Evidence for a dipeptide porter in the lysosome membrane

Small neutral dipeptides such as Gly-Gly are known to cross the lysosome membrane rapidly. The mode of dipeptide translocation was studied, using an osmotic-protection method. Results with dipeptide analogues, such as omega-amino aliphatic acids and taurine, indicated that dipeptides do not cross the rat liver lysosome membrane by unassisted diffusion. Using seven pairs of dipeptide stereoisomers, the penetration of the L-isomer was always found to be much more rapid than that of the D-analogue. It is concluded that the lysosome membrane contains a porter that recognizes and transports L-dipeptides.

Mechanism of cystine reaccumulation by cystinotic fibroblasts in vitro

It is well established that when cystine-depleted cystinotic cells are cultured in cystine-containing medium, they reaccumulate cystine within their lysosomes more rapidly than when cultured in cystine-free medium. This has been a puzzling result, since the lysosome membrane of cystinotic cells is impermeable to cystine. To probe the mechanism of cystine reaccumulation, we have measured reaccumulation in the presence of colchicine, an inhibitor of pinocytosis, or of glutamate, a competitive inhibitor of cystine transport into human fibroblasts. Colchicine had no effect, thus eliminating pinocytosis as a putative mechanism for cystine translocation from the culture medium to the lysosomes. Glutamate, however, strongly inhibited cystine reaccumulation. It is concluded that the true mechanism is as follows. 1. Exogenous cystine crosses the plasma membrane on the cystine-glutamate porter. 2. Cystine is reduced in the cytoplasm by GSH. 3. The cysteine that is generated enters the lysosome, where it becomes cystine by participating in the reduction of cystine residues during intralysosomal proteolysis, or by autoxidation.

Cell physiology of the rat visceral yolk sac: a study of pinocytosis and lysosome function

The rat visceral yolk sac is active in pinocytosis. Macromolecules accumulated by the tissue are, in general, routed to the lysosomes, where they either accumulate (if non-digestible by the lysosomal enzymes) or are degraded to their monomeric components. The yolk sac cells engage in adsorptive pinocytosis, which leads to the preferential uptake of macromolecules bearing certain surface features, such as a hydrophobic or a cationic domain. Substrates that enter the yolk sac by adsorptive pinocytosis can in some cases act as bivalent ligands, carrying in a second substance by "piggy-back" pinocytosis. Pinocytosis and intralysosomal digestion of plasma proteins by the organogenesis-stage rat embryo play an important nutritional role, supplying a high proportion of the embryo's amino acid requirement. Teratogenic effects can be induced by substances that inhibit either pinocytosis or intralysosomal proteolysis at this sensitive stage of gestation.

Sources of amino acids for protein synthesis during early organogenesis in the rat. I. Relative contributions of free amino acids and of proteins

Rat conceptuses on the 10th day of gestation were cultured for 27 h in whole rat serum. An addition of either [3H]leucine or [3H]leucine-labelled rat serum proteins was made once during the culture period, and the acid-soluble and acid-insoluble radioactivities of embryo and visceral yolk sac measured at harvesting. The extent of radiolabel incorporation into embryonic and yolk-sac proteins increased linearly with the duration of exposure of the conceptus to the radiolabelled leucine or radiolabelled serum proteins, indicating roughly constant rates of incorporation, per unit mass of tissue, throughout the culture period. The incorporation rates, expressed as clearances, were 0.73 and 0.78 microliter/mg tissue protein/h for embryo and yolk sac, respectively, when the source was [3H]leucine; and 1.8 and 1.3 microliters/mg tissue protein/h, for embryo and yolk sac, respectively, when the source was [3H]leucine-labelled serum proteins. It is estimated, from the known leucine and protein concentrations in serum, that protein contributed over 99 per cent of the leucine supplied to the conceptus for its protein synthesis. In parallel experiments, measurements were made on cultures conducted in the presence of an antiserum against rat visceral yolk sac (100 micrograms/ml). Antiserum profoundly inhibited incorporation of radioactivity into embryo and yolk-sac proteins, when the source was 3H-labelled protein, a result consistent with the known ability of the antiserum to inhibit pinocytosis in the yolk sac. Antiserum also decreased incorporation from [3H]leucine in the yolk sac, suggesting that a proportion of the free leucine entering the yolk sac does so by pinocytosis. The failure of antiserum to affect incorporation of [3H]leucine into the embryo probably indicates that leucine can enter the embryo without the mediation of yolk-sac pinocytosis. The primacy of protein, as a source of amino acids for the organogenesis-stage embryo, is consistent with the serious effects, in terms of embryonic death and malformation, that result from the interruption of amino acid supply when either pinocytosis or lysosomal proteolysis in the yolk sac is inhibited.

Mixed monolayers of natural and polymeric phospholipids: structural characterization by physical and enzymatic methods

This study has focused on physical characterization and enzymatic hydrolysis of mixed monolayers of a natural phospholipid substrate and a polymerizable phospholipid analogue. Such a mixed system presents the possibility to stabilize model biomembranes, vary the molecular environment within the layer through polymerization and simultaneously examine these influences on monolayer structure. Phospholipase A2 was used here as a sensitive probe of the molecular environment within these mixed, polymerizable monolayers to complement information obtained from isotherm and isobar data. The results clearly show a strong influence of molecular environment on phospholipase A2 activity, even if differences in the physical state of mixed monolayers are not detectable with isotherm and isobar measurements. Physical characterization indicated that both monomeric and polymeric mixed monolayers were phase-mixed. Enzyme hydrolysis, however, showed large differences in the ability of the enzyme to selectively hydrolyze the natural phosphatidylcholine component from the monomeric as opposed to the polymeric mixtures. This demonstrates a high sensitivity of phospholipase A2 to distinguish subtle differences in molecular arrangement within mixed monolayers on a molecular level.

