A combined assay of three lysosomal marker enzymes: acid phosphatase, beta-D-glucuronidase, and beta-N-acetyl-D-hexosaminidase

Identification and characterization of a lysosomal membrane protein of Dictyostelium discoideum with monoclonal antibodies

Lysosomes are responsible for the degradation of macromolecules derived from the cell exterior by endocytosis, or from within the cell by autophagy. While our knowledge of the biosynthesis and targeting of lysosomal hydrolases is considerable, much less is known about the lysosomal membrane itself. To identify the lysosomal membrane proteins that mediate these functions, we have isolated lysosomes from amebae and injected them into mice to produce monoclonal antibodies (MAbs). We produced nine MAbs against Dictyostelium lysosomes from the batches of fused cells. Among them, three MAbs were specific to lysosomal membrane and gave a strong signal, and thus used in this study. The MAbs specifically reacted with a single protein band of 27 kd and stained a lysosome-like structure by immunofluorescence microscopy. To identify the antigen that the MAbs recognize, we processed differential centrifugation with whole-cell extract of Dictyostelium and traced p27 protein by activity assay of organelle marker enzyme. We showed that p27 is one component of the lysosomal system on the basis of comigration with a lysosomal marker enzyme. We also demonstrated that the 27-kd lysosomal protein is a tightly bound integral membrane protein by using a phase separation method of Triton X-114.

Subcellular distribution of glycosylphosphatidylinositol-specific phospholipase D in rat liver

Glycosylphosphatidylinositol (GPI)-hydrolysing enzymes have been described in many mammalian tissues and body fluids; however, their site(s) of action and in vivo functions have remained unclear. In order to identify a possible intracellular site of GPI hydrolysis, we studied the subcellular distribution of GPI-hydrolysing activity in rat liver. We found that purified fractions from rat liver hydrolysed the GPI moieties of two GPI-anchored proteins with the specificity of a phospholipase D. This GPI-specific phospholipase D (GPI-PLD) activity was found to be highly enriched in a lysosomal fraction and showed a similar intracellular distribution to that of typical lysosomal enzymes. Our results indicate that lysosomes may represent a possible intracellular site of GPI-PLD action.

Intralysosomal glycerophospholipid catabolism in liver: hydrolysis of amino alcohol-containing phospholipids and their metabolites

Liver lysosomes were isolated from untreated rats and rats pretreated with Triton WR-1339. Purified lysosomes were also separated into lysosomal matrix and membrane fractions. With freshly prepared and frozen biological material, the lysosomal catabolism of various stereospecifically radiolabeled amino alcohol-containing glycerophospholipids and their potential metabolites was studied. Basically there was no qualitative difference in the formation of phospholipid metabolites in both preparations: after long-term incubation, free fatty acids, lysophospholipids, acyl-free phosphodiesters were detected, and to a far lesser extent, amino alcohol-containing phosphomonoesters and only traces of free amino alcohols. These findings indicate the presence of lysosomal phospholipases A as well as C and lysophospholipase(s), with pH optima of about 4.5, and they clearly exclude phospholipase D activity. Unfractionated lysosomes and their soluble as well as particulate subfractions were not capable of hydrolysing the acyl-free amino alcohol-containing phosphodiesters. These compounds must therefore be considered one of the end products of the intralysosomal catabolism of amino alcohol-containing phosphoglycerides. They are presumably cleared from the lysosomal compartment by an as yet unknown transport system in the lysosomal membrane. In liver, the extralysosomal site of their (Mg(2+)-dependent) hydrolysis seems to be the plasma membrane. By contrast, hydrolysis of glycero-3-phosphate and the amino alcohol-containing phosphomonoesters was catalysed in the lysosomal compartment, with a pH optimum of about 5.0, although at considerably lower rates than that of glycero-2-phosphate, a model substrate for lysosomal acid phosphatase.

