Characterization of insulin degradation products generated in liver endosomes: in vivo and in vitro studies

The degradation products generated from A14 and B26 125I-labelled insulins in liver endosomes in vivo and in vitro have been isolated by high-performance liquid chromatography and cleavages in the B chain have been identified by automated radiosequence analysis. In rats sacrificed various times after injection of each of the 125I-labelled insulins, two major degradation products slightly less hydrophobic than intact iodoinsulins were identified; these accounted, at 8 min. for about 45% (A14 125I-labelled insulin) and 15% (B26 125I-labelled insulin) of the total radioactivity recovered, respectively. The products generated from A14 125I-labelled insulin contained an intact A chain, whereas those generated from B26 125I-labelled insulin contained a B chain cleaved at the B16-B17 bond. With B26 125I-labelled insulin, two minor products, with cleavages at the B23-B24 and B24-B25 bonds, were also observed. In vivo chloroquine treatment did not alter the nature but caused a decrease in the amount of insulin degradation products associated with endosomes. When endosomal fractions isolated from iodoinsulin injected rats were incubated at 30 degrees C in isotonic KCl, a rapid degradation of iodoinsulin, maximal at pH 6, was observed. With A14 125I-labelled insulin, the two major degradation products identified in vivo were generated along with monoiodotyrosine, but with B26 125I-labelled insulin monoiodotyrosine was the main product formed. Addition of ATP, presumably by decreasing the endosomal pH, shifted the medium pH for maximal iodoinsulin degradation to about 7-8. These studies have allowed a direct identification of two previously suggested cleavage sites in the B chain. They have also shown that the degradation products generated in cell-free endosomes under conditions that promote endosomal acidification are similar to those identified in vivo.     

Initial site of insulin cleavage by insulin protease.

Exposure of insulin to insulin protease (insulinase, EC 3.4.22.11), a degradative enzyme with considerable specificity toward insulin, results in alterations in the properties of the insulin molecule. Limited degradation by the enzyme results in a decrease in the ability of insulin to bind to membrane receptors with less change in the immunoprecipitability or trichloracetic acid precipitability of the hormone. Limited degradation by insulin protease also alters insulin so that the molecule becomes susceptible to attack by nonspecific endopeptidases which have no effect on unaltered insulin. These data demonstrate the production of an intermediate in the proteolytic degradation of insulin. By labeling with [14C]dansyl chloride, an insulin intermediate with three amino-terminal residues, glycine, phenylalanine, and leucine, was identified. Analysis of this intermediate demonstrated that it was composed of an intact A chain and a B chain cleaved between residues B16 and B17, with the three peptide chains held together by disulfide bonds. Based on these findings, we hypothesize that a stepwise degradation of insulin occurs in vivo and that an early step in the process is the cleavage between B16 and B17 that renders the molecule sucseptible to further degradation by nonspecific proteases.     

High performance liquid chromatographic analysis of insulin degradation by rat skeletal muscle insulin protease

The degradation of [125I]iodoinsulin (A14) by insulin protease (EC 3.4.22.11) was studied using HPLC. A reverse phase HPLC method is presented which allows the separation and quantitation of insulin degradation products. After incubation of [125I]iodoinsulin (A14) with insulin protease, there was an initial rapid loss of radioactivity from the [125I] iodoinsulin (A14) peak, which was quantitatively accounted for by the appearance of radioactivity in 11 different peaks, but was not accompanied by a proportional increase in the solubility of the sample in trichloroacetic acid. Two of the peaks showed appreciable accumulation before the others, and all but the first-eluted peak plateaued by 20 min. After 20 min of incubation, the amount of radioactivity present as the first-eluted peak, solubility in trichloroacetic acid, and insulin loss continued to increase at a steady, but slowed, rate. The order of appearance suggests that insulin protease acts on insulin in an ordered sequence of steps to generate a number of intermediates that are precipitable by trichloroacetic acid, but are subsequently degraded to material that is soluble in trichloroacetic acid. Sulfitolysis of 5 major peaks and subsequent HPLC analysis of the fragments showed none of the peaks to possess intact A chains. Peptide sequencing of 2 of the peaks indicates that the A-chain is cleaved in at least 2 positions, one beyond the 14th position, and one between the 13th and 14th amino acids (leucine and tyrosine).     

