Respective roles of kallikrein and endopeptidase 24.11 in the metabolic pathway of atrial natriuretic peptide in the rat.

The metabolism of atrial natriuretic peptide (ANP) and Cys-105-Phe-106-cleaved ANP (ANP) was studied during constant infusion of 125I-labelled peptides in rats. Analysis of circulating radioactivity indicated rapid clearance of ANP and ANP', with mean half-lives of 0.42 and 1.04 min respectively. H.p.l.c. fractionation of plasma taken during the infusion of labelled ANP revealed the presence of three radioactive fragments, the major one co-eluting with 125I-ANP'. These fragments correspond to cleavage products previously found to be generated in vitro by the action of endopeptidase 24.11 (E-24.11). On evaluating the effects of peptidase inhibitors, a significant increase in the half-life of ANP was observed with phosphoramidon (t1/2 7.8 min) and aprotinin (t1/2 5.4 min). A maximal inhibition of ANP degradation was obtained when both inhibitors were given simultaneously (t1/2 15 min). In blood samples taken during infusion of 125I-ANP and phosphoramidon, the intact peptide accounted for more than 90% of total circulating radioactivity, and no cleavage product was present in detectable amounts. Phosphoramidon had no effect on the metabolism of infused ANP'. In contrast, when 125I-ANP' was infused together with aprotinin, the rate of degradation of the infused peptide was reduced by more than 80%. It is proposed that two different peptidase activities, E-24.11 and a kallikrein-like proteinase, are responsible for the cleavage of ANP in the circulation. The Cys-Phe-cleaved ANP would in turn be degraded by kallikrein and not by E-24.11.     

Binding of Brain and Atrial Natriuretic Peptides to Cultured Mouse Astrocytes and Effect on Cyclic GMP

125I-Porcine brain natriuretic peptide (125I-pBNP) bound to mouse astrocytes in primary culture in a time-dependent manner (t1/2 = 4.5 min), similar to 125I-human atrial natriuretic peptide (125I-hANP) (t1/2 = 5 min). Binding was saturable and reached equilibrium after 90 min at 22 degrees C for both radioligands. Scatchard analysis suggested a single class of binding sites for pBNP with a binding affinity and capacity (KD = 0.08 nM; Bmax = 78.3 fmol/mg of protein) similar to those of hANP1-28 (KD = 0.1 nM; Bmax = 90.3 fmol/mg of protein). In competition binding studies, pBNP or human/rat atrial natriuretic peptide (ANP) analogues [hANP1-28, rat ANP1-28 (rANP1-28), and rANP5-28] displaced 125I-hANP, 125I-pBNP, and 125I-rANP1-28 completely, all with IC50 values of less than nM (0.14-0.83 nM). All four peptides maximally stimulated cyclic GMP (cGMP) production by 10 min at 22 degrees C at concentrations of 1 microM with EC50 values ranging from 50 to 100 nM. However, maximal cGMP induction by brain natriuretic peptide (BNP) (25.9 +/- 2.1 pmol/mg of protein) was significantly greater than that by hANP1-28 (11.5 +/- 2.2 pmol/mg of protein), rANP1-28 (16.5 +/- 2.0 pmol/mg of protein), and rANP5-28 (15.8 +/- 2.2 pmol/mg of protein). These studies indicate that BNP and ANPs act on the same binding sites and with similar affinities in cultured mouse astrocytes. BNP, however, exerts a greater effect on cGMP production. The difference in both affinity and selectivity between binding and cGMP production may indicate the existence of receptor subtypes that respond differentially to natriuretic peptides despite similar binding characteristics.     

Kinetics of four 11C-labelled enkephalin peptides in the brain, pituitary and plasma of rhesus monkeys