Preparation and developmental toxicity of monoclonal antibodies against rat visceral yolk sac antigens

Thirty clones producing monoclonal antibodies (MCAs) to rat visceral yolk sac (VYS) antigens have been prepared. These MCAs localized by immunofluorescence in the VYS endoderm in vitro and were tested for developmental toxicity by intraperitioneal injection of ascites fluid into pregnant rats on day 9 of gestation. Five of the hybridomas produced MCAs that induced embryonic death, malformation, and growth retardation; the other MCAs had no developmental toxicity. Five MCAs, three teratogenic and two nonteratogenic, were tested for their ability to inhibit pinocytosis in the isolated day 17-VYS. Only the teratogenic MCAs were inhibitory, providing further evidence for the hypothesis that teratogenic antibodies interfere with the nutritional supply to the embryo.

The effects of decreased growth temperature on the cystine content of cystinotic fibroblasts

Cystinotic fibroblasts transferred from 37 degrees C to 28 degrees C accumulated additional cystine over the period from 4 to 7 days of incubation at 28 degrees C, after which the additional cystine was lost; warming (to 37 degrees C) of cells with elevated cystine stores led to rapid cystine loss. These results, taken together with previously published data showing cystine release from cystinotic fibroblasts incubated at above-normal temperature, are interpreted as indicating the presence in the cystinotic fibroblast lysosome membrane of a cystine-porter whose efficacy is increased by an increase in membrane fluidity. This porter may be the residual activity of the cystine porter that is known to be deficient in cystinosis, or it may be a second as yet unrecognized porter. It is further proposed that this porter is responsible for the presumed efflux of cystine from cystinotic lysosomes.

Passive diffusion of non-electrolytes across the lysosome membrane

An osmotic-protection method has been used to study the permeability of rat liver lysosomes to 43 organic non-electrolytes of formula weights ranging from 62 to 1000. A lysosome-rich centrifugal fraction of rat liver homogenate was resuspended in an unbuffered 0.25 M solution of test solute, pH 7.0, and incubated at 25 degrees C for 60 min. The free and total activities of 4-methylumbelliferyl N-acetyl-beta-D-glucosaminidase were measured after incubation for 0, 30 and 60 min. Three patterns of results were seen. In pattern A the percentage free activity remained low throughout the 60 min incubation, indicating little or no solute entry into the lysosomes. In pattern B, the percentage free activity was initially low, but rose substantially during the incubation, indicating solute entry. In pattern C there was not even initial osmotic protection, indicating very rapid solute entry. The rapidity of solute entry into the lysosomes showed no correlation with the formula weight, but a perfect inverse correlation with the hydrogen-bonding capacity of the solutes. The results, which can be used to predict the ability of further compounds to cross the lysosome membrane by unassisted diffusion, are discussed in the context of metabolite and drug release from lysosomes in vivo.

Forensic applications of the determination of benzodiazepines in blood samples by microcolumn cleanup and high-performance liquid chromatography with reductive mode electrochemical detection

Recently described microcolumn cleanup and high-performance liquid chromatography with electrochemical reductive mode detection have facilitated the determination of benzodiazepines in contaminated and degraded blood samples. Examples from an extensive application of the procedure to British forensic science casework are given here.

Monocyte-to-macrophage transition in vitro. A systematic study using human cells isolated by fractionation on Percoll

Improved density-gradient methods, using Percoll or Nycodenz, have recently been introduced for the isolation of human monocytes, but the capacity of cells thus isolated to differentiate into macrophages has not been systematically studied. We have compared Percoll and Nycodenz methods for the isolation of monocytes from human blood. The Nycodenz method yielded a monocyte population of high purity, but the yield was low. The Percoll method gave almost quantitative yield of monocytes, and the contaminating cells, mostly lymphocytes, were readily washed away after allowing the monocytes to adhere to a plastic surface. The Percoll method was then successfully scaled up, providing a simple method to obtain the monocytes from 180 ml blood. These monocytes were maintained in culture and their capacity to mature into macrophages was studied, using the following criteria: increase in cell size and protein content, increase in specific activity of hexosaminidase, differential hexosaminidase release on exposure to opsonized zymosan and unopsonized polystyrene beads, loss of peroxidase activity, and development of fluoride-insensitivity by the cells' cytochemically demonstrable esterase. The cells also displayed morphological changes typical of the monocyte-to-macrophage transition. The procedures reported constitute a simple and reliable method for the production of human macrophages in increased yield.

Glycogen metabolism in the rat visceral yolk sac. I. Glycogen content and gestational age

The glycogen content of the rat visceral yolk sac was determined between 13.5 and 20.5 days of gestation by the best available colorimetric method. The concentration of glycogen in the tissue increased ten-fold between 13.5 and 18.5 days, to reach a value similar to that for mammalian muscle, but then decreased by 50 per cent between 18.5 and 20.5 days. Determination of the iodine-iodide spectra and fractionation of the glycogen particles by a novel sodium citrate centrifugation method indicated broad similarities between the structures of glycogen particles, isolated by a mild phenol-water method, from the yolk sac and the liver of the rat. However, the proportion of 'high'-molecular-weight glycogen in the yolk sac increases between 18.5 and 20.5 days, as a result of the preferential loss of 'low'-molecular-weight glycogen, so that at term the proportion approaches that found in liver glycogen.