Naturally processed heterodimeric disulfide-linked insulin peptides bind to major histocompatibility class II molecules on thymic epithelial cells.

We determined whether disulfide-linked insulin peptides that are immunogenic in vitro for CD4+ T cells bind to major histocompatibility complex class II in vivo. Radiolabeled recombinant human insulin (rHI) was injected into BALB/c mice, and processed rHI peptides bound to I-Ad molecules on different thymic antigen-presenting cells were characterized. The A6-A11/B7-B19 and A19-A21/B14-B21 disulfide-linked I-Ad-bound rHI peptides were isolated from thymic epithelial cells but not dendritic cells. While both thymic epithelial cells and dendritic cells present rHI to HI/I-Ad-specific T cells, these antigen-presenting cells do not present the reduced or nonreduced forms of the disulfide-linked rHI peptides. Thus, a naturally processed disulfide-linked peptide can bind to major histocompatibility complex class II in vivo. The potential role of these peptides in immunological tolerance is discussed.     

The trafficking and processing of insulin and insulin receptors in cultured rat hepatocytes

The processing and trafficking of insulin in cultured rat hepatocytes were studied. A time course of binding of radiolabeled insulin to hepatocytes at 37 C revealed a rapid rise in cell-associated radioactivity that reached a steady state by 20 min. Using an acid medium to extract insulin bound to surface receptors, the time courses of receptor binding and internalization of the ligand were characterized. The earliest event in insulin processing was the binding of insulin to surface receptors, reaching steady state by 20 min with a t1/2 of 4 min. The internalization rate of ligand was initially slower than the binding rate, with a t1/2 of 6 min. Similar internalization rates of the insulin receptor were found by measuring the trypsin sensitivity of hepatocyte insulin receptors covalently occupied with a photo-affinity-labeled derivative of insulin [( 125I]B2 (2-nitro-4-azido-phenylacetyl)Des-PheB1-insulin). At steady state, the internalized ligand and receptor comprised approximately 40-45% of the cell-associated radioactivity. The time course of intracellular degradation was assessed by trichloroacetic acid (TCA) precipitability and Sephadex G-50 gel chromatography of solubilized cells containing only internalized radioactivity. Intracellular TCA-soluble and low mol wt degradation products first appeared by 5 min and were released from the cell 3 min later. Chloroquine (100 microM) completely inhibited the formation of intracellular low mol wt degradation products as well as their appearance in the medium. The release of intracellular radioactivity was assessed by first removing surface-bound insulin with acid extraction. Eighty percent of the intracellular radioactivity was released in 45 min with a t1/2 of 8 min. The released radioactivity was assessed by TCA precipitability and gel chromatography. These results demonstrate that after 20 min, 43% of the released intracellular radioactivity is intact insulin. The percentage of intact insulin released increases in a dose-dependent fashion as the amount of insulin bound and internalized increases. In conclusion, the earliest event in insulin processing is binding to surface receptors. After a short delay, insulin and its receptor are internalized and trafficked into either a chloroquine-sensitive degradative pathway or a chloroquine-insensitive retroendocytotic pathway. The amount of insulin that traverses the nondegradative retroendocytotic pathway is proportional to the amount of insulin bound and internalized by the cell.     

Biological activity of des-(B26-B30)-insulinamide and related analogues in rat hepatocyte cultures

Summary Short-term and long-term biological activities were studied in adult rat hepatocytes cultured in the presence of the insulin analogues des-(B26-B30)-insulinamide, [Tyr^B25]des-(B26-B30)-insulinamide and [His^B25]des-(B26-B30)-insulinamide. When compared to insulin, full potency of des-(B26-B30)-insulinamide has been reported in rat adipocytes and an enhanced potency has been reported for the other analogues. Steady state binding characteristics of the analogues to hepatocytes were indistinguishable from those of native insulin with half-maximal binding occurring at concentrations of about 0.8 nmol/l. Half-maximal effects for the stimulation of glycolysis and inhibition of basal and glucagon-activated glycogenolysis required identical concentrations for insulin and all 3 analogues. Induction of the key glycolytic enzymes glucokinase and pyruvate kinase as well as the inhibition of glucagon-dependent induction of phosphenolpyruvate carboxykinase also required identical concentrations of insulin and the 3 analogues. These data confirm that in cultured hepatocytes the C-terminal amidation of des-(B26-B30)-insulin results in a molecule with full in vitro potency. In contrast to data obtained in adipocytes, the des-(B26-B30)-insulin-amidated analogues with tyrosine or histidine substitutions at position B25 are equally as potent as native insulin in eliciting biological responses in rat hepatocyte culture.     