The kinetics of four 11C-labelled enkephalin peptides: Tyr-Gly-Gly-Phe-Met (Met-enkephalin), Tyr-D-Met-Gly-Phe-Pro-NH2 [D-Met2,Pro5)-enkephalinamide), Tyr-D-Ala-Gly-Phe-Met-NH2 (DALA) and Tyr-D-Ala-D-Ala-Phe-Met-NH2 (TAAFM) all labelled at the methyl group of methionine was studied in the Rhesus monkey. After intravenous administration, the regional kinetics in the head, lungs, liver and kidneys were followed by means of positron emission tomography (PET). The total radioactivity in blood and urine was measured and the composition of 11C-labelled peptide fragments in plasma in vivo and in vitro was analysed by liquid chromatography. With PET, an increased radioactivity was observed in the brain and pituitary over the 60-90 min investigation period after i.v. injection of the peptides. The highest radioactivities were noted for Met-enkephalin, followed by DALA and D-Met2, Pro5-enkephalinamide, while very low radioactivities were found for TAAFM. The uptake of Met-enkephalin- and DALA-derived radioactivity was of the same order as has previously been shown for morphine in the brain and considerably higher than that of D-Met2,Pro5-enkephalinamide and TAAFM, respectively. A large fraction of the brain radioactivity derived from Met-enkephalin and DALA probably emanated from [11C]methionine as indicated by plasma and urine analysis. Met-Enkephalin was rapidly eliminated from plasma in vitro with an half-life of less than two minutes, whereas DALA was stable suggesting clearance by other tissues than plasma. In conclusion, both Met-enkephalin and DALA, were rapidly hydrolyzed in vivo to [11C]methionine. [11C]Methionine was probably taken up in the brain, as the radioactivity increased with time in different brain regions as measured with PET.D-Met2,Pro5-Enkephalinamide and TAAFM were virtually stable in vivo and at least part of the radioactivity observed in the brain may have represented the intact peptide.     

Molecular characteristics and evidence for internalization of vasoactive-intestinal-peptide (VIP) receptors in the tumoral rat-pancreatic acinar cell line AR 4-2 J

1. Vasoactive intestinal peptide (VIP) receptors were investigated in the tumoral acinar cell line AR 4-2 J derived from rat pancreas [125I]Iodo-VIP binding to cell membranes showed the following IC50 values for unlabeled peptides: VIP, 0.3 nM; peptide His-IleNH2, 2 nM; helodermin, 30 nM; secretin, 100 nM. After incubation with 20 nM dexamethasone, the binding capacity increased twofold but affinities were unchanged. External [125I]iodo-VIP binding to intact cells reached steady state after 5 min at 37 degrees C, while the sequestration-internalization of the [125I]iodo-VIP-receptor complex (tested by cold acid washing) increased progressively, reaching 75% of total binding after 1 h. This phenomenon was blocked at 4 degrees C. Further data with dexamethasone, tunicamycin, cycloheximide, low temperature, and/or phenylarsine oxide, suggested a half-life of 2 days for VIP receptors and the necessity of N-glycosylation for proper translocation. 2. For chemical [125I]iodo-VIP cross-linking bis[2-(succinimidooxycarbonyloxy)ethyl]sulfone gave the best yield when compared with five other bifunctional reagents. In membranes, the main specifically cross-linked peptide had Mr 66,000 under nonreducing conditions, and migrated with lower velocity (-5%) under reducing conditions. Cross-linking was suppressed by VIP, peptide His-IleNH2 and helodermin (competitively) and also by GTP. In intact cells, the Mr of [125I]iodo-VIP-cross-linked peptides depended on the mode of cell solubilization. After direct solubilization, the major cross-linked radioactivity migrated as a smear of Mr 130,000-180,000 but an Mr-66,000 peptide was also detectable. In contrast, the solubilization of cross-linked cells detached by mild trypsinisation gave mainly the Mr-66,000 labeled peptide. This suggests that most VIP receptors in intact, attached cells were in a high-Mr complex and that mild cell treatment was sufficient to disrupt this complex.     

Binding of enterotoxin from Clostridium perfringens type A to liver cells in vivo and in vitro. The enterotoxin causes membrane leakage

Enterotoxin from Clostridium perfringens was shown to retain its biological activity after labelling with 125I. When injected intravenously into mice and rats, most of the radioactivity in the organs was present in the form of intact toxin. Studies of the tissue distribution of labelled enterotoxin showed the largest amounts in the liver, where the activity reached a maximum 10--15 min after administration. The highest concentration per g tissue was found in liver and kidneys. The radioactivity was excreted in the urine as a mixture of intact labelled toxin and low molecular weight degradation products. In vitro studies with purified parenchymal liver cells showed rapid release of lactate dehydrogenase (LDH) during treatment with enterotoxin, thus indicating severe membrane damage.     