Detection and determination of common benzodiazepines and their metabolites in blood samples of forensic science interest. Microcolumn cleanup and high-performance liquid chromatography with reductive electrochemical detection at a pendent mercury drop electrode

Benzodiazepines in the blood samples typical of forensic science work are recovered from 100-250 microliters amounts of blood (diluted with aqueous sodium octyl sulphate to suppress protein binding) onto microcolumns of Porapak-T, and finally eluted into 60-microliters volumes of aqueous acetonitrile. The eluates may be taken directly for analysis by high-performance liquid chromatography (HPLC) with reductive amperometric detection at a pendent mercury drop electrode held at potentials down to -1.2 V vs. Ag/AgCl. For high sensitivity work the electrode is preceded by a coulometric detector fitted with porous carbon electrodes held at 0 V (proprietary reference electrode). The technique detects all of the commonly encountered benzodiazepines and others except clobazam, which contains no azomethine group. The detection limits generally are in the range 1-5 ng/ml (40-200 pg HPLC-injected) in hemolyzed human blood, with recovery values of 84-95%, depending on the actual benzodiazepine, over the range examined (less than or equal to 2.14 micrograms/ml). The respective values for the metabolites of nitrazepam are 8-12 ng/ml and 75-84%. The technique is very much less susceptible to the interferences afflicting other commonly applied techniques, and facilitates considerably the analysis of degraded samples.

Carrier-mediated uptake of hexoses by the rat visceral yolk sac

The rat visceral yolk sac is shown to possess a sodium-independent, phloretin-sensitive, and phlorizin- and ouabain-insensitive transport system for hexoses. The rate of uptake of (3H)2-deoxy-D-glucose was measured in vitro and shown to be greatest on the 12th day, decreasing progressively with increasing gestational age up to the 20th day. Little uptake of 3-O-methyl-D-glucose, alpha-methylglucoside or L-glucose occurred. On uptake by the visceral yolk sac, 2-deoxy-D-glucose was phosphorylated, leading to considerable accumulation of this sugar. Several sugars inhibited 2-deoxy-D-glucose uptake as follows: D-glucose = mannose greater than fructose greater than galactose greater than xylose greater than fucose.

Anticancer agents coupled to N-(2-hydroxypropyl)methacrylamide copolymers. II. Evaluation of daunomycin conjugates in vivo against L1210 leukaemia

DBA2 mice were inoculated i.p. with 10(5)L1210 cells. Animals subsequently treated with daunomycin (single i.p. dose, 0.25-5.0 mg kg-1) all died. The maximum increase in mean survival time observed was approximately 135%. Animals treated with N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers conjugated to daunomycin (DNM) showed a significant increase in mean survival time when the polymer-drug linkage was biodegradable (i.e., Gly-Phe-Leu-Gly). Such treatment also produced a number of long term survivors (greater than 50 days). In contrast, HPMA copolymer conjugated to DNM via a non-degradable linkage (Gly-Gly) produced no increase in survival time relative to untreated control animals. The effect observed with biodegradable HPMA copolymer-DNM conjugates was dependent on the concentration of conjugated drug administered (optimum greater than 5 mg kg-1); the frequency of administration (multiple doses were more effective than single); the timing of administration (single doses given on days 1 and 3 were most effective); and the site of tumour inoculation and route of drug administration. Biodegradable HPMA copolymer-DNM conjugates administered i.p. were active against L1210 inoculated s.c. at higher doses than required to curb a peritoneal tumour. Under certain experimental conditions polymer-DNM conjugates containing fucosylamine or galactosamine proved more active than conjugates without the carbohydrate moeity. The mechanism of drug-conjugate action in vivo is at present unclear. Radioiodination of polymer showed approximately 75% of polymer-drug conjugate to be excreted 24 h after i.p. administration. Synthesis of HPMA conjugates containing [3H]DNM showed that polymer containing Gly-Gly-[3H]DNM was excreted (60% of radioactivity in the urine, 24 h) in macromolecular form. In contrast polymer containing Gly-Phe-Leu-Gly-[3H]DNM was largely excreted in the form of low molecular weight species.

Translocation of sugars into rat liver lysosomes. Evidence against a common carrier for D-glucose and D-ribose

A conflict exists in the literature concerning the mode of translocation of D-glucose and D-ribose across the lysosome membrane. The more rapid net uptake of ribose, when measured by the osmotic-protection technique, has been attributed either to its smaller size and lower hydrogen-bonding capacity, or to a lower affinity for a transport system shared by both sugars. The latency of acid beta-hexosaminidase in isolated rat liver lysosomes was measured after preincubation for periods up to 1 h in various solutions containing glucose and/or ribose, and in some cases sucrose. After confirmation of the superior osmotic protection afforded by glucose (than by ribose), it was shown that a solution 0.125 M in both glucose and ribose provided protection intermediate between that given by 0.25 M-glucose and that given by 0.25 M-ribose. This result is inconsistent with the common-carrier hypothesis.

