Effect of Aeromonas proteases on the binding of Aeromonas hydrophila strains to connective tissue proteins

125I-labelled connective tissue protein binding to cells of Aeromonas hydrophila, A. caviae, and A. sobria strains isolated from diseased fish, was correlated with the Aeromonas protease degradation of 125I-labelled collagen types I and IV, fibronectin and laminin, immobilized on tissue culture microtitre plates. An inverse relation between 125I-labelled connective tissue protein binding to cells of Aeromonas strains and proteolytic degradation of immobilized connective tissue proteins by Aeromonas proteases was established. Inhibition of the Aeromonas proteolytic activity by protease inhibitors enhances the 125I-labelled connective tissue protein binding to cells of Aeromonas hydrophila strains. Culture conditions were found to influence both expression of proteolytic activity and binding properties.     

Properties of follicle-stimulating-hormone receptor in cell membranes of bovine testis.

A simple method for preparing plasma membranes from bovine testes is described. Bovine testicular receptor has a high affinity and specificity for 125I-labelled human FSH (follicle-stimulating hormone). The specific binding of 125I-labelled human FSH to the plasma membranes is a saturable process with respect to the amounts of receptor protein and FSH added. The association and dissociation of 125I-labelled human FSH are time- and temperature-dependent, and the binding of labelled human FSH to bovine testicular receptor is strong and not readily reversible. Scatchard [Ann. N.Y. Acad. Sci. (1949) 51, 660-672] analysis indicates a dissociation constant, Kd, of 9.8 X10(-11)M, and 5.9 X 10(-14)mol of binding sites/mg of membrane protein. The testicular membrane receptor is heat-labile. Preheating at 40 degrees C for 15 min destroyed 30% of the binding activity. Specific binding is pH-dependent, with an optimum between pH 7.0 and 7.5. Brief exposure to extremes of pH caused irreversible damage to the receptors. The ionic strength of the incubation medium markedly affects the association of 125I-labelled human FSH with its testicular receptor. Various cations at concentrations of 0.1M inhibit almost completely the binding of 125I-labelled human FSH. Nuclectides and steroid hormones at concentrations of 1mM and 5mu/ml respectively have no effect on the binding of FSH to its receptor. Incubation of membranes with and chymotrypsin resulted in an almost complete loss of binding activity, suggesting that protein moieties are essential for the binding of 125I-labelled human FSH. Binding of 125I-labelled human FSH to bovine testicular receptor does not result in destruction or degradation of the hormone.     

Properties of a prolactin receptor from the rabbit mammary gland

Receptors for human, simian, ovine, bovine and murine prolactin, human growth hormone and human placental lactogen have been identified in plasma-membrane-containing subcellular particles isolated from rabbit mammary glands. The association and dissociation of (125)I-labelled prolactin are time- and temperature-dependent processes, both being maximal at 37 degrees C. (125)I-labelled prolactin prepared by the enzymic iodination procedure with lactoperoxidase binds better to receptors than does the preparation obtained by using chloramine-t as the oxidizing agent. The binding of (125)I-labelled prolactin to receptors is strongly influenced by pH and ionic composition but not by many low-molecular-weight compounds tested, e.g. steroids, nucleotides and several drugs. Receptor activity is sensitive to trypsin and phospholipase C digestion, suggesting that protein and phospholipid moieties are essential for the binding of (125)I-labelled prolactin. The binding of (125)I-labelled prolactin to receptors is a saturable and reversible process. Scatchard and Lineweaver-Burk analyses suggest that (125)I-labelled prolactin has a high affinity for its receptor. Binding of (125)I-labelled prolactin to receptors does not result in the destruction of the hormone. Considerable prolactin-binding activity is also observed in subcellular fractions isolated from the adrenal gland, liver, ovary and kidney of the pregnant rabbit, a finding that is consistent with other reported actions of prolactin in these organs.     

Low density lipoprotein receptors and catabolism in primary cultures of rabbit hepatocytes.