Des-serine-proline brain natriuretic peptide 3-32 in cardiorenal regulation

Brain natriuretic peptide (BNP 1-32) plays an important physiologic role in cardiorenal homeostasis. Recently, it has been reported that BNP 1-32 is rapidly cleaved by the ubiquitous enzyme dipeptidyl peptidase IV to BNP 3-32, which lacks the two NH2-terminal amino acids of BNP 1-32. The bioactivity of BNP 3-32 in cardiorenal regulation is unknown. We hypothesized that BNP 3-32 has reduced vasodilating and natriuretic bioactivity compared with BNP 1-32 in vivo. Synthetic human BNP 3-32 and BNP 1-32 were administered to eight anesthetized normal canines. After baseline measurements, BNP 1-32 at 30 ng x kg(-1) x min(-1) was administered, followed by a washout, a postinfusion clearance, and a clearance with an equimolar dose of BNP 3-32. In four studies, the sequence of BNP 1-32 and BNP 3-32 infusion was reversed. Peptides were compared by analyzing the changes from the respective preinfusion clearance to the respective infusion clearance. *P < 0.05 between peptides. BNP 3-32, unlike BNP 1-32, did not decrease mean arterial pressure (0 +/- 1 vs. -7 +/- 2* mmHg, respectively) and did not increase renal blood flow (+12 +/- 10 vs. +52 +/- 10* ml/min). Effects on heart rate and cardiac output were similar. Urinary sodium excretion increased 128 +/- 18 microeq/min with BNP 3-32 and 338 +/- 40* microeq/min with BNP 1-32. Urine flow increased 1.1 +/- 0.2 ml/min with BNP 3-32 and 2.8 +/- 0.4* ml/min with BNP 1-32. Plasma BNP immunoreactivity was lower with BNP 3-32, suggesting accelerated degradation. In this study, BNP 3-32 showed reduced natriuresis and diuresis and a lack of vasodilating actions compared with BNP 1-32.     

Neurotensin high affinity binding sites and endopeptidase 24.11 are present respectively in the meningothelial and in the fibroblastic components of human meningiomas

The presence of neurotensin receptors and endopeptidase 24.11 (E-24.11) in 16 human meningioma specimens, obtained at surgery, was assessed by measuring the binding of ¹²⁵I-[tyrosyl³]neurotensin(1–13) (¹²⁵I-NT) and the inhibitor ³H-N((2RS)-3-hydroxyaminocarbonyl-2-benzyl-1-oxopropyl)glycine (³H-HACBO-Gly), for the receptor and enzyme, respectively. E-24.11 activity was also measured. Autoradiography, on the 16 meningiomas, showed that specific ¹²⁵I-NT labeling (nonspecific labeling was assessed in the presence of excess NT) was exclusively located in the meningothelial regions. In contrast, specific ³H-HACBO-Gly labeling (nonspecific labeling was assessed in the presence of an excess of the E-24.11 inhibitor thiorphan) was exclusively found in fibroblastic regions. No specific labeling of either ligand was found on collagen or blood vessels. In vitro binding assays were performed on membranes of 10 of the 16 meningiomas. In the 4 meningiomas rich in meningothelial cells, ¹²⁵I-NT specifically bound to one population of sites with B_max ranging from 57 to 405 fmol/mg protein and K_d around 0.3 nM. These sites share common properties with the brain NT receptor, since the carboxy terminal acetyl NT(8–13) fragment bound to the same sites but with a higher affinity. The carboxy terminal analogue of NT, neuromedin N, also bound to the same sites with a 10-fold lower affinity and the sites were bradykinin and levocabastine insensitive. In the 4 meningiomas rich in fibroblastic cells, ³H-HACBO-Gly specifically bound to one population of sites with B_max ranging from 251 to 739 fmol/mg protein and K_d around 2.8 nM. In agreement with the binding data, E-24.11 activity, expressed in fmol ³H-[D-Ala²]leucine enkephalin degraded/min/mg protein, ranged from 102 to 281 and was specifically inhibited by the E-24.11 inhibitor retrothiorphan R, indicating the presence of biologically active E-24.11 in the meningiomas. In the 2 meningiomas poor in tumoral cells and rich in collagen bundles, no specific binding was found with either ligand. The presence, in abundance, of NT receptors and E-24.11 on the meningothelial components and on the fibroblastic components of the meningiomas, respectively, is a new indicator of the duality of the arachnoid cell from which these tumors arise. These markers may be useful for the classification of the histologic phenotypes of the meningiomas, and for clinical diagnosis of small meningiomas using SPECT and for the treatment of surgically inaccessible meningiomas.     