Cytoprotective effect of copper(II) complexes against ethanol-induced damage to rat gastric mucosa

The cytoprotective effect of various copper(II) complexes on the gastric mucosa damage induced by acute intragastric administration of ethanol was investigated. For in vitro experiments, the following copper(II) complexes were tested: Cu(II)(L-Trp)(L-Phe), Cu(II)(L-Leu)Cu(II)(L-Leu-Leu)(L-Leu), Cu(II)(L-Phe-L-Leu), Cu(II)(Gly-His-Lys), and Cu(II)(cyHis)2(ClO4)2. Inorganic copper such as CuSO4 was also tested. The free radical generating system, acting for 2 hr on cardial and fundic mucosa scrapings or mucosal microsomes, was Fe++ (20 microM)/ascorbate (0.25 mM). We found a marked inhibition to 75% of lipid peroxidation in the range 10-100 mM, regardless of whether copper was given in complexed or inorganic form. The results suggest that nontoxic copper(II)-amino acid complexes are able to neutralize oxygen-derived free radicals. In addition, copper(II) complexes suppressed membrane lipid peroxidation when mucosa homogenates were exposed to t-butyl hydroperoxide (1-20 microM) plus Fe++ (50 microM). In vivo experiments on rat stomachs, pretreated p.o. by gavage either with Cu(II)(L-Trp)(L-Phe) as paradigmatic agent or with copper sulphate at equivalent doses in the range 3-30 mg/kg body weight showed a significant decrease (30 min after 95% ethanol administration) in the number and severity of mucosal hemorrhagic lesions. In the gastric mucosa scrapings of copper-treated rats after ethanol exposure, we found that malondialdehyde and conjugated diene levels were unchanged compared to those of untreated controls; five enzyme activities released from lysosomes were near control values. In isolated mucosal cells, whether or not pretreated with 200 microM solution of either Cu(II)(L-Trp)(L-Phe) or CuSO4, the release of cathepsin D activity was also unmodified. The results suggest that the cytoprotective effect of Cu(II) complexes against ethanol-induced mucosal lesions was not associated in vivo to lipid peroxidation.

Partial characterisation and subcellular distribution patterns of endothelin-1, -2 and -3 binding sites in human liver

For the first time, endothelin-1, -2 and -3 (ET-1, -2, -3) binding sites were characterized in human liver and shown to differ significantly in their respective dissociation constants and densities. In addition, subcellular distribution patterns of these binding sites in biochemically analysed fractions obtained after differential centrifugation were shown to be heterogeneous. Thus, the bulk of ET-1 and ET-3 binding sites seemed to be present in plasma membranes, although their partial presence in a compartment sedimenting together with the endoplasmic reticulum cannot be excluded. In contrast, a major proportion of the ET-2 binding sites appeared to be associated with a compartment sedimenting together with mitochondria, suggesting a special accumulation of ET-2 binding sites in human liver. A significant portion of ET-1, -2 and -3 binding sites seems to be localized also in lysosomes, presumably indicating their participation in the internalisation process.

Insulin processing in primary endosomes is not responsible for insulin resistance observed in parametrial adipocytes from lactating rats

The fate of [125I insulin and the insulin receptor after internalization was characterized in parametrial adipocytes from virgin rats. Parallel experiments were carried out on parametrial adipocytes from 2-4-day lactating rats, which are insulin resistant. Similar results were obtained in adipocytes from either group of animals. Insulin caused 10% of the plasma membrane insulin receptor to be translocated to a compartment resistant to extracellular trypsin. The intracellularly located insulin receptor rapidly recycled to the plasma membrane at 37 degrees C. An endosomal compartment involved in both the endocytosis and subsequent recycling of [125I]insulin and the insulin receptor to the plasma membrane was identified on sucrose density floatation gradients. [125I]Insulin internalized at 37 degrees C accumulated in a fraction of modal density 1.12 g/ml. Crosslinking experiments revealed the presence of intact [125I]insulin-insulin receptor complexes in endosomes. After a pulse with [125I]insulin, 55-60% of the 125I radioactivity recovered in the endosome compartment was intact [125I]insulin. The remainder was composed of low molecular weight degradation products. Endosomal 125I radioactivity was rapidly retroendocytosed to the medium with a mean half-life of 6 min. These results suggest: (1) [125I]insulin and the insulin receptor are internalized by parametrial adipocytes into an early endosomal compartment (primary endosomes), from which the receptor, intact [125I]insulin, and [125I]tyrosine are returned to the cell surface; and (2) the damping of the insulin signal observed in parametrial adipocytes from lactating rats is not expressed at the level of altered endocytotic processing of [125I]insulin and the insulin receptor.