Rabbit 125I-labelled low density lipoproteins (LDL) were incubated with primary monolayer cultures of rabbit hepatocytes in studies designed to assess the role of liver in LDL catabolism at the cellular level. After hepatocytes were preincubated for 20 h in lipoprotein-free medium, they exhibited time- and concentration-dependent interaction with 125I-labelled DLD at concentrations to 1 mg LDL protein/ml and times to 24 h. After a 3 h (37 degrees C) incubation with 50 microgram LDL protein/ml, hepatocytes bound 400 ng (LDL protein)/mg (cell protein), internalized 280 ng/mg, and degraded 660 ng/mg. Internalization and degradation may be greater than indicated by these values since pulse studies suggested the presence of a deiodinase which attacks cell associated 125I-labelled LDL. The amounts of LDL bound to hepatocytes after 3 h (37 degrees C) were similar to amounts for fibroblasts, but DLD internalization and degradation were considerably less. Rabbit hyperlipidemic 125I-labelled DLD showed the same amount of binding but 1.39 times more internalization and degradation than normolipidemic 125I-labelled LDL. Binding of both control and hyperlipidemic LDL was 3-fold greater at 24 and 42 h than at O or 3 h but addition of a 50-fold molar excess of high density lipoproteins (HDL) prevented increased LDL binding with time. Induction of specific high affinity receptors for binding LDL was shown to occur by preincubation of hepatocytes for increasing periods in lipoprotein-free medium and then measuring 125I-labelled LDL binding at 4 degrees C in the presence and absence of excess unlabelled LDL. Finally, hepatocytes took up 40 times more LDL than sucrose or dextran over a 24-h period, an indication that the uptake of LDL occurs via some mechanism other than simple bulk fluid endocytosis.     

The labelling of proteins to high specific radioactivities by conjugation to a 125I-containing acylating agent

1. A new method is described for labelling proteins to high specific radioactivities with (125)I. The protein is treated with a (125)I-labelled acylating agent, iodinated 3-(4-hydroxyphenyl)propionic acid N-hydroxysuccinimide ester, which reacts with free amino groups in the protein molecule to attach the (125)I-labelled groups by amide bonds. 2. Three protein hormones have been labelled by this method, human growth hormone, human thyroid-stimulating hormone and human luteinizing hormone. Specific radioactivities of up to 170, 120 and 55muCi/mug respectively have been obtained for these hormones. 3. The immunoreactivity of these labelled hormones has been investigated by using a radioimmunoassay system specific for each hormone. These preparations have also been compared with and found to be equal or superior to labelled hormones prepared by chemical substitution of (125)I into tyrosine residues of the proteins by using the chloramine-t-oxidation procedure. 4. With some antisera the immunoreactivity of the antigen was diminished by the introduction of a single I atom into the tyrosyl groups, whereas antigen containing a single (125)I-labelled 3-(4-hydroxyphenyl)propionamide group showed the same immunoreactivity as the unmodified antigen.     

Low density lipoprotein binding, internalization, and degradation in human adipose cells.

Human adipose tissue derives its cholesterol primarily from circulating lipoproteins. To study fat cell-lipoprotein interactions, low density lipoprotein (LDL) uptake and metabolism were examined using isolated human adipocytes. The 125I-labelled LDL (d = 1.025-1.045) was bound and incorporated by human fat cells in a dose-dependent manner with an apparent Km of 6.9 + 0.9 microgram LDL protein/mL and a Vmax of 15-80 microgram LDL protein/mg lipid per 2 h. In time-course studies, LDL uptake was characterized by rapid initial binding followed by a linear accumulation for at least 4 h. The 125I-labelled LDL degradation products (trichloroacetic acid soluble iodopeptides) accumulated in the incubation medium in a progressive manner with time. Azide and F- inhibited LDL internalization and degradation, suggesting that these processes are energy dependent. Binding and cellular internalization of 125I-labelled LDL lacked lipoprotein class specificity in that excess (25-fold) unlabelled very low density lipoprotein (VLDL) (d less than 1.006) and high density lipoprotein (HDL) (d = 1.075-1.21) inhibited binding and internalization of 125I-labelled LDL. On an equivalent protein basis HDL was the most potent. The 125I-labelled LDL binding to an adipocyte plasma membrane preparation was a saturable process and almost completely abolished by a three- to four-fold greater concentration of HDL. The binding, internalization, and degradation of LDL by human adipocytes resembled that reported by other mesenchymal cells and could account for a significant proportion of in vivo LDL catabolism. It is further suggested that adipose tissue is an important site of LDL and HDL interactions.     