Parathyroid hormone degradation by chymotrypsin-like endopeptidase in the opossum kidney cell

Cathepsin-D has been previously reported to cleave intact PTH into PTH-(1-34) and -(35-84) in membranous fractions of rat and bovine kidney. Whether PTH degradation occurs by intact kidney cells, however, has not been examined in detail. We have, therefore, examined this possibility using an opossum kidney (OK) cell line which possesses the characteristics of proximal renal tubules and responds to PTH. PTH radioimmunoreactivity recovered in trichloroacetic acid-soluble products and in fractions eluted from reverse phase HPLC was measured using an antibody directed to the midregion and C-terminus of PTH. In this study, intact OK cells, but not extracellular enzymes, cleaved human (h) PTH-(1-84) into three discrete fragments which were released into the medium in a time- and temperature-dependent fashion. Half-maximal velocity of PTH-degrading activity (PTHDA) was observed at 9 nM hPTH-(1-84). A 1000-fold molar excess of PTH antagonists [hPTH-(3-34) and [Tyr34]hPTH-(7-34)amide] markedly inhibited PTHDA, whereas ACTH, glucagon, or big gastrin did not suppress it, suggesting an involvement of the PTH receptor in PTHDA. This PTHDA was strongly inhibited by phenylmethylsulfonylfluoride and chymostatin, but not by trypsin inhibitor, elastatinal, or inhibitors of aspartic, cysteine, or metalloproteinases, suggesting that it is due to a seryl chymotrypsin-like endopeptidase. Analysis of chymotrypsin-digested products of hPTH-(1-84) eluted from HPLC exhibited five fragments detected by UV absorbance (210 nm), three of which were measurable by PTH RIA, and each corresponded to the three PTH fragments produced by OK cells. All three fragments were predominantly suppressed in the presence of chymostatin, suggesting that chymotrypsin-like activity is solely responsible for PTHDA in intact OK cells. To further explore the cleavage sites of PTH by chymotrypsin, amino acid analysis of chymotrypsin-cleaved products was performed. The results strongly support the conclusion that a chymotrypsin-like enzyme in OK cells cleaved the hormone between residues 23-24, and 34-35 to produce, at least, hPTH-(24-84) and -(35-84). Lysosomal blockers (chloroquine, ammonium chloride, or monensin) did not affect this PTHDA. Our present study indicates that chymotrypsin-like endopeptidase, but not other endopeptidase or lysosomal enzymes, is responsible for the limited hydrolysis of PTH by intact OK cells.     