Acid phospholipase A1 in liver--a brief survey

Acid phospholipase A1 activity in liver (rat, human) is predominantly localized in lysosomes. A minor proportion (less than 3% of the total activity) is also present in the Golgi apparatus and the endoplasmic reticulum, presumably due to enzymatically active precursors of the corresponding lysosomal enzyme. Lysosomal phospholipase A1 is the most important enzyme initiating the intralysosomal catabolism of diacylphosphoglycerides. It has been purified 50,600-fold, with a yield of about 26%. The enzyme prefers phosphatidyl-ethanolamine as a substrate, which at 200 microM and pH 4.5, is hydrolysed at a rate of approximately 8.2 U/mg. Lysosomal phospholipase A1 is a glycoprotein of about 29 kDa with an isoelectric point of pH 5.3. Unspecific extralysosomal endogenous inhibitors of this enzyme are pH range, inorganic cations, and various proteins. Divalent cations are more potent inhibitors than monovalent ones. Most endogenous intra- and extracellular proteins inhibit the enzyme, the cationic species exhibiting high inhibitory potencies, glycoproteins only little. Inhibitory proteins act through their binding to the substrate. Lysosomal phospholipase A1 seems to be an important target in drug-induced lipidosis. This lipid storage disease is caused by various cationic amphiphilic drugs that are trapped intralysosomally by protonation. In lysosomes such compounds raise the pH, interact with the polar lipids to be degraded and the lysosomal lipolytic enzymes, such as phospholipase A1. These mechanisms result in impaired intralysosomal phospholipid degradation and hence in intralysosomal phospholipid accumulation.

Rat skeletal muscle lysosomes contain glycogen

1. A lysosome- and mitochondria-enriched fraction was obtained from rat skeletal muscle using a differential centrifugation procedure. This fraction contained a proportion (3-4%) of the tissue glycogen content. 2. Lysosomes and mitochondria were separated from one another by centrifugation of a cell-free extract upon discontinuous Ficoll-sucrose gradients. Little glycogen was associated with the resulting mitochondrial fraction. The lysosomal fraction, however, contained a significant amount of glycogen, accounting for 5% of the skeletal muscle glycogen. 3. The lysosomal glycogen was purified and found to be enriched in high molecular weight material. 4. The compartmentation of skeletal muscle glycogen metabolism is suggested.

Acid phospholipase A activities in rat hepatocytes

Cultured rat hepatocytes exhibit acid phospholipase A activity. On the basis of product formation from stereospecifically radiolabeled phosphatidylethanolamine substrates, phospholipases A1 and A2 have been identified with optimal activities at pH 4.5. According to subcellular fractionation studies, the acid phospholipases in hepatocytes appear to be located in the lysosomal compartment. Application of specific inhibitors of the biosynthesis, glycosylation, and translocation of lysosomal enzymes in hepatocyte cultures suggests a half-life of approx. 1 day for the acid lysosomal phospholipase A1. About the same value for the half-life was obtained for the lysosomal marker enzymes, acid phosphatase and beta-N-acetyl-D-hexosaminidase.

Changes of phospholipid-metabolizing and lysosomal enzymes in hypoglossal nucleus and ventral horn motoneurons during regeneration of craniospinal nerves

In order to study the biochemical changes associated with the cell body response to axonal crush injury, two systems, hypoglossal nucleus and spinal cord ventral horn, were used. The time intervals chosen were 7, 14, and 28 days after unilateral crushing of the right hypoglossal nerve and cervicothoracic nerves of the rabbit. Non-crushed, contralateral nerves were used as controls. Three groups of enzyme activities were tested: (a) phospholipase A2, acyl CoA:2-acyl-sn-glycero-3-phosphocholine acyltransferase, and choline phosphotransferase, as indicators of phospholipid degradation and biosynthesis; (b) seven hydrolases, namely, beta-D-glucuronidase, beta-N-acetyl-D-hexosaminidase, arylsulfatase A, galactosylceramidase, GM1-ganglioside beta-galactosidase, and acid RNase, as indicators of lysosomal activity; and (c) free and inhibitor-bound alkaline RNase, as an index of RNA metabolism. Changes could be grouped into three distinct patterns. Compared to contralateral control, choline phosphotransferase showed a slight increase, whereas phospholipase A2 and most lysosomal hydrolases showed a significant increase of activity, especially evident in the ventral spinal cord neurons 14-28 days after crushing. These changes correlate with known increases of membrane and organelle numbers, including lysosomes, in motor and sensory neurons during peripheral regeneration. In contrast, free and acid alkaline RNase activity significantly decreased in the injured sides compared to the controls. This change can probably be correlated with a stabilization of RNAs needed for increased protein synthesis. No changes in total alkaline RNase and acyltransferase activities in either regeneration model were observed.

