Glycosphingolipid β-Galactosidases

Chaperone therapy update: Fabry disease, GM1-gangliosidosis and Gaucher disease

Chaperone therapy is a newly developed molecular therapeutic approach to lysosomal diseases, a group of human genetic diseases causing severe brain damage. Based on early molecular studies during the last decade of the 20th century and early years of the 21st century, mainly on Fabry disease and GM1-gangliosidosis, we found some mutant enzyme proteins were unstable in the cell, and unable to express catalytic activities. Subsequently galactose and other active-site binding substrate analogs were found stabilized and enhance the mutant enzyme activity in culture cells. We concluded that the mutant misfolding enzyme protein and substrate analog competitive inhibitor (chemical chaperone) form a stable complex to be transported to the lysosome, to restore the catalytic activity of mutant enzyme after spontaneous dissociation under the acidic condition. This gene mutation-specific molecular interaction is a paradoxical phenomenon that an enzyme inhibitor in vitro serves as an enzyme stabilizer in situ. First we developed a commercially available compound 1-deoxygalactonojirimycin (DGJ) for Fabry disease, and confirmed the above molecular phenomenon. Currently DGJ has become a new candidate of oral medicine for Fabry disease, generalized vasculopathy involving the kidneys, heart and central nervous system in the middle age. This drug development has reached the phase 3 of human clinical study. Then we found two valienamine derivatives, N-octyl-4-epi-β-valienamine (NOEV) and N-octyl-β-valienamine (NOV), as promising therapeutic agents for human β-galactosidase deficiency disorders (GM1-gangliosidosis and Morquio B disease) and β-glucosidase deficiency disorders (phenotypic variations of Gaucher disease), respectively. Originally NOEV and NOV had been discovered as competitive inhibitors, and then their paradoxical bioactivities as chaperones were confirmed in cultured fibroblasts from patients with these disorders. Subsequently GM1-gangliosidosis model mice have been used for confirmation of clinical effectiveness, adverse effects and pharmacokinetic studies. Orally administered NOEV entered the brain through the blood-brain barrier, enhanced β-galactosidase activity, reduced substrate storage, and improved neurological deterioration clinically. Computational analysis revealed pH-dependent enzyme-chaperone interactions. Our recent study indicated chaperone activity of a new DGJ derivative, MTD118, for β-galactosidase complementary to NOEV. NOV also showed the chaperone effect toward several β-glucosidase gene mutants in Gaucher disease. Furthermore a commercial expectorant drug ambroxol was found to be a chaperone for β-glucosidase. A few Gaucher patients responded to this drug with remarkable improvement of oculomotor dysfunction and myoclonus. We hope chaperone therapy will become available for some patients with Fabry disease, GM1-gangliosidosis, Gaucher disease, and other lysosomal storage diseases particularly with central nervous system involvement.

Molecular basis of GM1 gangliosidosis and Morquio disease, type B. Structure-function studies of lysosomal beta-galactosidase and the non-lysosomal beta-galactosidase-like protein

GM1 gangliosidosis and Morquio B disease are distinct disorders both clinically and biochemically yet they arise from the same beta-galactosidase enzyme deficiency. On the other hand, galactosialidosis and sialidosis share common clinical and biochemical features, yet they arise from two separate enzyme deficiencies, namely, protective protein/cathepsin A and neuraminidase, respectively. However distinct, in practice these disorders overlap both clinically and biochemically so that easy discrimination between them is sometimes difficult. The principle reason for this may be found in the fact that these three enzymes form a unique complex in lysosomes that is required for their stability and posttranslational processing. In this review, I focus mainly on the primary and secondary beta-galactosidase deficiency states and offer some hypotheses to account for differences between GM1 gangliosidosis and Morquio B disease.

Hydrolysis of lactosylceramide by human galactosylceramidase and GM1-beta-galactosidase in a detergent-free system and its stimulation by sphingolipid activator proteins, sap-B and sap-C. Activator proteins stimulate lactosylceramide hydrolysis