The degradation of monoiodotyrosyl insulin isomers by insulin protease

The four single-site monoiodotyrosyl insulin isomers were synthesized by lactoperoxidase-catalyzed iodination of porcine insulin and were separated from one another by high performance liquid chromatography. The susceptibility of the four isomers (A14-, A19-, B16-, and B26-monoiodotyrosyl insulin) to degradation by purified insulin protease was examined using several different assay methods, including trichloroacetic acid precipitation, immunoprecipitation, and Sephadex G-50 chromatograpy. Using trichloroacetic acid precipitation, isomer susceptibility, determined from the initial rate of hydrolysis, was highest with the A14 isomer, lowest with the A19 isomer, and intermediate and roughly equal with the two B-chain-labeled isomers. Based upon the initial rate of isomer hydrolysis, the Michaelis Menten constant (Km) of insulin protease was higher for the B16 isomer (55 nM) than for the other three isomers, whose Km values were not different from one another (A14 = 24 nM; A19 = 35 nM; B26 = 29 nM). In addition, the values for maximum velocity (Vmax) were higher for the A14 and B26 isomers than for the A19 and B16 isomers. However, during incubation, the order of isomer susceptibility to insulin protease changed to B26 greater than A14 greater than A19 greater than B16. This change in apparent isomer susceptibility was prevented by including in the incubation mixture a rat renal peptidase, which did not degrade the intact isomers, suggesting that insulin protease converted the isomers to trichloroacetic acid-soluble products via trichloroacetic acid-precipitable intermediates. Using the immunoprecipitation assay, the susceptibility of isomers to hydrolysis did not change during incubation, but remained highest with the A14 isomer, lowest with the A19 isomer, and intermediate with the two B-chain-labeled isomers, of which the B16 isomer was degraded more rapidly. Each isomer was converted more rapidly to nonimmunoprecipitable products than to trichloroacetic acid-soluble products, implying that insulin protease converted the isomers to trichloroacetic acid-precipitable, nonimmunoprecipitable intermediates, which it then converted to trichloroacetic acid-soluble form. Using Sephadex G-50 chromatography (SGC) assay, the susceptibility of isomers to hydrolysis did not change during incubation, but remained highest with the A14 isomer, lowest with the A19 isomer, and intermediate with the two B-chain-labeled isomers, of which the B16 isomer was hydrolyzed more rapidly. With the exception of the A19 isomer, isomer hydrolysis appeared faster with SGC assay than with either of the other two assays.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of modified insulin B21-B26 fragments on glycogenesis and on insulin-receptor complex fate in cultured fetal hepatocytes

Biological activity and interference with insulin receptor complex fate of two modified sequences of insulin B21-B26, beta-Ala-Arg-Gly-Phe-Phe-Tyr-NH2 (DP-432) and beta-Ala-Arg-Pro-Phe-Phe-Tyr-NH2 (DP-640), were studied in cultured 18-day-old fetal rat hepatocytes known to respond to insulin by an acute stimulation of glycogenesis. The two derivatives stimulated [14C]glucose incorporation into glycogen in the absence of insulin independently of the deprivation of serum in the medium. The maximal effect of 3 mM DP-640 after 2 h, more pronounced than with 3 mM DP-432, was of the same order as that obtained with 10 nM insulin alone (stimulation index: 4.7 +/- 0.7, 2.5 +/- 0.2 and 3.6 +/- 0.9, n = 4, with DP-640, DP-432 and insulin, respectively) whereas insulin B-chain decreased glycogen labeling. Simultaneous addition of derivatives and insulin at maximal concentrations produced nearly additive effects. DP-640, as well as DP-432, increased the amount of [125I](A14) or (B26) human insulin associated with cells at 37 degrees C and inhibited intracellular insulin degradation with differences depending on the kind of insulin isomer and derivative, while the rapid insulin receptor cycle was not affected. Thus, the two derivatives specifically modified the cellular processing of insulin in cultured fetal hepatocytes, and exerted an insulin-like effect on glycogenesis clearly enhanced through modification of DP-432 by substitution of glycine for proline (DP-640).     

Identification of insulin domains important for binding to and degradation by endosomal acidic insulinase

The endosomal compartment of hepatic parenchymal cells contains an acidic endopeptidase, endosomal acidic insulinase (EAI), which hydrolyzes internalized insulin at a limited number of sites. Although the positions of these cleavages are partially known, the residues of insulin important in its binding to and proteolysis by EAI have not been defined. To this end, we have studied the degradation over time of native human insulin and three insulin-analog peptides using a soluble endosomal extract from rat liver parenchyma followed by purification of the products by HPLC and determination of their structure by mass spectrometry. We found variable rates of ligand processing, i.e. high ([Asp(B10)]- and [Glu(A13),Glu(B10)]-insulin), moderate (insulin) and low (the H2-analog). On the basis of IC(50) values, competition studies revealed that human and mutant insulins display nearly equivalent affinity for the EAI. Proteolysis of human and mutant insulins by EAI resulted in eight cleavages in the B-chain which occurred in the central region (Glu(B13)-Leu(B17)) and at the C-terminus (Arg(B22)-Thr(B27)), the latter region comprising the initial cleavages at Phe(B24)-Phe(B25) (major pathway) and Phe(B25)-Tyr(B26) (minor pathway) bonds. Except for the [Glu(A13),Glu(B10)]-insulin mutant, only one cleavage on the A-chain was observed at residues Gln(A15)-Leu(A16). Analysis of the nine cleavage sites showed a preference for hydrophobic and aromatic amino acid residues on both the carboxyl and amino sides of a cleaved peptide bond. Using the B-chain alone as a substrate resulted in a 30-fold increase in affinity for EAI and a 6-fold increase in the rate of hydrolysis compared with native insulin. A similar role for the C-terminal region of the B-chain of insulin in the high-affinity recognition of EAI was supported by the use of the corresponding B(22)-B(30) peptide, which displayed an increase in EAI affinity similar to the entire B-chain vs. wild-type insulin. Thus, we have identified a highly specific molecular interaction of insulin with EAI at the aromatic locus Phe(B24)-Phe(B25)-Tyr(B26). Analytical subfractionation of a postmitochondrial supernatant fraction showed that a pulse of internalized [(125)I]Tyr(A14)-H2-analog, a protease-resistant insulin analog, undergoes a greater lysosomal transfer and lesser degradation than [(125)I]Tyr(A14)-insulin, confirming that endosomal sorting is regulated directly or indirectly by endosomal proteolysis.     