Fate of injected 125I-labeled cholera toxin taken up by rat liver in vivo. Generation of the active A1 peptide in the endosomal compartment

Subcellular fractionation techniques have been used to assess the localization of injected 125I-labeled cholera toxin (125I-CT) taken up by rat liver in vivo, and to determine whether internalization of the toxin is required for the generation of the active A1 peptide. The uptake of injected 125I-CT into the liver is maximal at 5 min (about 10% injected dose/g). At this time the radioactivity is for the most part recovered in the microsomal (P) fraction, but later on it progressively associates with the mitochondrial-lysosomal (ML) and supernatant fractions. The radioactivity is enriched 7-fold in plasma membranes at 5-15 min, and 15-60-fold in Golgi-endosome (GE) fractions at 15-60 min. On analytical sucrose gradients the radioactivity associated with the P fraction is progressively displaced from the region of 5'-nucleotidase (a plasma membrane marker) to that of galactosyltransferase (a Golgi marker). On Percoll gradients, however, it is displaced towards acid phosphatase (a lysosomal marker). Density-shift experiments, using Triton WR 1339, suggest that some radioactivity associated with the P fraction (at 30 min) and all the radioactivity present in the ML fraction (at 2 h) is intrinsic to acid-phosphatase-containing structures, presumably lysosomes. Comparable experiments using 3,3'-diaminobenzidine cytochemistry indicate that the radioactivity present in GE fractions is separable from galactosyltransferase, and thus is presumably associated with endosomes. The fate of injected 125I-labeled cholera toxin B subunit differs from that of the whole toxin by a more rapid uptake (and/or clearance) of the ligand into subcellular fractions, and a greater accumulation of ligand in the ML fraction. Analysis of GE fractions by SDS/polyacrylamide gel electrophoresis shows that, up to 10 min after injection of 125I-CT, about 80% of the radioactivity is recovered as A subunit and 20% as B subunit, similarly to control toxin. Later on there is a time-dependent decrease in the amount of A subunit and, at least with the intermediate GE fraction, a concomitant appearance of A1 peptide (about 15% of the total at 60 min). In contrast the radioactivity associated with plasma membranes remains indistinguishable from unused toxin. It is concluded that, upon interaction with hepatocytes, 125I-CT (both subunits A and B) sequentially associates with plasma membranes, endosomes and lysosomes, and that endosomes may represent the major subcellular site at which the A1 peptide is generated.     

Adipocyte insulin receptor. Generation of a cryptic domain of the alpha-subunit during internalization of hormone-receptor complexes.

The dynamics of the internalization of photoaffinity-labelled insulin-receptor complexes was investigated in isolated rat adipocytes by using tryptic proteolysis to probe both the orientation and cellular location of the labelled complexes. In cells that were labelled at 16 degrees C and not prewarmed, 150 micrograms of trypsin/ml rapidly degraded the labelled 125 kDa insulin-receptor subunit into a major proteolytic fragment of 70 kDa and minor amounts of 90- and 50-kDa fragments. With milder trypsin treatment conditions (100 micrograms of trypsin/ml, 15 s at 37 degrees C), the 90 kDa peptide (different from the 90 kDa beta-subunit of the insulin receptor) appeared as a major intermediate proteolytic product, but this species was rapidly and completely converted into the 70- and 50-kDa fragments with continued exposure to trypsin, such that it did not accumulate to appreciable amounts in cells that were not prewarmed before trypsin exposure. By contrast, trypsin treatment of cells prewarmed to 37 degrees C for various times showed that: first, a proportion of the labelled 125 kDa receptors was internalized (became trypsin-insensitive); secondly, the 90 kDa tryptic peptide was formed in large amounts, with proportionate decreases occurring in the amounts of the 70- and 50-kDa tryptic peptides. The increased accumulation of the 90 kDa tryptic peptide from cells preincubated at 37 degrees C, but not at 16 degrees C, indicated that trypsin cleavage sites within the 90 kDa segment of the insulin-receptor alpha-subunit that were exposed at 16 degrees C were made inaccessible by incubation at 37 degrees C, a finding that is consistent with generation of a cryptic domain of the receptor subunit. The tryptic generation of the 90 kDa peptide at 37 degrees C was rapid, becoming half-maximal in 4.4 +/- 0.6 min and maximal in 15-20 min, preceded the intracellular accumulation of labelled receptors (half-maximal in 12.6 +/- 0.7 min and maximal in 30-40 min), was highly correlated with receptor internalization, and was not observed in cultured IM-9 lymphocytes, a cell line in which photolabelled insulin receptors are primarily lost by shedding into the incubation media. These results show that, in adipocytes incubated at 37 degrees C, rapid masking of a previously (at 16 degrees C) accessible domain of the insulin-receptor alpha-subunit occurs and that this dynamic process happens at an early stage in the internalization of insulin-receptor complexes.     