The fate of homologous 125I-labelled immunoglobulin G within rat visceral yolk sacs incubated in vitro.

When 125I-labelled rat IgG (immunoglobulin G) is incubated in vitro with visceral yolk sacs from 17.5-day-pregnant rats, the protein is readily degraded. The major radioactive digestion product that accumulates in the medium is [125I]iodo-L-tyrosine. When rotenone (10 microM) is also present in the incubation medium, the rate of digestion of IgG is inhibited to the same extent as the rate of pinocytosis of 125I-labelled polyvinylpyrrolidone. Proteolysis is likewise inhibited when either NH4Cl (30 mM) or leupeptin (30 micrograms/ml) is present in the medium. The above findings strongly suggest that the observed proteolysis occurs within lysosomes. Normally, yolk sacs that have been exposed in vitro to radiolabelled substrates release radioactivity slowly when the tissue is re-incubated, unless the substrate can be degraded within lysosomes and released in the form of low-molecular-weight hydrolysis products. However, in such experiments 125I-labelled rat IgG shows quite exceptional behaviour in being rapidly released in an apparently intact form (as well as being degraded). If an agent that inhibits pinocytosis (e.g. rotenone or 2,4-dinitrophenol) is present in the incubation medium during exposure of the tissue to 125I-labelled rat IgG, it abolishes release of macromolecular radioactivity on re-incubation of the tissue. Enhanced tissue accumulation of 125I-labelled rat IgG, induced by the presence of leupeptin in the medium during the uptake phase, resulted in no concomitant increase in the amount of 125I-labelled IgG released in macromolecular form on re-incubation of the tissue. These findings indicate that the observed rapid release of 125I-labelled IgG is unlikely to represent release from lysosomes and is more compatible with release from a separate class of vesicle that does not fuse with lysosomes.     

Urokinase binds to a plasminogen activator inhibitor type-2-like molecule in placental microvillous membranes

Placental microvillous membranes exhibited saturable binding of urokinase-type plasminogen activator with plateau achieved by 30 min at 4 degrees C and 10 min at 37 degrees C. The binding was essentially irreversible. The capacity was about 8 pmol urokinase per mg membrane protein. Half-maximal displacement of 125I-labelled urokinase was achieved with about 1.0 nM unlabelled urokinase when using 75 micrograms membrane protein/ml. 125I-labelled urokinase did not bind when treated with diisopropylfluorophosphate to block the catalytic activity. Single-chain urokinase (prourokinase), devoid of catalytic activity, did not bind. Catalytically active tissue-type plasminogen activator did compete with 125I-labelled urokinase for binding although less efficiently than urokinase. Binding activity remained in the 100,000 x g pellet after treatment of the membranes with 3 M KCl, alkaline stripping at pH 12 or extraction by the detergent Triton X-100. The binding was essentially blocked by antibodies against plasminogen activator inhibitor-type-2 (PAI-2). Sodium dodecyl sulfate polyacrylamide gel electrophoresis of solubilized membranes with bound 125I-labelled urokinase showed that the urokinase-PAI-2 complexes largely migrated in fractions corresponding to a very large Mr although no clearly defined peaks were observed. It is suggested that PAI-2 occurs in a form anchored to syncytiotrophoblast microvilli, possibly to the cytoskeleton.     