Insulin degradation: mechanisms, products, and significance

Although much remains to be learned, our understanding of the mechanisms and processes by which insulin is degraded has advanced considerably over the past few years. The roles of receptor binding and internalization in mediating insulin degradation have been clarified, and the endosomal pathway for intracellular insulin degradation has been established and partially characterized. The importance of IP (IDE) in cellular insulin degradation has been established and the importance of lysosomal degradation questioned. Studies on IP have identified the degradation products resulting from insulin metabolism by this enzyme and shown that the degradation products by IP are identical with those produced by isolated hepatocytes. A major remaining question for future investigation is the potential role of insulin degradation and intracellular processing in insulin action.     

Ontogenesis of insulin processing in fetal rat hepatocytes

Summary We studied insulin processing and hepatic glycogenesis in cultured hepatocytes isolated from rat fetuses of 17, 19, and 21 days of gestation. Steady-state insulin binding increased by 250% between days 17 and 19, from 145±8 to 361±52 fmol/mg protein, and by an additional 40% (405±69 fmol/mg protein) by 21 days of gestation. At 37°C, ¹²⁵I-insulin was rapidly (t_1/2<5 min) internalized by hepatocytes at all three ages, reaching maximal levels (63–76% of the total cell-associated radioactivity) by 15 min. ¹²⁵I-labelled degradation products appeared rapidly (t_1/2<15 min) within the cells. Yet, the majority (68–77%) of the intracellular radioactivity consisted of intact ¹²⁵I-insulin, even after 4 h at 37°C. Hepatocytes pre-loaded with ¹²⁵I-insulin and then acid-stripped of surface-bound radioactivity, rapidly released both intact ¹²⁵I-insulin (retroendocytosis) and its radiolabelled degradation products. While intact insulin was initially released more rapidly (t_1/2<6 min), and reached a plateau after 15–30 min, the degradation products continued to accumulate in the medium for at least 4 h. Methylamine inhibited intracellular ¹²⁵I-insulin degradation at all three gestational ages and also blocked insulin-stimulated glycogenesis in 19- and 21-day hepatocytes, without altering basal glycogen synthesis. Insulin-stimulated glycogenesis was not induced in 17-day fetal rat hepatocytes in control or methylamine-treated cultures. We conclude that both degradative and retroendocytotic pathways for processing insulin are present in fetal rat hepatocytes by 17 days of gestation. Further, insulin-receptor processing was functionally related to the glycogenic action of insulin in responsive 19- and 21-day fetal rat hepatocytes     

Identification of intracellular degradation intermediates of aldolase B by antiserum to the denatured enzyme.

