Isoelectric-focusing behavior of acid hydrolases in rat kidney lysosomes. Effects of the pH gradient, autolysis and neuraminidase

Supramolecular assemblies from lysosomal matrix proteins and complex lipids

Most lysosomal hydrolases are soluble enzymes. Lamp-II (lysosome-associated membrane protein-II) is a major constituent of the lysosomal membrane. We studied the aggregation of a series of lysosomal molecules. The aggregation-sensitive lysosomal marker enzymes were optimally aggregated at intralysosomal pH. A similar pH dependence was recorded for aggregation of Lamp-II. The pH-dependent loss of solubility of isolated Lamp-II required components of the lysosome extract. Conditions of mild acid pH promoting aggregation triggered the formation of complexes with lipids of lysosomal origin. We fractionated a membrane-free lysosome extract by gel-filtration chromatography and could reconstitute assemblies in vitro from separated fractions. We found some selectivity in the lysosomal proteins binding to complex lipids, phosphatidylcholine, sphingomyelin, and phosphatidylethanolamine being most effective. We propose that the formation at pH 5.0 of such supramolecular assemblies between lysosomal proteins and lipids occurs within the intralysosomal environment. Some possible consequences of such an intralysosomal matrix formation on organelle function are discussed.

Separation and identification of 2',3'-cyclic nucleotide 3'-phosphodiesterase on isoelectric focusing gels

A method is presented for the separation and detection of the myelin marker enzyme 2',3'-cyclic nucleotide 3'-phosphodiesterase on isoelectric focusing gels and by immunoblotting. The gel staining procedure is a modification of a method used to demonstrate enzyme activity on blots after sodium dodecyl sulfate-polyacrylamide gel electrophoresis and two-dimensional polyacrylamide gel electrophoresis. The results show that immunologically active 2',3'-cyclic nucleotide 3'-phosphodiesterase can be separated under equilibrium conditions on isoelectric focusing gels with an expanded alkaline pH range after solubilization in a mixture of nonionic/zwitterionic detergents and urea. Enzymatically active 2',3'-cyclic nucleotide 3'-phosphodiesterase focused as two closely spaced bands at pIapp 8.1 and 8.8, respectively, while 2',3'-cyclic nucleotide 3'-phosphodiesterase immunoreactivity was detected as four distinct bands at pIapp 4.2, 7.4, 8.8, and 9.3 and a diffuse band at pIapp 7.9-8.2. By two-dimensional separation these five bands showed molecular weights of about 43-47 kDa, i.e., corresponding to reported values for immunologically active 2',3'-cyclic nucleotide 3'-phosphodiesterase. Since enzyme activity is associated with only two of the bands, nonspecific and artifactual banding due to, e.g., detergent micelle formation, is unlikely.

Properties and prenatal ontogeny of beta-D-mannosidase in selected goat tissues

beta-D-Mannosidase activity in selected normal adult, neonatal and foetal goat tissues and in tissues from animals affected with caprine beta-mannosidosis was examined with the use of 4-methylumbelliferyl beta-D-mannopyranoside as substrate. The enzyme in normal adult thyroid, kidney and brain exhibited a sharp unimodal pH optimum at pH 5.0, whereas the enzyme in both normal adult and mutant liver exhibited broad pH ranges of activity (pH 4.5-8.0). No residual enzyme was detectable in mutant kidney or brain; in contrast, residual activity in mutant liver was 52% of that in a neonatal control. Concanavalin A-Sepharose 4B (Con A-Sepharose) fractionation of normal adult liver beta-D-mannosidase resolved the enzyme into an unbound (non-lysosomal) from (52%) with a broad pH range of activity (pH 4.5-8.0) and a bound (lysosomal) form (48%) with a sharp pH optimum of 5.5. The enzyme in mutant liver consisted entirely of the unbound (non-lysosomal) form. Beta-D-Mannosidase activity in normal adult thyroid, kidney and brain was resolved by chromatofocusing into two major isoenzymes, with pI 5.5 and 5.9, and traces of a minor isoenzyme, with pI 5.0. In normal adult liver the enzyme was also resolved into three isoenzymes with similar pI values; however, that with pI 5.0 predominated. The predominant form of the enzyme in 60-day-foetal liver was bound by Con A, exhibited a unimodal pH optimum (5.0) and was resolved into two isoenzymes, with pI 5.4 and 5.8; only traces of an isoenzyme with pI 5.0 were detectable. Total hepatic beta-D-mannosidase activity increased progressively towards adult values during the last 90 days of gestation as a result of increasing non-lysosomal isoenzyme activity (pI 5.0). Lysosomal beta-D-mannosidase was shown to occur in all normal goat tissues studied as multiple isoenzymes, which are genetically and developmentally distinct from the non-lysosomal isoenzyme occurring predominantly, if not exclusively, in liver.