High-density lipoprotein binding to bovine adrenal cortex membranes

Binding studies were performed with bovine adrenal cortex membranes, human 125I-labelled high-density lipoprotein (HDL) and modified photoactivable derivatives of 125I-labelled HDL, namely 125I-labelled HDL-amidinophenylazide and 125I-labelled HDL-amidopropionyldithiophenylazide. The purity of the apolipoprotein composition of the 125I-labelled HDL and photoactivable 125I-labelled HDL used in the binding studies was determined by Coomassie blue and silver staining, and by measuring 125I-labelled cpm after SDS-polyacrylamide gel electrophoresis. About 45% of the 125I-labelled HDL binding to the membranes occurred in the presence of excess EDTA and only unlabelled HDL competed for the binding site. The 125I-labelled interaction with this binding site on the membranes did not require calcium. In addition, 40% of the 125I-labelled HDL binding was to an EDTA-sensitive site, and unlabelled HDL and low-density lipoprotein (LDL) competed for the binding site. Consequently, adrenal cortex membranes have binding sites which show cross reactivity for both HDL and LDL. Modification of 58% of the apolipoprotein lysine residues of 125I-labelled HDL with methylazidophenylimidate, a reagent which maintains the positive charge at lysine residues, had little affect on binding to EDTA-sensitive and insensitive sites. In contrast, modification of 35% of apolipoprotein lysine residues of 125I-labelled HDL with N-succinimidyl(4-azidophenyldithio)propionate, a reagent which converts charged amino lysines to amide bonds, showed binding properties which were almost totally inhibited by EDTA.     

Expanding the scorpion toxin alpha-KTX 15 family with AmmTX3 from Androctonus mauretanicus.

A novel toxin, AmmTX3 (3823.5 Da), was isolated from the venom of the scorpion Androctonus mauretanicus. It showed 94% sequence homology with Aa1 from Androctonus australis and 91% with BmTX3 from Buthus martensi which, respectively, block A-type K+ current in cerebellum granular cells and striatum cultured neurons. Binding and displacement experiments using rat brain synaptosomes showed that AmmTX3 and Aa1 competed effectively with 125I-labelled sBmTX3 binding. They fully inhibited the 125I-labelled sBmTX3 binding (Ki values of 19.5 pm and 44.2 pm, respectively), demonstrating unambiguously that the three molecules shared the same target in rat brain. The specific binding parameters of 125I-labelled AmmTX3 for its site were determined at equilibrium (Kd = 66 pm, Bmax = 22 fmol per mg of protein). Finally, patch-clamp experiments on striatal neurons in culture demonstrated that AmmTX3 was able to inhibit the A-type K+ current (Ki = 131 nm).     

The intracellular distribution of high density lipoproteins taken up by isolated rat hepatocytes

The subcellular distribution of 125I-labelled HDL taken up by rat hepatocytes in vivo and in vitro has been studied with subcellular fractionation techniques: differential centrifugation and isopycnic centrifugation in sucrose gradients. 125I-labelled HDL bind to plasma membranes both in vivo and in vitro and part of the membrane-bound 125I-labelled HDL can be dissociated by the addition of unlabelled HDL. The hepatocytes also internalize 125I-labelled HDL. The 125I-labelled HDL accumulate, however, at different intracellular sites in the in vivo and in vitro situation. The subcellular distribution pattern of 125I-labelled HDL taken up by the cells in vivo is similar to that of the lysosomal marker enzyme acid phosphatase. Peak activity was found at a density of 1.20 g/ml. In vitro 125I-labelled HDL accumulate in an organelle with a medium density of about 1.13 g/ml. This distribution was similar to that of the plasma membrane marker 5'-nucleotidase. The subcellular distribution of radioactivity taken up in vivo was changed to lower density by incubating the cells with chloroquine, a drug known to render the lysosomes more boyant. Chloroquine had no effect on the distribution of 125I-labelled HDL taken up by hepatocytes in vitro.     