A method has been developed that enables us to identify intracellular degradation intermediates of fructose-bisphosphate aldolase B (D-fructose-1,6-bisphosphate D-glyceraldehyde-3-phosphate-lyase, EC 4.1.2.13). This method is based on the use of antibody against thoroughly denatured purified aldolase. This antibody has been shown to recognize only denatured molecules, and it did not interact with "native" enzyme. supernatants (24,000 X g for 30 min) of liver and kidney homogenates were incubated with antiserum to denatured enzyme. The antigen-antibody precipitates thus formed were subjected to NaDodSO4/PAGE, followed by electrotransfer to nitrocellulose paper and immunodecoration with antiserum to denatured enzyme and 125I-labeled protein A. Seven peptides with molecular weights ranging from 38,000 (that of the intact subunit) to 18,000, which cross-reacted antigenically with denatured fructose-bisphosphate aldolase, could be identified in liver. The longest three peptides were also present in kidney. The possibility that these peptides were artifacts of homogenization was ruled out as follows: 125I-labeled tagged purified native aldolase was added to the buffer prior to liver homogenization. The homogenates were than subjected to NaDodSO4/PAGE followed by autoradiography, and the labeled enzyme was shown to remain intact. This method is suggested for general use in the search for degradation products of other cellular proteins.     

Characterization of insulin-degrading activity of intact and subcellular components of human fibroblasts

We have studied insulin degrading activity (IDA) in cultured human fibroblasts and assessed the effect of various inhibitors of insulin processing on IDA. To evaluate the role of three enzymes of insulin degradation (neutral protease, microsomal glutathione insulin transhydrogenase, and lysosomal acid protease), we subfractionated homogenized fibroblasts into membrane (and nuclei) cytosol, mitochondria, microsomes, and lysosomes. Greater than 90% of IDA was found to be present in the cytosolar fraction containing neutral protease. IDA in intact fibroblasts was completely inhibited by 1 mM N-ethylmaleimide and partially by 0.5 mM dansylcadaverine (75%), 0.5 mM chloroquine (48%), 1 mg/ml bacitracin (32%) and Trasylol (30%). Lidocaine (5 mM) and glucagon (10(-6)M) exhibited about 15% inhibition with minimal inhibition (7%) by nonsuppressible insulin-like activity. Study of similar inhibitors on subfractionated components indicated inhibition of cytosolar enzyme by N-ethylmaleimide (100%), glucagon (30%), chloroquine (41%), nonsuppressible insulin-like activity (30%), Lidocaine (25%), dansylcadaverine (16%), and bacitracin (11%). Incubation of ammonium sulfate-fractionated cytosolar enzyme at 37 C with A14-125I-insulin resulted in generation of two intermediate peaks as early as 1 min. These peaks could be identified by HPLC but not by molecular sieve chromatography. These intermediates exhibited less immunoprecipitability with antiinsulin antibody and receptor binding with liver membrane preparations than intact insulin. Further incubation of A14-125I-insulin with the cytosolar enzyme(s) resulted in reduction of these peaks as well as insulin and formation of 125Iodotyrosine peak. We conclude that human fibroblast is capable of metabolizing cell-associated A14-125I-insulin in a time- and temperature-dependent manner. This process is inhibited by various inhibitors of insulin processing. The bulk of IDA consists of soluble neutral protease(s) with properties similar to other more purified neutral insulin protease preparations. This fraction, similar to the intact fibroblast degrades insulin to two intermediates with similar molecular weight to that of intact insulin but with more hydrophilicity and less binding affinity to antiinsulin antibody and liver membrane than intact insulin.     

The role of lysosomes in hepatic metabolism of insulin

We have studied the suitability of the insulin-receptor complex as a substrate for hepatic lysosomal and cytoplasmic insulin-degrading enzymes. Broken lysosome preparations degraded receptor-bound insulin more slowly than free insulin; most of the degradation of bound insulin could be accounted for by prior dissociation of the complex and degradation of the freed insulin. At pH 7.6 insulin showed rapid specific and nonspecific binding to intact lysosomes; no degradation products appeared in the medium. The associated insulin could be recovered by disrupting the lysosomes or by dissociation which was rapid and complete, particularly at low pH (5.5); in both cases more than 75% of the recovered insulin was intact. Insulin did not show specific binding to lysosomal membrane, suggesting that the insulin bound to intact lysosomes was intralysosomal. Free insulin but not receptor-bound insulin was rapidly degraded by cytosolic enzymes. It is hypothesized that if receptor-bound insulin were introduced into lysosomes from endocytic vesicles it would be rapidly dissociated at the prevailing intralysosomal pH; most of the insulin would be rapidly released from the lysosomes and would be available for intracellular binding and for degradation by cytosolic insulin protease.     