In vitro degradation of endothelin-1 by endopeptidase 24.11 (enkephalinase).

The breakdown of endothelin-1 by crude membrane preparations of human kidney and choroid plexus was investigated. 125I-labeled endothelin-1 was degraded by both tissues in a phosphoramidon-sensitive way, suggesting a role of endopeptidase 24.11 in the in vitro metabolism of this peptide. Identification of the cleavage sites of purified human renal endopeptidase 24.11 in the sequence of endothelin-1 revealed that bonds involving the amino side of the hydrophobic amino acids (Ser4, Leu6, Val12, Phe14, His16, Leu17, Ile19) were susceptible to cleavage. Endothelin-1 appears thus to be degraded at multiple sites by endopeptidase 24.11 in vitro, producing inactive fragments.     

Biological half-life and organ distribution of [3H]8-arginine vasopressin following administration of vasopressin receptor antagonist OPC-31260

The effects of the antidiuretic (V(2)) non-peptide receptor antagonist OPC-31260 on the plasma vasopressin level and the biological half-life and organ distribution of radiochemically pure, biologically active [(3)H]8-arginine vasopressin [spec. act.: 15.9 mCi/mmol (588 GBq/mmol)] were studied in Wistar rats. The plasma vasopressin level increased significantly throughout the whole experimental period (24 h). There was no change in the fast phase of the curves of total radioactivity disappearance from the plasma after the administration of [(3)H]arginine vasopressin (control: 1.51+/-0.17 min, OPC-31260-treated: 1.42+/-0.12 min, n=10). The fast phase of the disappearance curves of intact [(3)H]arginine vasopressin did not change either following the administration of OPC-31260 in a dose of 30 mg/kg p.o. (control: 1.06+/-0.19 min, OPC-31260-treated: 1.00+/-0.15 min, n=6). The slow phase of the biological half-life, which is characteristic for the examined compound, proved to be significantly longer (total radioactivity control: 9.29+/-0.61 min, OPC-31260-treated: 12.33+/-0.42 min, P<0.05, n=10; [(3)H]arginine vasopressin radioactivity: control: 5.96+/-0.58 min, OPC-31260-treated: 8.90+/-0.37 min, P<0.05, n=6). In the control rats, the radioactivity was accumulated to the greatest extent in the neurohypophysis, adenohypophysis and kidney. Following OPC-31260 administration, significantly more radioactive compounds accumulated in the kidney (control: 0.30+/-0.052 total radioactivity %/100 mg organ weight, OPC-31260-treated: 0.50+/-0.133 total radioactivity %/100 mg organ weight, P<0.05, n=10) and neurohypophysis (control: 0.37+/-0.053 total radioactivity %/100 mg organ weight, OPC-31260-treated: 0.52+/-0.076 total radioactivity %/100 mg organ weight, P<0.05, n=10). Our results permit the conclusion that the antidiuretic antagonist OPC-31260 not only blocks the V(2) receptors, but also increases the biological half-life of vasopressin. The longer biological half-life of vasopressin following OPC-31260 administration may play a role in the elevation of the plasma vasopressin level.     