Two exo-beta-galactosidases are involved in the lysosomal degradation of glycosphingolipids: GM1-beta-galactosidase (EC 3.2.1.23) and galactosylceramidase (EC 3.2.1.46). Analyses were performed with both enzymes, using lactosylceramides with varying acyl chain lengths as substrates that were inserted into unilamellar liposomes and naturally occurring sphingolipid activator proteins sap-B and sap-C, rather than detergents, to stimulate the reaction. While sap-B was a better activator for the reaction catalyzed by GM1-beta-galactosidase, sap-C preferentially stimulated lactosylceramide hydrolysis by galactosylceramidase. The enzymic hydrolysis of liposome-integrated lactosylceramides was significantly dependent on the structure of the lipophilic aglycon moiety of the lactosylceramide decreasing with increasing length of its fatty acyl chain (C2 > C4 > C6 > C8 > C10 > C18). However, in the presence of detergents the degradation rates were independent of the acyl chain length. Hydrolysis of liposomal lactosylceramide was compared with sap-B-stimulated hydrolysis of liposomal ganglioside GM1 by GM1-beta-galactosidase and sap-C-stimulated degradation of liposomal galactosylceramide by galactosylceramidase. Kinetic and dilution experiments indicated that sap-B forms water-soluble complexes with both lactosylceramide and GM1. These complexes were recognized by GM1-beta-galactosidase as optimal substrates in the same mode, as postulated for the hydrolysis of sulfatides by arylsulfatase A [Fischer, G. and Jatzkewitz, H. (1977) Biochim. Biophys. Acta 481, 561-572]. GM1-beta-galactosidase was more active on these complexes than on glycolipids (GM1 and lactosylceramides) still residing in liposomal membranes. On the other hand, dilution experiments indicated that degradation of galactosylceramide and lactosylceramide by galactosylceramidase proceeds almost exclusively on liposomal surfaces: both activators, sap-C and sap-B, stimulated the hydrolysis of lactosylceramide analogues with long acyl chains more than the hydrolysis of lactosylceramides with short acyl chains.

Chloride binding proteins: mechanistic implications for the oxygen-evolving complex of Photosystem II

Chloride plays a key role in activating the photosynethetic oxygen-evolving complex (OEC) of Photosystem II, but the OEC is only one of many enzymes affected by this anion. Some of the mechanistic features of Cl(-) involvement in water-splitting resemble those of other proteins whose structure and chemistry are known in detail. An overview of the similarities and differences between these Cl(-)-binding systems is presented.

Microassay for GM1 ganglioside beta-galactosidase activity using high-performance liquid chromatography

A simple and sensitive assay for GM1 ganglioside (GM1) beta-galactosidase activity was devised by direct measurement of released D-galactose using high-performance liquid chromatography (HPLC). GM1 beta-galactosidase activity in crude samples such as brain homogenates could be measured by this method. After incubation of brain homogenate for 1 h with GM1 at 37 degrees C and pH 4.4 in the presence of sodium taurodeoxycholate, the reaction was terminated by heating at 100 degrees C for 2 min and the supernatant from the centrifuged sample was analysed directly by HPLC. D-Galactose isolated by HPLC was converted into a fluorescent compound by a post-column reaction with arginine at 150 degrees C and the fluorescence intensity at 430 nm was measured with excitation at 320 nm. By this method 10 pmol of D-galactose could be measured and the fluorescence intensity was linear up to 1 mmol of D-galactose. Using this method, the optimal conditions for the activity of this enzyme were re-examined. As an application, the enzyme activity in the brain of a patient with GM1 gangliosidosis was examined. This method can be applied to any natural substrates, glycolipids or glycoproteins, the terminal galactose of which is hydrolysed by this enzyme.

Properties and kinetics of a neutral beta-galactosidase from rabbit kidney

A neutral beta-galactosidase has been purified by concanavalin A-Sepharose affinity chromatography, DEAE-cellulose chromatography, Sephadex G-200 gel filtration and hydroxylapatite chromatography. The enzyme was purified 126-fold with a yield of about 21%. This form has a neutral optimal pH (7.5) and it is located in the cytosolic fraction. It shows a wide pH stability from pH 4.5 to 8.0, but it is very unstable at low pH values. Its isoelectric point is 4.9 and this value does not change on neuraminidase treatment. The estimated molecular weight was 47 000. The neutral form shows beta-D-galactosidase, beta-D-fucosidase and beta-D-glucosidase activities, all of them associated in a single peak in all the purification steps. p-Nitrophenyl beta-D-galactosides, p-nitrophenyl beta-D-fucosides and p-nitrophenyl beta-D-glucosides competed fully for a common active site in mixed-substrate experiments. Using gamma-D-galactonolactone as competitive inhibitor the Ki values were always coincident for the three activities. The effect of NaCl, methyl mannoside and some sugars (fucose, galactose and glucose) was studied.

A fluorometric microassay procedure for monitoring the enzymatic activity of GM1-ganglioside beta-galactosidase by use of high-performance liquid chromatography

For the measurement of the enzymatic activity of GM1-ganglioside (II3 NeuAcGgOse4Cer, galactosyl-N-acetylgalactosaminyl-(N-acetylneuraminosyl) galactosyl-glucosylceramide) beta-galactosidase in crude enzyme samples, a microassay using nonradioisotopic GM1-ganglioside was devised. To reduce the volume of the reaction mixture and eliminate the interferences due to the fluorescent contaminants in the reaction mixture, NADH, a product after the oxidation of the released galactose with NAD and beta-galactose dehydrogenase, was fluorometrically estimated by use of high-performance liquid chromatography. By this method, as little as 10 pmol of galactose can be detected. Using rat brain homogenates as an enzyme sample, the several parameters were reexamined to define the optimal conditions for the assay. This assay method was also applied to human cultured skin fibroblast homogenates, and it was found that this method can be used for the diagnosis of GM1-gangliosidosis, instead of the usual method using the radioisotope-labeled natural substrate.