Rates of pinocytic capture of simple proteins by rat yolk sacs incubated in vitro.

The rates of pinocytic uptake of a number of small 125I-labelled simple proteins (insulin, ribonuclease A and lysozyme) by rat yolk sacs incubated in vitro were determined both before and after treating these proteins with reagents that are known to increase the rate of capture of 125I-labelled bovine serum albumin. Uptake of the untreated forms of all three proteins was extremely rapid, indicating that adsorptive pinocytosis is the principal mechanism by which yolk-sac cells capture these simple proteins, but these rates show no simple correlation with molecular charge. In contrast with albumin, the rates of uptake of treated proteins were either unchanged or lower than that of the corresponding untreated protein preparations; polymeric forms of 125I-labelled lysozyme larger than dimers were ingested at rates significantly lower than that of the monomer.     

The preparation and properties of purified 125I-labelled antibodies to insulin

1. A procedure is described for the preparation of an insulin-immunoadsorbent containing 105-185mg. of insulin/g. of matrix, and with an antibody-binding capacity of 660mg./g. of insulin-immunoadsorbent. 2. Purified non-precipitating (125)I-labelled antibodies were prepared by iodination of the antibodies while they were isolated on the insulin-immunoadsorbent in order to protect at least one antigen-binding site. 3. The radioactive antibody was simply and rapidly separated from other radioactive material and damaging reagents, and found to be up to 94% pure as judged by its reaction with insulin-immunoadsorbent and unfixed insulin. 4. The (125)I-labelled antibody preparations had iodine contents of 0.5-64atoms/mol. of protein and specific radioactivities of 0.3-125mc/mg. of protein. 5. It is suggested that this is a simple method of producing a useful reagent.     

Lactate, alanine and glutamine metabolism in isolated canine pup liver cells

125I-Labelled alpha 2-macroglobulin-trypsin complex (125I-labelled alpha 2-macroglobulin X trypsin) was associated to isolated rat adipocytes and hepatocytes with a half-time of about 60 min at 37 degrees C. The association of 0.5 micrograms/ml 125I-labelled alpha 2-macroglobulin X trypsin was inhibited by unlabelled alpha 2-macroglobulin X trypsin with a half-inhibition constant of about 8 micrograms/ml (11 nM). 125I-Labelled alpha 2-macroglobulin became cell-associated to a smaller extent (10-40% of that of alpha 2-macroglobulin X trypsin) and the half-inhibition constant was about 35 micrograms/ml in adipocytes. The cell association of 125I-labelled alpha 2-macroglobulin X trypsin was markedly inhibited by dansylcadaverine, bacitracin, omission of Ca2+ from the medium or pretreatment of the cells with trypsin. After incubation for 180 min more than 60% of the cell-associated 125I-labelled alpha 2-macroglobulin X trypsin was not removed by treatment of the cells with trypsin-EDTA and represented probably internalized material. 125I-Labelled alpha 2-macroglobulin X trypsin was degraded to trichloroacetic acid-soluble fragments by suspensions of both cell types but only to a negligible extent by incubation media preincubated with these cells. The rate of degradation of 0.5 micrograms/ml 125I-labelled alpha 2-macroglobulin was approx. 40% of that of 125I-labelled alpha 2-macroglobulin X trypsin. Degradation of 125I-labelled alpha 2-macroglobulin X trypsin was abolished by a high concentration (0.5 mg/ml) of alpha 2-macroglobulin X trypsin. It is concluded that alpha 2-macroglobulin X trypsin by a specific and saturable mechanism is bound to, internalized and degraded by isolated rat adipocytes and hepatocytes.     

Uptake and degradation of 125I-labelled high density lipoproteins in rat liver cells in vivo and in vitro.