The effect of lysosomotropic detergents on the permeability properties of the lysosome membrane

Compounds such as N-dodecylimidazole and N-dodecylmorpholine kill cells in culture. Their cytotoxicity has been attributed to accumulation in lysosomes where protonation confers detergent properties resulting in membrane destabilization. This hypothesis has been tested by examining the ability of N-dodecylimidazole and N-dodecylmorpholine to decrease the latency of alpha-glucosidase in isolated rat liver lysosomes. No effect was observed. Nor was N-dodecylimidazole apparently able to increase the permeability of isolated rat liver lysosomes to L-alanine, as no diminution of the disruptive effect of L-alanine methyl ester was seen. N-Dodecylimidazole (10-20 micrograms per ml) caused lactate dehydrogenase release from cystinotic fibroblasts, but marginally toxic concentrations failed to induce cystine release, as might have been expected if lysosome membrane damage had occurred. It is concluded that the cytotoxic effects of lysosomotropic detergents may be mediated by a non-lysosomal mechanism.

Anticancer agents coupled to N-(2-hydroxypropyl)methacrylamide copolymers. I. Evaluation of daunomycin and puromycin conjugates in vitro

During recent years N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers have been developed as targetable drug carriers. These soluble synthetic polymers are internalized by cells by pinocytosis and they can be tailor-made to include peptidyl side-chains degradable intracellularly by specific lysosomal enzymes. Thus they provide the opportunity fo achieve controlled intracellular delivery of anticancer agents. The anthracycline antibiotic daunomycin, and protein synthesis inhibitor puromycin, were bound to HPMA copolymers via several different peptide side-chains, including Gly-Gly, Gly-Phe-Leu-Gly and Gly-Phe-Phe-Leu. Incubation of polymer-drug conjugates with isolated lysosomal enzymes (either a mixture of rat liver lysosomal enzymes or purified thiol-dependent lysosomal proteinases, cathepsins L and B) showed that significant release of drug occurred over 20 h, more than 20% of daunomycin and more than 80% of puromycin being liberated. To test their pharmacological activity conjugates were incubated with either the mouse leukaemia L1210, or the human lymphoblastoid leukaemia CCRF in vitro. The conjugates tested were all less effective than free daunomycin, but they showed differential toxicity against L1210 depending on the aminoacid sequence of their drug-polymer linkage. Inclusion of fucosylamine-terminating side-chains into the HPMA copolymer structure increased the affinity of conjugates for the L1210 cell membrane and resulted in increased toxicity. In contrast HPMA-daunomycin conjugates with or without fucosylamine affected CCRF cells equally, but this cell line was more sensitive than the mouse leukaemia to both free and polymer-bound daunomycin. Incubation of L1210 cells in polymer-bound daunomycin for 72 h, followed by plating cells out in low density in drug-free medium, showed that a concentration of polymer-bound drug (184 micrograms ml-1) could be selected to achieve a cytotoxic effect.

Pinocytosis and degradation of exogenous proteins by cystinotic fibroblasts

Lysosomes of cystinotic human fibroblasts contain over 100-times the normal concentration of cystine. The high cystine concentration (probably in the millimolar range) might be expected to inhibit intralysosomal protein breakdown. A comparison of pinocytosis and degradation of five 125I-labelled proteins (bovine serum albumin, formaldehyde-denatured bovine serum albumin, bovine pancreatic ribonuclease A and porcine lactate dehydrogenase isoenzymes H4 and M4) by human fibroblasts has been made, using one cystinotic and two normal cell-lines. The proteins each entered fibroblasts by adsorptive pinocytosis and were then degraded within the lysosomes by enzymes susceptible to leupeptin, the thiol-proteinase inhibitor. Each protein was captured by the fibroblasts at a characteristic rate, which was not different in cystinotic cells. Normal and cystinotic fibroblasts did not differ in their proteolytic capacity, as measured in extracts of disrupted cells. In intact fibroblasts, four of the five proteins were rapidly and fully digested following pinocytosis, in both cystinotic and normal cells. However, with formaldehyde-denatured albumin, the most resistant to degradation of the proteins tested, or with some other proteins in the presence of leupeptin, when the proteolytic capacity of lysosomes is diminished, intralysosomal degradation of pinocytosed protein was incomplete. Moreover, under these conditions, cystinotic cells demonstrated a lower rate of protein digestion than normal cells. It is concluded that pinocytic capture, rather than intralysosomal proteolysis, is commonly the rate-limiting step in the overall process of uptake and degradation of proteins by fibroblasts, and that intralysosomal cystine inhibits digestion of pinocytosed protein only in circumstances when degradation becomes the rate-limiting step.

Disulphide reduction in lysosomes. The role of cysteine

Published evidence indicates that cystine-containing proteins have their disulphide bonds reduced during proteolysis in lysosomes. However, the intralysosomal accumulation of cystine in the cells of patients with cystinosis has been seen as evidence that protein cystine residues are not reduced. The data are reconcilable and fully in harmony if it is postulated that cysteine from the cytoplasm is the physiological reducing agent.