Alteration of beta-hexosaminidase activity and isoenzymes in human leukemic cells

beta-Hexosaminidase (EC 3.2.1.20; Hex) activity and isoenzyme characteristics were analyzed in human normal and leukemic leukocytes. Unseparated CLL and CML cells had a specific activity that was lower, whereas ALL and AML blasts had a higher specific activity than normal lymphocytes and granulocytes. CLL B-cells had a lower specific activity compared with that in normal non-T-lymphocytes; CLL T-cells and normal T-cells had similar activity. Isoenzyme separation was performed by chromatofocusing on PBE-94 coupled with an automated enzyme assay. When using a single linear pH elution gradient, normal leukocytes and all leukemia cells contained two forms of isoenzyme (B and A). When a double pH elution gradient was performed, an extra distinct form of Hex (I) was recorded. Hex I was present in small amounts in normal granulocytes and PHA-stimulated normal lymphocytes; isoenzyme I was found in high amounts in all leukemias tested. The activity ratios I/B and I/A, as well as the I isoenzyme profile, may facilitate differentiation between normal and leukemic cells and between lymphoblastic and myeloblastic leukemias.

A major phosphotyrosyl-protein phosphatase from bovine heart is associated with a low-molecular-weight acid phosphatase

The phosphotyrosyl [Tyr(P)]-immunoglobulin G (IgG) phosphatase activity in the extracts of bovine heart, bovine brain, human kidney, and rabbit liver can be separated by DEAE-cellulose at neutral pH into two fractions. The unbound fraction exhibits a higher activity at acidic than neutral pH while the reverse is true for the bound fraction. Of all tissues examined, the Tyr(P)-IgG phosphatase activity in the unbound fraction measured at pH 5.0 is higher than that in the bound fraction measured at pH 7.2. The acid Tyr(P)-IgG phosphatase activity has been extensively purified from bovine heart. It copurified with an acid phosphatase activity (p-nitrophenyl phosphate (PNPP) as a substrate) throughout the purification procedure. These two activities coelute from various ion-exchange and gel filtration chromatographies and comigrate on polyacrylamide gel electrophoresis, indicating that they reside on the same protein molecule. The phosphatase has a Mr = 15,000 by gel filtration and exhibits an optimum between pH 5.0 and 6.0 when either Tyr(P)-IgG-casein or PNPP is the substrate. It is highly specific for Tyr(P)-protein with little activities toward phosphoseryl [Ser(P)]- or phosphothreonyl [Thr(P)]-protein. The enzyme activities toward Tyr(P)-casein and PNPP are strongly inhibited by microM molybdate and vanadate but insensitive to inhibition by L(+)-tartrate, NaF, or Zn2+. The molecular and catalytic properties of the acid Tyr(P)-protein phosphatase purified from bovine heart are very similar to those of the low-molecular-weight acid phosphatases of Mr = 14,000 previously identified and purified from the cytosolic fraction of human liver, placenta, and other animal tissues.