Disintegration of isolated cells

A simple device has been developed for the disintegration of isolated cells in isotonic or hypertonic media. It is based on the extrusion of a cell suspension through a small orifice under controlled pressure. The diameter of this interchangeable orifice has to be adapted to the cell type to be disrupted. The conditions fulfill the requirements for an efficient disruption of the plasma membranes and for good preservation of the particularly fragile subcellular components, lysosomes and peroxisomes, of rat hepatocytes and rat Leydig cells.

Fractionation, biochemical characterization and lysosomal phospholipases of human liver

Human liver was homogenised and fractionated by differential centrifugation, and the subcellular fractions were characterised biochemically. Absolute values and distribution patterns of protein and marker enzyme activities obtained from human liver have also been compared with those from rat liver. In addition, acid phospholipase activities have been studied in human liver. On the basis of product formation from stereo-specifically radiolabeled phosphatidylethanolamine substrates, lysosomal phospholipases A1 and A2 with optimal activities at pH 4.7 have been identified in human liver. Acid phospholipase C and lysophospholipase activities, however, were not found in human liver. Cationic amphiphilic drugs inhibited the activities of the acid phospholipases A in human and rat liver lysosomes to about the same extent.

A putative protein-sequestration site involving intermediate filaments for protein degradation by autophagy. Studies with transplanted Sendai-viral envelope proteins in HTC cells

Reconstituted Sendai-viral envelopes (RSVE) were fused with hepatoma tissue-culture (HTC) cells, thereby introducing viral membrane glycoproteins into the plasma membrane [Earl, Billett, Hunneyball & Mayer (1987) Biochem. J. 241, 801-807]. Fractionation of homogenized cells on Nycodenz gradients shows that much of the viral 125I-labelled HN and F proteins were rapidly sequestered into a dense fraction distinct from fractions containing plasma membrane, lysosomes and mitochondria. Electron microscopy (results not shown) indicates that the dense fraction contains nuclear residues, multivesicular structures, dense bodies and fibrous structures. Both the dense fraction and a hexosaminidase-enriched fraction contain trichloroacetic acid-insoluble radioactivity, including intact 125I-labelled viral proteins. The viral proteins are progressively transferred from the dense fraction to the hexosaminidase-enriched fraction; the transfer is retarded by 50 micrograms of leupeptin/ml. Trichloroacetic acid-soluble radiolabel is progressively released into the culture medium as the proteins are degraded. Within 5 h after transplantation of viral HN and F proteins into recipient cells, a proportion (approx. 45%) of the 125I-labelled glycoproteins cannot be extracted by sequentially treating cells with digitonin (1 mg/ml), Triton X-100 (1%, w/v) and 0.3 M-KI. HN and F proteins in the non-extractable residue are tightly associated with nuclear-intermediate-filament (vimentin) material, as shown by Western blots and electron microscopy. The viral proteins are progressively transferred out of the nuclear-intermediate-filament residue; the transfer is slowed when cells are cultured with leupeptin. The data are consistent with the notion that transplanted viral HN and F proteins are sequestered to a perinuclear site in tight association with intermediate filaments before transfer into the autophagolysosomal system for degradation.

A putative protein-sequestration site involving intermediate filaments for protein degradation by autophagy. Studies with microinjected purified glycolytic enzymes in 3T3-L1 cells