Turnover and tissue uptake of rabbit ferritin from rabbit plasma

Using a two-site immunoradiometric assay for rabbit liver ferritin normal NZW rabbits were found to have very low plasma ferritin concentrations (less than 4 micrograms/l). Purified preparations of rabbit liver and kidney ferritin were labelled with 125I and injected into rabbits. Clearance from plasma was extremely rapid with an initial half-life of 1-2 min as measured by immunoprecipitation of labelled ferritin. The rate of clearance was unaffected by the labelling procedure and by the method of ferritin purification. Autoradiography and organ uptake studies showed that 125I-rabbit liver ferritin was removed mainly by liver reticuloendothelial cells, although on a weight basis, spleen had the greatest radioactivity. These studies indicate that rabbit ferritin released into the circulation is promptly cleared by the RES.     

Biological half-life, organ distribution and excretion of 125-I-labelled toxic peptide from the blue-green alga Microcystis aeruginosa

M. aeruginosa is a bloom-forming cyanobacterium which is common in fresh-water lakes. It contains a potent hepatotoxin which when purified has been shown to be a heptapeptide of molecular weight 1019. The toxin was iodinated with 125I using the lactoperoxidase method, the labelled toxin administered intravenously to adult female rats and the half-life and organ distribution measured. The blood half-life after redistribution into extracellular pools was 42 min. The liver and kidneys showed accumulation of 21.7 +/- 1.1 and 5.6 +/- 0.2% of the dose respectively after 30 min. Little accumulation was observed in other organs and tissues. Small-intestinal contents and urine contained 9.4 +/- 6.1 and 2.9 +/- 1.2% of the dose respectively after 120 min. It was concluded that the liver is the main target organ for both accumulation and excretion of the toxin.     

Characterization of ligand binding and processing by bombesin receptors in an insulin-secreting cell line.

Bombesin is a tetradecapeptide which stimulates insulin secretion in vivo by isolated islets and by HIT-T15 cells, a clonal line of hamster pancreatic-islet cells. In the present study we have used [125I-Tyr4]bombesin to characterize bombesin receptors in HIT-T15 cells. [125I-Tyr4]Bombesin binding was time- and temperature-dependent: maximum binding occurred after 45 min, 90 min and 10 h at 37, 22 and 4 degrees C respectively. Thereafter, cell-associated radioactivity declined at 37 degrees C and 22 degrees C but not at 4 degrees C. Scatchard analysis of [125I-Tyr4]bombesin binding measured at 4 degrees C showed that HIT-T15 cells contain a single class of binding sites (approximately equal to 85000/cell) with an apparent Kd of 0.9 +/- 0.11 nM. Structurally unrelated neuropeptides did not compete for [125I-Tyr4]bombesin binding. However, the relative potencies of bombesin and four bombesin analogues in inhibiting the binding of [125I-Tyr4]bombesin correlated with their ability to stimulate insulin release. Receptor-mediated processing of [125I-Tyr4]bombesin was examined by using an acid wash (0.2 M-acetic acid/0.5 M-NaCl, pH 2.5) to dissociate surface-bound peptide from the cells. Following [125I-Tyr4]bombesin binding at 4 degrees C, more than 85% of the cell-associated radioactivity could be released by acid. When the temperature was then increased to 37 degrees C, the bound radioactivity was rapidly (t1/2 less than 3 min) converted into an acid-resistant state. These results indicate that receptor-bound [125I-Tyr4]bombesin is internalized in a temperature-dependent manner. In fact, the entire ligand-receptor complex appeared to be internalized, since pretreatment of cells with 100 nM-bombesin for 90 min at 37 degrees C decreased the subsequent binding of [125I-Tyr4]bombesin by 90%. The chemical nature of the cell-associated radioactivity was determined by reverse-phase chromatography of the material extracted from cells after a 30 min binding incubation at 37 degrees C. Although 70% of the saturably bound radioactivity was co-eluted with intact [125I-Tyr4]bombesin 90% of the radioactivity subsequently dissociated from cells chromatographed as free iodide. At least some of the degradation of receptor-bound [125I-Tyr4]bombesin appeared to occur in lysosomes, since chloroquine increased the cellular accumulation of [125I-Tyr4]bombesin at 37 degrees C and slowed the release of radioactivity.(ABSTRACT TRUNCATED AT 400 WORDS)     