Molecular forms of beta-N-acetylhexosaminidase in Epstein-Barr virus-transformed lymphoid cell lines from normal subjects and patients with Tay-Sachs disease

In whole leukocytes and in lymphocytes from normal subjects, the percentage activity of heat-stable beta-N-acetylhexosaminidase (30 +/- 5% and 45 +/- 5%, respectively) was higher than in the transformed lymphoid cell line (19 +/- 3%). In Tay-Sachs transformed cells as well as non-transformed beta-N-acetylhexosaminidase was almost completely heat-stable (95 - 98%). In the transformed cells from normal subjects, the beta-N-acetylhexosaminidase B (Hex B) activity (5% of total) was significantly lower than in blood lymphocytes (average 25 - 30% of total activity), whereas Hex A and Hex I were similar in the either cell type. Blood lymphocytes and lymphoid cell lines established from a Tay-Sachs patient lacked heat-labile Hex A and expressed high heat-stable Hex I and Hex B activities (3-6-fold). After neuraminidase treatment, Hex A peak sharpened while Hex I peaks switched to higher pI than normal Hex I, in the region of Hex B. PreHex A/S pI was not affected. Hydrolytic properties using the both substrates (4-methylumbelliferyl-2-acetamido-2-deoxy-beta-D-glucopyranoside and 4-methylumbelliferyl-2-acetamido-2-deoxy-beta-D-galactopyranoside) of each molecular form were similar in transformed and non-transformed cells. Data derived from the use of a mixture of substrates were consistent with the model which proposes a common active site for either substrate in the case of preHex A, Hex B and Hex I, but not for Hex A. Thus Epstein-Barr virus-transformed lymphoid cell lines represent an accurate model system for studies on Tay-Sachs disease.

Characterization of the isoenzymes of pig-liver esterase. 1. Chemical Studies

Three different subunits of highly purified pig liver esterase (EC 3.1.1.1) can be separated by analytical dodecyl sulfate electrophoresis, though their relative mobilities are very similar. The same subunit bands are obtained with microsomes, in which the esterases have been labeled with the specific active-site-directed inhibitor bis(4-nitro-[14C]phenyl)phosphate. The heterogeneity of the native trimeric enzyme is much more complex, as is demonstrated by isoelectric focussing and polyacrylamide gel electrophoresis. Fractions of esterase which were partially separated by preparative isoelectric focussing show differences in their subunit composition, their amino acid analyses, their tryptic peptide maps, and their C-terminal amino acids. From these experiments various features of the differing esterase subunits can be deduced. Based on the chemical results and on various experiments which did not indicate any secondary modification of the protein side-chains, the molecular basis of the esterase heterogeneity is discussed. We conclude that the native trimeric esterase is a mixture of numerous hybrids of at least three protein subunits with differing but closely related primary sequences. A comparison of the relative specificity of various preparations of pig liver microsomes indicates that genetic differences concerning the composition of liver esterase exist between individuals.

Subcellular distribution of low- and high-molecular-weight acid phosphatases

Acid phosphatases (orthophosphoric-monoester phosphohydrolases (acid optimum), EC 3.1.3.2) of low and high molecular weight were separated by Sephadex G-75 filtration from extracts of rat brain, liver and kidney. The proportion of each phosphatase in the extract depends critically on the method employed for homogenate preparation, and no interconversion between high and low molecular weight forms was detected. In extracts obtained from subcellular organelles only high-molecular-weight acid phosphatase was detected, which is of lysosomal origin. Low-molecular-weight acid -phosphatase is restricted to the cell sap. Low- and high-molecular-weight acid phosphatases were characterized by their elution volumes, specific inhibition and activity with two substrates. It is suggested that the distribution pattern found om rat tissues could be common to all eukaryotic cells.