Pinocytosis and phagocytosis: the effect of size of a particulate substrate on its mode of capture by rat peritoneal macrophages cultured in vitro

Both phagocytosis (of particles) and pinocytosis (of solutes) occur in macrophages. It is not known, however, whether particles, if they are small enough, can enter by pinocytosis, nor whether there is a minimum size of particle capable of triggering phagocytic uptake. These questions have been investigated by studying, in vitro, the uptake by rat peritoneal macrophages of particles ranging in diameter from 30 nm to 1100 nm. Percoll (30 nm diameter) and polystyrene beads (100, 300, 600, 800 or 1100 nm diameter) were 125I-iodinated and their uptake by macrophages was measured in the absence or presence of metabolic and cytoskeletal inhibitors. Since uptake, expressed as an Endocytic Index (microliter/10(6) cells per h), increased steadily with the duration of incubation and was inhibited by low temperature or metabolic inhibitors, it was concluded that true endocytosis, and not a superficial cell-association, was being measured. Rates of clearance increased with increasing particle diameter. The rate of uptake of Percoll was 10-times, and of 100 nm polystyrene beads 100-times, the rate of fluid-phase pinocytosis, as measured by the uptake of 125I-labelled polyvinylpyrrolidone. Polystyrene beads of 1100 nm diameter were captured at 700-times this rate. The differential effects of colchicine and cytochalasin B on the uptake of 125I-labelled polyvinylpyrrolidone and of 1100 nm polystyrene beads were taken as indicators of their effects on pinocytosis and phagocytosis respectively. It is concluded that Percoll, although particulate, is captured by pinocytosis. The pattern of inhibition of uptake of polystyrene particles suggests that there is no radical discontinuity between pinocytic and phagocytic uptake, but that the contribution of phagocytosis steadily increases with increasing particle diameter. The results are discussed.

Evidence that suramin and aurothiomalate are teratogenic in rat by disturbing yolk sac-mediated embryonic protein nutrition

Suramin (250 mg/kg) and sodium aurothiomalate (100 mg/kg) both induced congenital malformations in the offspring following treatment of pregnant rats at either 8.5 or 9.5 days of gestation. Conceptuses from 9.5-day pregnant rats were cultured for 48 h in homologous serum to which either suramin or sodium aurothiomalate was added for the final 6 h. The presence of suramin up to 5 mg/ml had no effect on the protein content of yolk sacs at harvesting, but at 10 mg/ml caused a significant decrease. In contrast sodium aurothiomalate increased the protein content of yolk sacs at harvesting, in a concentration-dependent manner up to 100 micrograms/ml. Neither suramin nor sodium aurothiomalate significantly affected embryo protein content. When 125I-labelled polyvinylpyrrolidone was added to the culture serum for the final 6 h of culture, radioactivity was found in the yolk sac at harvesting, but not in the embryo. When suramin (2-10 mg/ml) was present for the final 6 h of culture, the quantity of radioactivity measured in the yolk sac at harvesting was significantly decreased in a concentration-dependent manner. No radioactivity was detected in the embryos. Sodium aurothiomalate had no effect on the uptake of 125I-labelled polyvinylpyrrolidone. When rat serum whose proteins were labelled with [3H]leucine was used as culture medium, radioactivity was found in the conceptus (both yolk sac and embryo) at harvesting. Suramin (5 mg/ml), present for the final or penultimate 6 h, significantly decreased the uptake of radioactivity into conceptuses and caused a significant increase in the proportion of the captured radiolabel that was associated with the yolk sac. Sodium aurothiomalate (25 or 500 micrograms/ml) had no effect on the total uptake of radio-label but caused a significant increase in the proportion of total radioactivity captured that was associated with the yolk sac. These data indicate that suramin, by interfering with both the uptake and intralysosomal digestion of protein, and sodium aurothiomalate, by inhibiting digestion of captured protein, disturb the normal pathway of yolk sac-mediated protein utilization with a consequent diminution of the supply of amino acids to the conceptus. The effects of suramin are seen only at high concentration, those of sodium aurothiomalate at much lower concentrations. It is likely that the two drugs exert their teratogenic action by their effects on the yolk sac nutritional pathway with resultant amino acid deprivation of the conceptus at a critical stage of development.

Fate of N-(2-hydroxypropyl)methacrylamide copolymers with pendent galactosamine residues after intravenous administration to rats

N-(2-Hydroxypropyl)methacrylamide (HPMA) copolymers bearing galactosamine residues accumulate in the liver after intravenous administration to rats (Duncan, R., Kopecek, J., Rejmanová, P. and Lloyd, J.B. (1983) Biochim. Biophys. Acta 755, 518-521). In this study HPMA copolymers bearing pendent galactosamine residues (1.0-11.6 mol%) were injected intravenously into rats and their rates of blood clearance and liver accumulation were measured. A level of substitution of 4 mol% was found to be sufficient to cause substantial deposition in the liver 30 min after administration. The most highly substituted polymer (11.6 mol%) was directed rapidly to the liver, 80-90% being recovered there less than 10 min after administration. Separation of liver into hepatocytes and non-parenchymal cells indicated that polymer was largely associated with the hepatocytes, and density-gradient subcellular fractionation of liver at various times after administration confirmed that polymer was internalized by liver cells and transported, with time, into the secondary lysosomes. Experiments using isolated rat hepatocytes indicated that HPMA copolymers with high galactosamine content have higher affinity for the hepatocyte plasma membrane. HPMA copolymers containing galactosamine and in addition glycylglycyltyrosinamide side-chains were used to demonstrate release of a drug analogue across the lysosomal membrane. These polymers were radioiodinated and, following intravenous administration to rats, the liver lysosomes were isolated and incubated at 37 degrees C in 0.25 M sucrose. Radioactivity was released from the lysosomes faster than the lysosomal enzyme arylsulphatase, an observation that indicates intralysosomal hydrolysis of the copolymer side-chain with subsequent passage of low molecular weight degradation product across the lysosomal membrane.