Fate of injected interleukin 1 in rats: Sequestration and degradation in the kidney

The tissue distribution and route of clearance of human recombinant interleukin 1 alpha (IL 1 alpha) injected intravenously in rats was studied. The plasma half-life was approximately 2.5 min, and this was increased after nephrectomy, the kidney being the major organ through which the IL 1 alpha was excreted. Two iodinated fragments of IL 1 alpha, of approximately 5 and 9 kDa, were excreted by the kidneys whereas only intact, 17-kDa IL 1 alpha was detected in plasma, suggesting that the protein was being degraded after uptake by the kidney. The results of in vivo experiments in which surface endopeptidase-24.11 was inhibited with phosphoramidon and in vitro experiments in which rat kidney homogenates were incubated with radiolabeled IL 1 alpha suggest that the cytokine was endocytosed and then hydrolysed by lysosomal proteinases.     

Analytical high-performance affinity chromatography: evaluation by studies of neurophysin self-association and neurophysin-peptide hormone interaction using glass matrices

Bovine neurophysin II (BNP II) was covalently immobilized on both nonporous and porous (200-nm pore diameter) glass beads and incorporated in a high-performance liquid chromatograph to evaluate analytical high-performance affinity chromatography as a microscale method for characterizing biomolecular interactions. By extension of the theoretical treatment of analytical affinity chromatography, both the self-association of neurophysin and its binding of the peptide hormone vasopressin were characterized by using a single chromatographic column containing immobilized neurophysin predominantly in the monomer form. Both [3H] [Arg8]vasopressin (AVP) and 125I-BNP II were rapidly eluted (less than 25 min). The relatively symmetrical elution peaks obtained allowed calculation of both equilibrium dissociation constants and kinetic dissociation rate constants. The dissociation constant measured chromatographically for the AVP-immobilized neurophysin complex, KM/L = 11 microM with porous glass beads and 75 microM with nonporous glass (NPG) beads, was in reasonable agreement with those previously obtained by curve fitting of Scatchard plots (16-20 microM) and from binding to [BNP II]Sepharose (50 microM). The values obtained are larger than that for dissociation of AVP from BNP II dimer, by a factor consistent with the intended nature of immobilized BNP II as monomers. Chromatography of BNP II on the [BNP II]NPG gave a dimer dissociation constant of 166 microM, a value in excellent agreement with that derived from equilibrium sedimentation studies (172 microM). In contrast to the agreement of chromatographic equilibrium binding constants with those measured in solution, the dissociation rate, k-3, determined from the variance of the affinity chromatographic elution profile with nonporous beads, was several orders of magnitude smaller than the solution counterpart.(ABSTRACT TRUNCATED AT 250 WORDS)     

Metabolism of very low density lipoproteins in rats with isotretinoin (13-cis retinoic acid)-induced hyperlipidemia

A significant rise in plasma triacylglycerols from the control level of 0.89 mmol/l to 1.88 mmol/l (P less than 0.001) was observed in male Sprague-Dawley rats treated for 11 days with isotretinoin (oral dosing; 10 mg/day). This rise was due to an increased level of plasma very low density lipoproteins (VLDL). When VLDL from untreated rats were labeled with 125I-labeled tyramine-cellobiose and injected intravenously into rats treated for 10 days with isotretinoin (n = 6) and in control rats (n = 6), it was found that the disappearance of radioactivity from the blood was dramatically retarded in the treated animals. The disappearance could be divided into two phases, a rapid (alpha) phase dominated the first 5 min and was followed by a slower (beta) phase. The half-life of the beta-phase increased significantly from 53 +/- 7 min in the controls, to 120 +/- 62 min after isotretinoin. VLDL prepared from isotretinoin-treated animals (n = 6) had about the same half-life in control animals (62 +/- 8 min) as had ordinary VLDL. The elimination of tracer from the blood was mainly due to uptake by the liver. The amount of radioactivity in the liver after 30 min of circulation was significantly reduced from 34 +/- 7% of injected dose in controls to 24 +/- 5% in the isotretinoin group (P = 0.013). The uptake in other organs was less than 3% per organ and was essentially unaffected by the treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma clearance and liver metabolism of native and asialo-human transcortin in the rat.

