in response to calcitonin, prostaglandin E2, or sodium fluoride. These studies support the hypothesis that the actions of parathyroid hormone and calcitonin on bone are mediated through distinct receptor sites, and the receptors for parathyroid hormone can be altered selectively with trypsin.

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...