Several glycolytic enzymes (lactate dehydrogenase, pyruvate kinase, glyceraldehyde-3-phosphate dehydrogenase) were radiolabelled by [125I]iodination, conjugation with 125I-labelled Bolton & Hunter reagent and reductive [3H]methylation, and their degradative rates after microinjection into 3T3-L1 cells compared with that of the extracellular protein bovine serum albumin. Although the albumin remains largely cytosolic in recipient cells, the glycolytic enzymes rapidly (less than 30 min) become insoluble, as measured by detergent and salt extractions. The microinjected glycolytic enzymes appear to form disulphide-linked aggregates, are found in a cell fraction rich in vimentin-containing intermediate filaments and histones (nuclear-intermediate-filament fraction), and are degraded slowly by a lysosomal mechanism, as judged by the effects of inhibitors (NH4Cl, leupeptin, 3-methyladenine). 125I-labelled bovine serum albumin appears to be degraded rapidly and non-lysosomally. Prolonged treatment (96 h) of cultured cells with leupeptin results in the accumulation of pulse-labelled ([35S]methionine for 24 h) endogenous cell proteins in the detergent-and salt-non-extractable residue, but NH4Cl and 3-methyladenine do not have this effect. The findings are in terms of the interpretation of experiments involving microinjection of proteins to study intracellular protein protein degradation by autophagy.

The role of superoxide anion and lysosomal enzymes in anti-listerial activity of elicited peritoneal macrophages

The in vitro effect of superoxide anion and lysosomal enzyme activity on the killing of Listeria monocytogenes EGD (listeria) by peritoneal macrophages (PM) was investigated. Generation of superoxide anion by PM stimulated with phorbol myristate acetate (PMA) was significantly increased by intraperitoneal injection of Lactobacillus casei YIT 9018 (LC9018) or Corynebacterium parvum (CP), but not by injection of peptone. However, superoxide anion generation by LC9018-elicited PM stimulated with listeria was not increased any more than that by peptone-elicited PM, and generation of superoxide anion by the PM was affected by the difference in stimuli. The killing of listeria by LC9018- or CP-elicited PM in vitro was significantly less than that by peptone-elicited PM or resident PM. Significant correlation was observed between the anti-listerial activity of PM and the intracellular killing of listeria by PM. On the other hand, beta-glucuronidase and beta-N-acetyl-D-glucosaminidase activities of LC9018-elicited, CP-elicited, or resident PM were significantly weaker than those of peptone-elicited PM, and no significant correlation was observed between the increase in beta-glucuronidase and beta-N-acetyl-D-glucosaminidase activities and the increase in anti-listerial activity. These results suggest that increase in the activity of lysosomal enzymes such as beta-glucuronidase and beta-N-acetyl-D-glucosaminidase is not correlated with the anti-listerial activity of PM, and that superoxide anion has very little effect on the anti-listerial activity of PM in vitro.

Diacylglycerol hydrolysis in rat liver lysosomes

The matrix of rat liver lysosomes exhibits high hydrolytic activity towards 1,2-diacylglycerol with an optimum at pH 4.0. The lipolytic reaction follows Michaelis-Menten kinetics (apparent Vmax 470 nmol hydrolysed/min per mg protein; apparent Km 71 microM 1,2-dioleoylglycerol). Formation of 1- and 2-monooleoylglycerols indicates an initial attack at both the primary and secondary ester bonds. The lysosomal matrix also catalyses (re)acylation reactions, i.e. the formation of 1,2-diacylglycerol from 2-monoacylglycerol and free fatty acid. However, (re)acylation proceeds at a far lower rate than deacylation of diacylglycerol. Lysosomal diacylglycerol hydrolysis is sensitive towards non-ionic detergents, cationic amphiphilic drugs and the lipase inhibitor RHC 80267.

Effects of antimalarial drugs on phospholipase A and lysophospholipase activities in plasma membrane, mitochondrial, microsomal and cytosolic subcellular fractions of rat liver

Activities of membrane-associated phospholipases A1 and A2, and membrane-associated as well as soluble lysophospholipases were measured in different subcellular fractions of rat liver, using suspensions of stereospecifically labelled radioactive phospholipids as substrates. Plasma membranes and endoplasmic reticulum were shown to contain phospholipase A1 and lysophospholipase activities, both of which could be stimulated by Ca2+, mitochondria Ca2+-dependent phospholipase A2 and cytosol Ca2+-independent lysophospholipase activities. Each of these lipolytic enzymes could be inhibited by antimalarial drugs (chloroquine, mepacrine, primaquine) at concentrations above 1 x 10(-4) M. Inhibition of the alkaline cytosolic lysophospholipase by these drugs was noncompetitive with respect to the substrate, and the inhibitory potency increased, when the pH was raised.