Characterization of the adsorptive pinocytic capture of a polyaspartamide modified by the incorporation of tyramine residues

Previously it has been shown (Duncan, R., Starling, D., Rypácek, F., Drobník, J. and Lloyd, J.B. (1982) Biochim. Biophys. Acta 717, 248-254) that incorporation of tyramine residues into poly (alpha, beta-(N-2-hydroxyethyl]-DL-aspartamide (PHEA) greatly increases its rate of pinocytic uptake by rat visceral yolk sacs cultured in vitro. Here we describe the relationship between the tyramine content (1.2-21.9 mol%) of modified PHEA and its rate of uptake by yolk sacs. Above a level of substitution of approximately 10 mol% the rate of uptake rises rapidly, and the concentration-dependence of capture is indicative of uptake by adsorptive pinocytosis. Serum proteins were shown to compete effectively for membrane binding sites, indicating a nonspecific interaction of PHEA-derivatives with the yolk sac membrane. PHEA derivatives of the same tyramine content, but of different mean molecular weights (Mr), were captured at the same rates.

pH-profile of cystine and glutamate transport in normal and cystinotic human fibroblasts

In the human recessive condition cystinosis, cystine transport has been reported to be normal in the plasma membrane but defective in the lysosome membrane. A possible explanation is that the transport systems at the two cellular sites are identical and that the defect in cystinosis affects the porter's ability to operate at the low pH of the lysosome. To test this hypothesis the uptake of 3H-labelled cystine and glutamate by normal and cystinotic human skin fibroblasts has been measured in vitro at pH 5.8, 6.5, 7.0, 7.4 and 8.0. Uptake of glutamate was more rapid than that of cystine. Uptake of cystine increased with increasing pH, but uptake of glutamate showed no marked pH-dependence. Transport in cystinotic cells was similar to that in normal cells, and similarly affected by pH. This finding is incompatible with the hypothesis proposed above. It is concluded that the cystine porters of the plasma membrane and the lysosome membrane are probably genetically distinct.

The role of pinocytosis in the cellular uptake of an amino acid

Uptake of L- and D-alanine by the rat visceral yolk sac has been studied in vitro in incubations of short duration. It is concluded that much of the uptake of D-alanine is due to fluid-phase pinocytosis and that the plasma membrane L-alanine porter is more stereospecific than has hitherto been supposed.

A study of the low beta-galactosidase activity in cystinotic fibroblasts: effects of cysteamine

beta-Galactosidase activity but not beta-glucuronidase, N-acetyl-beta-D-galactosaminidase or arylsulphatase A activity, is known to be significantly lower in cultured human skin fibroblasts from patients with cystinosis than in cells from control subjects. Incubation of cell homogenates with disulphide or thiol compounds did not affect beta-galactosidase activity, suggesting that decreased beta-galactosidase activity in affected cells was not caused by the presence of inhibiting substances or absence of activating substances. Incubating cells with 0.5 or 1.0 mmol/l cysteamine, a substance used in the clinical treatment of cystinosis because it depletes cells of excess cystine, greatly decreased beta-galactosidase activity in both cystinotic and normal cells. This effect is shown to result from enzyme instability in lysosomes with raised pH and increased thiol concentration. Thus, cysteamine, although effective in depleting cystinotic cells of excess cystine, may have the undesired side-effect of severely decreasing lysosomal beta-galactosidase.

Degradation of insulin by human fibroblasts: effects of inhibitors of pinocytosis and lysosomal activity

The role of the pinosome-lysosome pathway in the degradation of 125I-labelled bovine insulin by cultured human fibroblasts was examined by comparing the effects of various known inhibitors of pinocytosis and lysosomal degradation on the uptake and degradation of 125I-labelled polyvinylpyrrolidone, formaldehyde-denatured bovine serum albumin and bovine insulin by these cells. Fibroblasts incubated with polyvinylpyrrolidone steadily accumulate this substrate, whereas incubations with insulin or denatured albumin led to the progressive appearance in the culture medium of [125I]iodotyrosine. Inhibitors of pinocytosis (bacitracin, colchicine and monensin), metabolic inhibitors (2,4-dinitrophenol and NaF), lysosomotropic agents (chloroquine and NH4Cl) and an inhibitor of cysteine-proteinases (leupeptin) decreased the rate of uptake of polyvinylpyrrolidone and denatured albumin very similarly, but only bacitracin had an effect on the processing of insulin. Chloroquine, NH4Cl and leupeptin strongly inhibited the digestion of denatured albumin, but not of insulin. The different responses to the modifiers, with polyvinylpyrrolidone and denatured albumin on the one hand and insulin on the other, suggest that insulin degradation can occur by a non-lysosomal pathway. The very strong inhibitory effect of bacitracin on insulin processing by fibroblasts may point to an important role of plasma membrane proteinases in insulin degradation.

