Studies on the turnover of exogenous mannose-terminal glucocerebrosidase in rat liver lysosomes

We have cloned the full coding cDNA sequence of chicken annexin V and of a mutant lacking 8 amino acid residues of the N-terminal tail for prokaryotic expression. Both proteins were synthesized in Escherichia coli upon induction with isopropyl thio-β-D-galactoside, and were purified following two different protocols: one based on the ability of these proteins to interact reversibly with liposomes in the presence of calcium, and the other based on two sequential ion-exchange chromatographic steps. Spectroscopical analysis of recombinant annexin V revealed that binding of calcium did not change the circular dichroism spectra indicating no significant changes on the secondary structure; however, a conformational change affecting the exposition to the solvent of the tryptophan residue 187 was detected by analysis of fluorescence emission spectra. Recombinant annexin V binds with high affinity to collagen types II and X, and with lower affinity to collagen type I in a calcium-independent manner. Heat denaturing of collagen decreases this interaction while pepsin-treatment of collagen almost completely abolishes annexin V binding. Mutated annexin V interacts with collagen in a similar way as the nonmutated recombinant protein, indicating that the N-terminal tail of annexin V is not essential for collagen binding.     

The permeability of rat liver lysosomes to sugars. Evidence for carrier-mediated facilitated diffusion.

1. By the osmotic-protection method, the penetration of sugars through the rat liver lysosomal membranes was studied with a view of determining whether sugar uptake was by facilitated diffusion. 2. The following criteria for this type of transport were established: sugar specificity, the order of uptake being 2-deoxy-D-glucose less than D-glucose less than D-mannose less than D-galactose less than D-ribose less than 2-deoxy-D-ribose; stereospecificity, the uptake of L-glucose and L-ribose being 50% slower than their D-stereoisomers; inhibition by 1 MM-phlorrhizin and 1 M-cytochalastin B; competition between sugars for uptake, and a Q10 (rate difference over a 10 degrees C temperature range) for uptake of approx. 2.8. 3. It is proposed that sugar uptake into lysosomes from rat liver is by facilitated diffusion.     

5'-Nucleotidase in rat liver lysosomes

5'-Nucleotidase was found in purified rat liver tritosomes. When tritosomes were subfractionated into the membrane and soluble contents fractions, 73% of the total 5'-nucleotidase activity was found in the membrane fraction and 24% in the soluble contents fraction. Immunoblotting using specific polyclonal antibodies against the rat liver plasma membrane 5'-nucleotidase showed that the mobilities on SDS-polyacrylamide gel electrophoresis of both 5'-nucleotidases in the membrane and contents fractions were identical to that of the enzyme in the plasma membranes (Mr = 72,000). 5'-Nucleotidases in the membrane and contents fractions were sensitive to neuraminidase and converted into a form that was 4 kDa smaller after digestion, as observed in the case of plasma membrane enzyme. 5'-Nucleotidases, both from the membrane and contents fractions, were purified using immunoaffinity chromatography, and the isoelectric points, heat stability, and oligomeric structure of the purified enzymes were compared. Isoelectric focusing and the heat stability test indicated the resemblance of the soluble enzyme to the membrane-bound enzyme. However, the membrane-bound enzyme aggregated in the absence of Triton X-100, whereas the soluble enzyme behaved as a dimer. The topography of 5'-nucleotidase in the tritosomal membranes was studied using antibodies against 5'-nucleotidase and neuraminidase treatment. The inhibition of 5'-nucleotidase were not observed in the intact tritosomal fraction until the tritosomes had been disrupted by osmotic shock. These results show that the active sites and the oligosaccharide chains of 5'-nucleotidase are located on the inside surface of the tritosomal membranes.     

Sulfate transport by rat liver lysosomes

Sulfate transport was examined using membrane vesicles (pH 7.0 inside) prepared from rat liver lysosomes. Sulfate uptake was dependent upon external pH with increased uptake at lower buffer pH. The Km for uptake was 160 microM at pH 5.0 while at pH 7.0, a lower affinity system with a Km of 1.4 mM was present. The protonophore carbonyl cyanide m-chlorophenylhydrazone increased uptake at pH 5.0 while valinomycin/KCl had no effect. In contrast, at pH 7.0, valinomycin-induced changes in membrane potential stimulated uptake. Countertransport of sulfate at pH 7.0 was inhibited by 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene, N-(4-azido-2-nitrophenyl)-2-aminoethanesulfonic acid, and a variety of anions: SO4(2-) greater than MoO4(2-) greater than Cl- greater than HPO4- greater than HCO3-. Trans-stimulation of sulfate uptake at pH 7.0 was observed with MoO4(2-) and, to a lesser extent, with S2O3(2-) while Cl-, HPO4-, and HCO3- had little effect. However, chloride loading of vesicles resulted in marked stimulation of sulfate uptake at pH 5.0. It appears that sulfate and protons exit lysosomes in exchange for chloride by a specific, pH-regulated anion transport system.     

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.     

Turnover studies on proteins of rat liver lysosomes.