Plasma kinetics and liver metabolism of iodinated human corticosteroid-binding protein have been studied in ovariectomized female rats. 125I-labeled human corticosteroid-binding globulin prepared by a modified chloramine T reaction was shown to be physically intact and biologically active. Intravenously injected 125I-labeled human corticosteroid-binding globulin was shown to give a complex clearance pattern from the plasma, with half-lives of 7.5 and 51 min. Estrogen injections had no effect on plasma clearance rate. Direct involvement of liver plasma membrane receptors for asialoglycoproteins in 125I-labeled human corticosteroid-binding globulin metabolism was demonstrated in vivo and in vitro using asialofetuin as a competitive inhibitor. 125I labeled human asialo-corticosteroid-binding globulin was cleared from the plasma with a half-life of less than 1 min, while the simultaneous injection of 5 mg asialofetuin maintained the circulating plasma lebels. Asialofetuin also slowed the clearance of intact 125I-labeled human corticosteroid-binding globulin from the plasma (t1/2 = 90 min). Binding of 125I-labeled human asialo-corticosteroid-binding globulin to rat liver plasma membranes in vitro was inhibited in a dose-dependent manner by asialofetuin, but not by intact human corticosteroid-binding globulin or fetuin. 125I-labeled human corticosteroid-binding globulin did not bind significantly to the membranes. It is concluded that human corticosteroid-binding globulin clearance from rat plasma is rapid and that the carbohydrate moiety of human corticosteroid-binding globulin is involved in its clearance and catabolism by the liver.     

Binding and internalization of 125I-Bolton-Hunter-substance-P by pancreatic acinar cells

Binding of 125I-labelled Bolton-Hunter substance P (125I-BH-SP) to dispersed pancreatic acinar cells from guinea pigs was studied. Association of 125I-BH-SP to acinar cells was rapid, specific, and temperature-dependent. Dissociation of bound 125I-BH-SP was slow and 60 min after dilution only 12% of cell-associated radioactivity had dissociated from cells. Various c-terminal fragments of SP as well as structurally related substances inhibited binding. Bound 125I-BH-SP partly resisted acid washes of cells. After lysis of cells, cell-associated radioactivity was chromatographed on a Sephadex G-25 column. Part of radioactivity was eluted as apparently intact 125I-BH-SP. It is suggested that this material has been internalized into acinar cells.     

Metabolism of glucagon-like peptide-2 in pigs: role of dipeptidyl peptidase IV

Little is known about the metabolism of the intestinotropic factor glucagon-like peptide-2 (GLP-2); except that it is a substrate for dipeptidyl peptidase IV (DPP-IV) and that it appears to be eliminated by the kidneys. We, therefore, investigated GLP-2 metabolism in six multicatheterized pigs receiving intravenous GLP-2 infusions (2 pmol/kg/min) before and after administration of valine-pyrrolidide (300 mumol/kg; a well characterized DPP-IV inhibitor). Plasma samples were analyzed by radioimmunoassays allowing determination of intact, biologically active GLP-2 and the DPP-IV metabolite GLP-2 (3-33). During infusion of GLP-2 alone, 30.9+/-1.7% of the infused peptide was degraded to GLP-2 (3-33). After valine-pyrrolidide, there was no significant formation of the metabolite. Significant extraction of intact GLP-2 was observed across the kidneys, the extremities (represented by a leg), and the splanchnic bed, resulting in a metabolic clearance rate (MCR) of 6.80+/-0.47 ml/kg/min and a plasma half-life of 6.8+/-0.8 min. Hepatic extraction was not detected. Valine-pyrrolidide addition did not affect extraction ratios significantly, but decreased (p=0.003) MCR to 4.18+/-0.27 ml/kg/min and increased (p=0.052) plasma half-life to 9.9+/-0.8 min. The metabolite was eliminated with a half-life of 22.1+/-2.6 min and a clearance of 2.07+/-0.11 ml/kg/min. In conclusion, intact GLP-2 is eliminated in the peripheral tissues, the splanchnic bed and the kidneys, but not in the liver, by mechanisms unrelated to DPP-IV. However, DPP-IV is involved in the overall GLP-2 metabolism and seems to be the sole enzyme responsible for N-terminal degradation of GLP-2.