Stability in rat plasma and serum of lysosomally degradable oligopeptide sequences in N-(2-hydroxypropyl) methacrylamide copolymers

Soluble copolymers of N-(2-hydroxypropyl)methacrylamide (HPMA) were prepared containing either oligopeptide side chains terminating in rho-nitroaniline, or oligopeptide sequences forming crosslinks between polymer chains. Such copolymers have potential as targetable drug carriers and already it has been shown that oligopeptide side chains and oligopeptide crosslinks are degraded intracellularly by lysosomal enzymes. The susceptibility of these oligopeptide sequences to degradation on incubation with rat plasma or rat serum was evaluated by monitoring either the liberation of rho-nitroaniline or, with the crosslinked polymers, the change in molecular weight distribution. Release of rho-nitroaniline from some of the polymers was not detectable, and from others proceeded very slowly, the maximum rate being from the side chain Gly-Gly-Phe-Leu-Gly-Phe-NAp where 5.1% of the bound rho-nitroaniline was released by rat serum over a 5 h incubation period. No cleavage of crosslinked HPMA copolymers by plasma or serum was detectable even after a 24 h incubation period.

Phagocytic uptake of latex beads by rat peritoneal macrophages: comparison of morphological and radiochemical assay methods

Contrary to previous reports, commercially available 1000-nm latex beads were found to be labelable with 125I, yielding a product that retained its radiolabel on storage at 4 degrees C and when incubated in tissue-culture media. This finding permitted a radiochemical method to measure phagocytic uptake of latex particles by rat peritoneal macrophages cultured in vitro, and a direct comparison with the established method of particle counting by light microscopy. The two methods yielded closely similar data, demonstrating that the (much more convenient) radiochemical method for quantitating phagocytic uptake is both feasible and reliable. The kinetics of phagocytic uptake of the latex particles and the effect of low temperature and metabolic inhibitors (sodium fluoride and 2,4-dinitrophenol) are described. Ongoing phagocytosis did not alter the rate of fluid-phase pinocytosis by macrophages.

Tyrosinamide residues enhance pinocytic capture of N-(2-hydroxypropyl)methacrylamide copolymers

N-(2-Hydroxypropyl)methacrylamide ( HPMA ) copolymers have been proposed as a potential lysosomotropic drug delivery system. HPMA copolymers bearing tyrosinamide residues, bound either directly to the polymer backbone or via a glycylglycine spacer, were radiolabelled with [125I]iodide and the effect of tyrosinamide content on their rate of pinocytic uptake by rat visceral yolk sacs cultured in vitro was measured. Incorporation of tyrosinamide enhanced uptake of the copolymer, most markedly at substitutions above 10 mol%. 2,4-Dinitrophenol, an inhibitor of pinocytosis, was used to confirm that tissue association of 125I-radiolabelled copolymer was due to pinocytic uptake. The side-chain -Gly-Gly-Tyr-NH2 was degraded following the internalization of copolymers containing this spacer and degradation was partially sensitive to the lysosomal thiol-proteinase inhibitor leupeptin. It is postulated that the effect of tyrosinamide residues is to increase the hydrophobicity of poly( HPMA ) and thus to increase its capacity for nonspecific adsorptive pinocytosis.

Inhibition of proteolysis in rat yolk sac as a cause of teratogenesis. Effects of leupeptin in vitro and in vivo

Conceptuses from 9.5-day pregnant rats were cultured for 48 h in heat-inactivated homologous serum to which leupeptin, a specific inhibitor of the lysosomal cysteine-proteinases, was added for the final or the penultimate 6 h. The presence of leupeptin (25 micrograms/ml or above) increased the protein content of yolk sacs at harvesting to approximately twice the control value. The protein content of the embryo at harvesting was lower than that of controls. When 125I-labelled polyvinylpyrrolidone was added to the culture serum for the final 6 h of culture, radioactivity was found in the yolk sac at harvesting, but not in the embryo. The presence of leupeptin did not affect the rate of uptake of the radiolabelled macromolecule by the yolk sac, nor facilitate its entry into the embryo. When formaldehyde-denatured 125I-labelled bovine serum albumin was added to the culture medium for the final 6 h of culture, little radioactivity was found in the yolk sac at harvesting, and barely any was found in the embryo. Trichloroacetic acid-soluble radioactivity was found in the culture serum. The presence of leupeptin sharply increased the levels of radioactivity in the yolk sac (but not the embryo) and sharply decreased the acid-soluble radioactivity of the culture medium. When rat serum whose proteins were labelled with [3H]leucine was used as culture medium, radioactivity was found in both yolk sac and embryo at harvesting. The presence of leupeptin increased the amount found in the yolk sac and decreased that found in the embryo. These results are interpreted as follows. Leupeptin enters the lysosomes of the yolk sac, inhibiting their cysteine proteinases. The digestion of proteins pinocytosed by the yolk sac is consequently inhibited, resulting in the accumulation of protein by the yolk sac and a decreased flow of amino acids to the embryo. Leupeptin (50 mg/kg), injected into pregnant rats at either 8.5 days or 9.5 days of gestation, induced congenital malformation in the offspring. It is proposed that leupeptin exerts its teratogenic action by inhibiting proteolysis in the lysosomes of the yolk sac, and so depriving the developing embryo of its supply of amino acids at a critical stage of development.