1. The uptake of 125I-labelled high density lipoproteins (HDL) in various organs of the rat was determined after an intravenous injection. The uptake of 125I-labelled polyvinylpyrrolidone in the same organs was determined in order to assess uptake by fluid endocytosis. The uptake/organ was highest for the liver. The adrenals showed the highest uptake/unit weight of the organs studied. The liver, the kidneys and the spleen showed comparable values for uptake/g of tissue. The uptake of 125I-labelled HDL exceeded by far that of 125I-labelled polyvinylpyrrolidone in the liver, the kidneys, the spleen and the adrenals, indicating that the uptake of 125I-labelled HDL was mediated by adsorptive endocytosis. 2. The in vivo uptake of 125I-labelled HDL was determined in purified hepatocytes and non-parenchymal cells prepared by collagenase perfusion of livers from animals after intravenous injections of 125I-labelled HDL. When expressed per cell, the hepatocytes and the non-parenchymal liver cells took up about the same amount of 125I-labelled HDL. 3. The in vitro uptake and degradation of 125I-labelled HDL in isolated rat hepatocytes was studied. The uptake at increasing concentrations of 125I-labelled HDL was saturable indicating uptake mediated through binding sites. 125I-labelled HDL were easily degraded by contaminating proteases from the perfusate. 4. Subcellular fractionation by isopycnic centrifugation indicated that the accumulation of 125I-labelled HDL did not take place in the lysosomes, but rather on the plasma membrane and possibly in the endosomes (phagosomes). 5. 125I-labelled HDL were internalized into the cells and degraded in the lysosomes. Leupetin and chloroquine, inhibitors of the lysosomal function effectively inhibited the formation of 125I-labelled acid-soluble radioactivity by the cells. Chloroquine, but not the protease inhibitor leupeptin, reduced the hydrolysis of the cholesteryl ester moiety of HDL.     

Insulin regulates protein metabolism in mouse blastocysts

Mouse blastocysts, in vitro, endocytosed 100 μg/ml ¹²⁵I-labelled bovine serum albumin (BSA) at a rate equivalent to 192 ± 27 μl/hr/mg embryonic protein over the first 20 min. Insulin stimulated this initial uptake by 30% (P < 0.05). After this time, accumulation of ¹²⁵I-labelled BSA began to plateau as the endocytosed ¹²⁵I-labelled BSA was catabolized and ¹²⁵I was released from the cells. Insulin caused an ≈?72% (P < 0.05) increase in the amount of uncatabolized ¹²⁵I-labelled BSA remaining in insulin-treated blastocysts after 2 hr as compared to control blastocysts. Insulin partially inhibited catabolism of endocytosed ¹²⁵I-labelled BSA during the first 2 hr following transfer to nonradioactive medium. After this time, degradation ceased in both control and insulin-treated blastocysts, leaving a small, uncatabolized protein pool remaining in the embryos; however, as a result of insulin's inhibitory effects on the initial catabolic rate, the uncatabolized protein pool was 30% (P < 0.05) larger in insulin-treated blastocysts after the 4 hr chase. Insulin inhibited endogenous protein degradation in blastocysts by 37% (P < 0.05). Combined with previous studies showing a 90% increase in endogenous protein synthesis in blastocysts following short-term stimulation with insulin (Harvey and Kaye, 1988), these results suggest that insulin acts to increase the endogenous protein-reserves in the embryo. Dose-response studies indicated an EC₅₀ of 0.5 pM for insulin's stimulation of ¹²⁵I-labelled BSA accumulation, consistent with action via its own receptor. Insulin-like growth factor-1 (IGF-1) also stimulated protein accumulation at concentrations similar to those observed with insulin, suggesting that IGF-1 may act via its own receptor rather than the insulin receptor to exert its effects on endocytosis. © 1993 Wiley-Liss, Inc.     