Lack of metabolic effects of cholecystokinin on hepatocytes

We previously reported that the liver was the major organ that extracts small, biologically active, circulating forms of cholecystokinin. Although our work indicated extensive degradation of cholecystokinin extracted from plasma during its transit across the hepatocyte, it was unclear whether cholecystokinin might also have a physiological effect on this cell before its intracellular degradation. Therefore we tested the hypothesis that cholecystokinin has a direct biological effect on hepatocytes. Using freshly isolated or cultured hepatocytes, we studied whether cholecystokinin-octapeptide alters protein synthesis, affects amino acid transport or influences cytosolic free calcium concentrations. Using liver slices, we also determined the effect of cholecystokinin-octapeptide on cyclic nucleotide levels. Cholecystokinin-octapeptide, up to a concentration of 1 mumol/L, had no effect on the incorporation of radiolabeled amino acids into total hepatocyte protein; in contrast, comparable molar amounts of insulin stimulated protein synthesis by as much as 37% (ED50 = 1.5 x 10(-10) mol/L). Although insulin and glucagon stimulated the transport into hepatocytes of 14C-alpha-aminoisobutyric acid, a nonmetabolizable amino acid analog, cholecystokinin-octapeptide had no affect Cholecystokinin-octapeptide also did not affect either the concentration of calcium in individual hepatocytes, as measured by digitized video microscopy using Fura-2, or the levels of cyclic AMP or cyclic GMP in liver slices. Our results show that cholecystokinin has no effect on protein synthesis, on amino acid transport or on hepatocyte calcium and cyclic nucleotide levels. These and our previous data suggest that the primary outcome of hepatic extraction of cholecystokinin is hormone degradation.     

The effect of insulin concentration on retroendocytosis in isolated rat adipocytes

After internalization by isolated rat adipocytes, insulin can be degraded or released intact from the cell, a process termed retroendocytosis. To determine whether the amount of ligand entering the cell could modulate its ultimate intracellular disposition, adipocytes were incubated with 0.4-25 ng/ml radiolabeled insulin for 20 min at 37 C to reach steady state binding and internalization. After this, surface bound insulin was removed by acid extraction and the cells were reincubated in insulin-free 37 C buffer. The fractional rate of release of internalized cell associated radioactivity was similar at all insulin concentrations. However, insulin enhanced the appearance of trichloroacetic acid (TCA)-precipitable material in a dose-dependent manner reaching a maximum at an insulin concentration of 10 ng/ml. At 0.4 ng/ml insulin, 18 +/- 2% of the released radioactivity was TCA precipitable whereas 36 +/- 3% was precipitable at 25 ng/ml. Sephadex G-50 gel chromatography and reverse phase HPLC analysis of the reincubation medium confirmed TCA-precipitable material was intact insulin. To further investigate the dual pathways of intracellular insulin processing, adipocytes were incubated with 0.4 and 25 ng/ml insulin for 20 min, acid extracted to remove surface receptor insulin, and solubilized. Sephadex G-50 and HPLC analysis revealed that proportionately less insulin intermediates and low molecular weight degradation products are found in cells incubated at the higher insulin concentrations. In conclusion, as adipocytes internalize more insulin, less is converted into insulin intermediates and low molecular weight degradation products and more is diverted to retroendocytosis.     

In vitro and in vivo degradation of human gastrin by endopeptidase 24.11

The degradation of human unsulfated heptadecapeptide gastrin (G-17) by human kidney endopeptidase 24.11 has been studied in vitro, and some of the products of degradation have been identified in plasma after in vivo infusion of G-17. The enzyme cleaved G-17 at four peptide bonds: Trp4Leu5, Ala11Tyr12, Gly13Trp14, and Asp16Phe17. The cleavage at Gly-Trp was rapid and 1-13 G-17 was an important intermediate. All the products of cleavage of synthetic 1-13 G-17 were also found after degradation of intact G-17. When normal human volunteers received infusions of G-17, there appeared in their blood peptides with the properties of 1-11, 1-13, 1-16, and 5-17 G-17 on the basis of immunochemical and high-performance liquid chromatographic properties. These observations provide evidence that endopeptidase 24.11 is involved in gastrin metabolism in humans, and may be responsible for the generation of G-17 fragments in the peripheral circulation.