Characterization of human liver alpha-D-mannosidase purified by affinity chromatography

Human liver acidic alpha-D-mannosidase was purified 1400-fold by a relatively short procedure incorporating chromatography on concanavalin A-Sepharose and affinity chromatography on Sepharose 4B-epsilon-aminohexanoylmannosylamine. In contrast with the acidic enzymic activity the neutral alpha-mannosidase did not bind to the concanavalin A-Sepharose so the two types of alpha-mannosidase could be separated at an early stage in the purification. The only significant glycosidase contaminant after affinity chromatography on the mannosylamine ligand was alpha-L-fucosidase, which was selectively removed by affinity chromatography on the corresponding fucosylamine ligand. The final preparation was free of other glycosidase activities. The pI of the purified enzyme was increased from 6.0 to 6.45 on treatment with neuraminidase. Although the pI and the mol.wt. (220 000) suggested that alpha-mannosidase A had been purified selectively, ion-exchange chromatography on DEAE-cellulose indicated that the preparation consisted predominantly of alpha-mannosidase B. This discrepancy is discussed in relation to the basis of the multiple forms of human alpha-mannosidase. The purified enzyme completely removed the alpha-linked non-reducing terminal mannose from a trisaccharide isolated from the urine of a patient with mannosidosis. A comparison of the activity of the pure enzyme towards the natural substrate and synthetic substrates suggests that the same enzymic activity is responsible for hydrolysing all the substrates. These results validate the use of synthetic substrates for determining the mannosidosis genotype. They are also further evidence that mannosidosis is a lysosomal storage disease resulting from a deficiency of acidic alpha-mannosidase.

Changes in electronegativity of lysosomal hydrolases during intracellular transport. An isoelectric-focusing study in subcellular fractions of rat kidney

Isoelectric focusing was used to investigate the multiple forms of acid phosphatase, arylsulfatase, beta-glucuronidase, beta-galactosidase and beta-N-acetylhexosaminidase in the following, previously characterized subcellular fractions from rat kidney: a special rough microsomal fraction, enriched up to 9-fold over the homogenate in acid hydrolases; a smooth microsomal fraction; a Golgi membrane fraction enriched about 2.5-fold in acid hydrolases and 10- to 20-fold in several glycosyl transferases; and a lysosomal fraction enriched up to 25-fold in acid hydrolases. The electro-focusing behavior of the hydrolases in these fractions was markedly sensitive to the autolytic changes that occur under acidic conditions, even at 4 degrees C. Autolysis was minimized by extracting fractions in an alkaline medium (0.2% Triton X-100, 0.1 M sodium glycinate buffer, pH 10, 0.1 % p-nitrophenyloxamic acid) and adding p-nitrophenyloxamic acid (0.1 %), AN INHIBITOR OF LYSOSOMAL NEURAMINIDASE AND cathepsin D, to the pH gradient. The enzymes in the lysosomal fraction displayed a characteristic bimodal or trimodal distribution. Arylsulfatase, beta-glucuronidase and beta-N-acetylhexosaminidase occurred in an acidic form with an isoelectric point of 4.4, and a basic form with an isoelectric point of 6.2, 6.7 and 8.0, respectively. Acid phosphatase and beta-galactosidase occurred in an acidic, intermediate and basic form with isoelectric points of about 4. 1, 5.6 and 7.4, respectively. In the special rough microsomal fraction these enzymes were mostly in a basic form with isoelectric points between 7.5 and 9; these were 1-2 units higher than the corresponding basic forms in the lysosomal fraction. Treatment of extracts of the rough microsomal fraction with bacterial neuraminidase raised the isoelectric points of all five hydrolases by 1-2.5 units, indicating the presence of some N-acetylneuraminic acid residues in these basic glycoenzymes. The hydrolases in the Golgi fraction were largely in an acidic form with isoelectric points similar to or lower than those of the corresponding acidic components in the lysosomal fraction. The hydrolases in the smooth microsomal fraction showed isoelectric-focusing patterns intermediate between those in the rough microsomal and the Golgi fractions. These findings support the following scheme for the synthesis, transport and packaging of the lysosomal enzymes. Each hydrolase is synthesized in a restricted portion of the r