The turnover of rat liver lysosomal proteins was studied by a double isotope-labeling technique. The cellular fractions investigated included soluble lysosomal proteins, lysosomal membrane proteins, highly purified lysosomal beta-glucuronidase, and for comparison, microsomal proteins and soluble cytoplasmic proteins. Both "normal" lysosomes and Triton WR-1339-filled lysosomes (tritosomes) were studied, with similar results. It was found that (a) the turnover rate of lysosomal proteins, of both the soluble and membranous compartments, was very similar to that of the proteins of the microsomal and soluble cytoplasmic fractions, and (b) the turnover rate of lysosomal proteins was asynchronous. The latter conclusion was based on two lines of evidence: (a) lysosomal beta-glucuronidase had a distinctly slower turnover rate than the average rate of the soluble lysosomal proteins, and (b) subunits of the proteins of the soluble lysosomal fraction as separated by sodium dodecyl sulfate. Sephadex G-200 gel filtration showed different rates of degradation.     

Studies on a HCO3 minus-stimulated ATPase and carbonic anhydrase system in rat liver lysosomes

Lysosome-rich fractions were prepared by discontinuous sucrose gradient centrifugation of liver homogenates from rats pretreated with Triton-WR-1339. The lysosome-rich fraction contained 48% of the crude homogenate hexosaminidase applied to the centrifuge tube and its specific activity was 10-fold greater than the original homogenate. A Mg²⁺-requiring ATPase that was stimulated by 20 mm HCO₃^? was associated with the lysosomal-enriched fraction. Its specific activity was 50–65% of that compared with the mitochondrial-rich fractions.The properties of the HCO₃^?-stimulated ATPase from rat liver lysosomes were similar to those previously reported from gastric mucosa, submaxillary gland, and pancreas with respect to substrate specificity, anion stimulators, and inhibitors. Double-reciprocal plots were nonlinear with respect to ATP (n_H = 2.23) and HCO₃^? (n_H = 1.84), and the corresponding K_m values were estimated to be 0.33 and 7.25 mm.Carbonic anhydrase activity was also found associated with the lysosomes at activities comparable to those of the mitochondrial-rich fractions. The lysosomal carbonic anhydrase was inhibited 33% by 100–200 μm acetazolamide, whereas that in mitochondrial fractions was inhibited by 68–71% by 100-μm levels of the drug. The ATPase and carbonic anhydrase system of rat liver lysosomes represents a possible mechanism for the maintenance of intralysosomal proton gradients.     

Neutral-sugar transport by rat liver lysosomes.

Transport of D-glucose was studied in Percoll-gradient-purified rat liver lysosomes. D-Glucose uptake had a Km of 22 mM and a t1/2 of approx. 30 s. D-Fucose, 2-deoxyglucose and methyl alpha-glucoside were the most effective competitors for uptake of D-glucose, although D-galactose, D-mannose, D-xylose and L-fucose also appeared to compete for uptake. L-Glucose was a poor competitor for uptake. No competition was observed with N-acetyl-D-glucosamine, N-acetyl-D-galactosamine, D-glucuronic acid, N-acetylneuraminic acid, D-glucosamine or the amino acids L-glycine, L-lysine and L-proline. Uptake was unaffected by N-ethylmaleimide, dithiothreitol, KCl, NaCl, ATP/Mg or alteration of buffer pH. D-Glucose efflux from lysosomes was temperature-dependent, with a Q10 of 2.3, and was inhibited by cytochalasin B. Counter-transport could not be demonstrated. In contrast, L-fucose uptake had a Km of 65 mM and was largely unaffected by 5 M excess of neutral D-sugars. Both uptake and efflux of L-fucose were inhibited by cytochalasin B. It appears that lysosomes possess a facilitated transport system for D-glucose and perhaps other neutral D-sugars that is discrete from transport systems for acetylated and acidic sugars.     

Cardiolipin degradation by rat liver lysosomes.

The lysosomal subcellular fraction of rat liver contains acid hydrolases which can carry out the degradation of cardiolipin to yield water-soluble products and free fatty acids. The time course of appearance of the products indicates that the major catabolic route involves the sequential removal of three of the fatty acids, followed by hydrolysis to acylglycerophosphoryl glycerol (from which the fatty acid is subsequently removed) and d-glycerophosphate (which is hydrolysed to give free phosphate and glycerol). The phospholipase A activity responsible for removal of the first fatty acid is located in the lysosomal fraction.     

Comparison of the ability of phospholipids from rat liver lysosomes to reconstitute glucocerebrosidase

The in situ lipid activator of rat liver glucocerebrosidase was investigated. Rat liver lysosomes were purified (42.9-fold relative to the crude homogenate) by sequential isopycnic centrifugation in sucrose and metrizamide gradients. Lipids were extracted with chloroform:methanol (2:1) and phospholipids were separated by one-dimensional thinlayer chromatography. The phospholipid content of the lysosome preparation was 0.28 μmol lipid phosphorus/mg protein. Phosphatidylcholine was present as the major non-acidic phospholipid (39.3%). Of the acidic phospholipids, phosphatidylinositol and phosphatidylserine were present in the greatest amounts (12.0 and 19.7%, respectively). The resolved phospholipids were tested separately and in the presence of a heat-stable factor from Gaucher spleen for their ability to reconstitute butanol-delipidated rat liver glucocerebrosidase activity. Alone or in the presence of the heat-stable factor, phosphatidylserine and phosphatidylinositol were the most effective activators of glucocerebrosidase. Bis(monoacylglyceryl) phosphate derived from rat liver tritosomes or rabbit lung macrophages was also effective in reconstituting β-glucosidase activity.