Effect of suramin on pinocytosis by resident rat peritoneal macrophages: an analysis using four different substrates

The effect of suramin on pinocytosis and intralysosomal proteolysis by resident rat peritoneal macrophages cultured in vitro has been studied. Suramin had little effect on the rate of pinocytic uptake of two non-adsorptive substrates [14C]sucrose and [3H]dextran, but unexpectedly enhanced uptake of a third, 125I-labelled polyvinylpyrrolidone (PVP). Since this enhanced uptake was completely abolished by NaF at a concentration known to inhibit pinocytosis, it clearly represented an increased internalization of substrate and not merely a superficial binding to the cell surface. It was concluded that suramin (i) does not affect the rate of formation of pinocytic vesicles but (ii) acts as a bivalent ligand, binding to both the macrophage surface and the 125I-labelled polyvinylpyrrolidone, thus converting a non-adsorptive into an adsorptive substrate. Suramin (500 micrograms/ml) decreased both the rate of uptake of formaldehyde-denatured 125I-labelled bovine serum albumin (BSA) (an adsorptive substrate) and the rate of its subsequent intracellular degradation. Thus, depending on the substrate chosen to measure pinocytosis, the same modifier may stimulate or inhibit uptake or be without effect.

The role of microtubules in pinocytosis. Inhibition of fluid-phase pinocytosis in the rat visceral yolk sac by mitoclasic and related agents

Colchicine, demecolcine and vinblastine all effectively inhibited the pinocytic capture of 125I-labelled poly(vinylpyrrolidone) by rat visceral yolk sacs cultured in vitro. Complete inhibition did not occur until 2-3h after addition of mitoclasic agent and this delay appeared to be related to the mechanism of drug-microtubule interaction rather than any difficulty of drug permeation into cells. Inhibition of pinocytosis by demecolcine was reversible, whereas inhibition by colchicine or vinblastine was not. In terms of dose-response demecolcine was slightly more effective than colchicine, but both were considerably more active than trimethylcolchicinic acid. Of the nine benzimidazoles tested for ability to inhibit pinocytosis, four severely retarded uptake and three were partially inhibitory.

Targeting of N-(2-hydroxypropyl)methacrylamide copolymers to liver by incorporation of galactose residues

Soluble synthetic polymers have potential as targetable carriers of pharmacological agents. Here we report that incorporation into poly[N-(2-hydroxypropyl)methacrylamide)] of an oligopeptide side-chain terminating in galactose enhanced the polymer's pinocytic uptake from the rat bloodstream by the liver. Within the liver lysosomes enzymic digestion led to the intracellular release of a drug analogue also bound to oligopeptide side-chains of the polymer.

Evidence that protein ingested by the rat visceral yolk sac yields amino acids for synthesis of embryonic protein

[3H]Leucine-labelled haemoglobin was prepared from rat reticulocytes incubated in the presence of [3H]leucine. Conceptuses from 9-5-day pregnant rats were incubated in vitro tox 48 h, with [3H]leucine-labelled haemoglobin present for the final 12, 8, 4, 2 or 0.5 hours. Radioactivity accumulated in visceral yolk sac and in embryonic tissue. When exposure to labelled haemoglobin was for only a short period before harvesting, all the radioactivity found in the embryo and most of that found in the visceral yolk sac trichloroacetic acid-soluble (i.e. associated with free amino acid rather than with protein). After longer exposures the proportion of radioactivity that was acid-soluble decreased to minimum values of about 20%. SDS-polyacrylamide gel electrophoresis of the protein-associated radioactivity in visceral yolk sac and embryo was performed. After exposure to labelled haemoglobin for 1 h only prior to harvesting, the yolk sac contained a single peak of radioactivity coincident in mobility with haemoglobin. The embryo contained no protein-associated radioactivity. After exposure to labelled haemoglobin for 12h, many protein bands in both yolk sac and embryo were radiolabelled. Thus a single radiolabelled protein pinocytically captured by the visceral yolk sac can give rise to the presence of many labelled proteins in embryo and visceral yolk sac. These results indicate that the source protein underwent proteolytic digestion and that the amino acids generated were re-utilized for protein synthesis in both embryonic and visceral yolk-sac cells.

Adsorptive pinocytosis of polycationic copolymers of vinylpyrrolidone with vinylamine by rat yolk sac and rat peritoneal macrophage

Polycationic copolymers of vinylpyrrolidone and vinylamine (10:0.77) were prepared, and 125I-labelled with either Bolton-Hunter reagent or methyl 3,5-di-[125I]iodohydroxybenzimidate. The rate of pinocytic capture of the copolymer was compared with that of 125I-labelled polyvinylpyrrolidone, using rat visceral yolk sacs and rat macrophages cultured in vitro as test systems. Whereas polyvinylpyrrolidone was captured entirely by non-adsorptive pinocytosis, the cationic derivative was captured more efficiently, probably because it adsorbs to the cell surface. Copolymer of Mr 120 000 was internalized by macrophages somewhat more rapidly than copolymer of Mr 46 000, but was excluded from the yolk sac.

Degradation of side-chains of N-(2-hydroxypropyl)methacrylamide copolymers by lysosomal thiol-proteinases

N-(2-Hydroxypropyl)methacrylamide copolymers bearing oligopeptide side-chains terminating in p-nitroaniline (NAp) were incubated with rat liver lysosomal enzymes in the presence of the thiol glutathione, and the rate of p-nitroaniline release was measured. Twelve of the 16 side-chains investigated were hydrolysed to release p-nitroaniline and in all but one case degradation was partially or totally inhibited by leupeptin. The effect of substrate concentration on the degradation of the most readily cleaved side-chain, -Ala-Gly-Val-Phe-NAp, was measured.