Identification of specific FSH binding sites in the testes of adult monkeys of different species

The interaction of 125I-labelled hFSH with primate testicular tissue from 4 species of adult monkeys (Macaca mulatta, M. nemestrina, M. fascicularis and Papio cynocephalus) was investigated. 125I-labelled hFSH binding to a particulate fraction (P1, 40 000 g) of frozen testes was highly specific and saturable. Displacement curves generated using the P1 fraction of testes from the 4 species and 125I-labelled hFSH and unlabelled FSH were very similar. The binding of FSH to the monkey testicular receptor was not species specific because purified FSH from heterologous species such as horse, sheep, pig and rat were very effective in competing with 125I-labelled hFSH for binding. The equine FSH was about 10 times more active than hFSH in this respect. Similarly, 125I-labelled ovine FSH bound as well as labelled hFSH to the testes fractions of all 4 monkey species. In marked contrast to the high binding of 125I-labelled hFSH, binding of 125I-labelled hCG with rhesus monkey testis homogenates and P1 fractions was very low. The FSH receptor in the adult rhesus monkey testis was present in much larger quantity than the LH receptor and was more readily detectable. Our studies show that frozen primate testis can be utilized for investigating testicular-FSH interactions.     

The labelling of proteins to high specific radioactivities by conjugation to a 125I-containing acylating agent. Application to the radioimmunoassay

1. A new method is described for labelling proteins to high specific radioactivities with ¹²⁵I. The protein is treated with a ¹²⁵I-labelled acylating agent, iodinated 3-(4-hydroxyphenyl)propionic acid N-hydroxysuccinimide ester, which reacts with free amino groups in the protein molecule to attach the ¹²⁵I-labelled groups by amide bonds. 2. Three protein hormones have been labelled by this method, human growth hormone, human thyroid-stimulating hormone and human luteinizing hormone. Specific radioactivities of up to 170, 120 and 55μCi/μg respectively have been obtained for these hormones. 3. The immunoreactivity of these labelled hormones has been investigated by using a radioimmunoassay system specific for each hormone. These preparations have also been compared with and found to be equal or superior to labelled hormones prepared by chemical substitution of ¹²⁵I into tyrosine residues of the proteins by using the chloramine-t-oxidation procedure. 4. With some antisera the immunoreactivity of the antigen was diminished by the introduction of a single I atom into the tyrosyl groups, whereas antigen containing a single ¹²⁵I-labelled 3-(4-hydroxyphenyl)propionamide group showed the same immunoreactivity as the unmodified antigen.     

Studies on the proteins involved in the interaction of high-density lipoprotein with isolated human small intestine epithelial cells.

Treatment of 125I-labelled high-density lipoprotein ([125I]HDL3) with monospecific polyclonal antibodies against apolipoproteins A-I and A-II resulted in a dose-dependent inhibition of the [125I]HDL3 binding to isolated human small intestine epithelial cells by 25% and 50%, respectively. Both antibodies also inhibited intracellular degradation of [125I]HDL3 by 80%. Treatment of enterocytes with polyclonal antibody against apolipoprotein A-I binding protein, a putative HDL receptor, inhibited both binding and degradation of [125I]HDL3 by these cells by 50%. Antibodies to apolipoprotein A-I, A-II and apo A-I-binding protein also inhibited [125I]HDL3 binding to cholesterol-loaded cells.     

Mechanism of stimulation of pinocytosis by trypan blue

Trypan blue at 50 microgram/ml stimulates the pinocytic uptake of 125I-labelled PVP, but not of colloidal 198Au or formaldehyde-denatured 125I-labelled bovine serum albumin, by the 17.5-day rat visceral yolk sac incubated in vitro. Neither Trypan blue nor a combination of the dye with 125I-labelled PVP stimulated the rate of pinocytosis of liquid by the tissue. Trypan blue itself was shown to enter the yolk-sac cells by adsorptive pinocytosis. It is proposed that an interaction between Trypan blue and 125I-labelled PVP enables the latter substrate to enter the cells adsorptively by so-called 'piggy-back' pinocytosis.