Application of a galactosylceramidase microassay method to early prenatal diagnosis of Krabbe's disease

An enzymatic microassay method was devised to determine galactosylceramidase activities of microsamples prepared from primary cultures of amniotic fluid cells. A piece of freeze-dried cell mass (0.5--1 microgram dry weight) prepared from a colony on the bottom plate of a culture flask, was weighed and incubated in 4.02 microliter of the assay mixture. A small amount of galactose hydrolyzed from galactosylceramide was amplified about 10 000-fold and determined by using an enzymatic amplification reaction, NAD cycling. Microsamples were also prepared from freeze-dried leukocytes and skin fibroblasts. The present method was applied to prenatal diagnosis using amniotic cells as a sample in a case of high-risk pregnancy for Krabbe's disease. The result was confirmed by analysis of fibroblasts and organs from the aborted fetus.

GM1 gangliosidosis: enzymatic variation in a single family

Acid beta-galactosidase activity can be separated into multiple molecular forms by isoelectric focusing on cellulose acetate membranes. The residual acid beta-galactosidase in the juvenile form of GM1 gangliosidosis has three bands of enzyme activity with an apparent isoelectric pH (pI) range from 4.9 to 5.2, whereas that in the infantile form has a single band with an apparent pI of 5.2. Separation of residual acid beta-galactosidase into multiple molecular forms by analytical isoelectric focusing demonstrates enzymatic differences that can be correlated with the allelic mutations that affect the GM1 ganglioside beta-galactosidase locus.

Specificity of galactosylceramidase activation by phosphatidylserine

Bovine brain phosphatidylserine effectively activates human brain galactosylceramidase (Hanada, E. and Suzuki, K. (1979) Biochim. Biophys. Acta 575, 410-420). Its effect on the other beta-galactosidase (Gm1-ganglioside beta-galactosidase) in human tissues, genetically distinct from galactosylceramidase, was examined. When partially purified human brain beta-galactosidase preparations, pure with respect to each other, were used as the enzyme source and when lactosylceramide, a common glycosphingolipid substrate for both beta-galactosidases, was used as the substrate, phosphatidylserine activated only hydrolysis of lactosylceramide by galactosylceramidase but not by GM1-ganglioside beta-galactosidase. With either galactosylceramide or lactosylceramide as substrate, and with phosphatidylserine as the activator, diagnosis of globoid cell leukodystrophy was possible using whole homogenates of cultured fibroblasts. Since 80-90% of lactosylceramide-cleaving activity in normal fibroblasts is due to GM1-ganglioside beta-galactosidase and since fibroblasts of globoid cell leukodystrophy patients are genetically deficient in galactosylceramidase but normal in GM1-ganglioside beta-galactosidase, these rsults are also consistent with specific activation of galactosylceramidase by phosphatidylserine.

Activation of human brain galactosylceramidase by phosphatidylserine

Assays of sphingolipid hydrolases in vitro generally require bile salts or other detergents. A few 'activator proteins' have been reported that can partially replace the detergents in the assay mixture. We report here that phosphatidylserine from bovine brain is a relatively specific activator of human brain galactosylceramidase in the absence of sodium taurocholate (phosphatidylserine system). Activity similar to that obtained with the conventional assay system containing taurocholate and oleic acid (taurocholate system) could be obtained. Other lipids tested generally gave less than 10% of the taurocholate system activity, but sulfatide could activate human brain galactosylceramidase to 20--30% of the taurocholate system. The properties of the reaction in the phosphatidylserine system were examined with human brain whole homogenate, crude soluble post-concanavalin A preparations, and partially purified preparations as the enzyme source and compared with those obtained with the taurocholate system. The pH optimum shifted from 4.2 in the taurocholate system to 4.7 in the phosphatidylserine system. The phosphatidylserine system was superior in the linearity of the reaction with respect to the enzyme protein. Reasonably linear Lineweaver-Burk plots could be obtained. The Km values for the phosphatidylserine system were greater than those for the taurocholate system. The effect of phosphatidylserine was not additive to that of taurocholate. Additional phosphatidylserine to the taurocholate system was either without effect at lower concentrations or inhibitory at higher concentrations. The assays of galactosylceramidase with phosphatidylserine and without taurocholate do not necessarily provide pragmatic advantages but offer a potentially useful system with which to study the mechanism of in vivo degradation of the membrane-bound glycosphingolipid.

Lactosyl ceramidosis: deficient activity of neutral beta-galactosidase in liver and cultivated fibroblasts?

Neutral beta-galactosidase was partially purified from liver of normal controls, a patient with Niemann-Pick disease type A and the previously described patient with lactosyl ceramidosis using Concanavalin A-Sepharose adsorption and Sephadex G-100 gel filtration. The partially purified fractions were essentially free of galactosyl ceramide beta-galactosidase and GM1 beta-galactosidase activities. The normal and Niemann-Pick fractions were found to hydrolyze lactosyl ceramide, in the presence of sodium taurodeoxycholate, at a pH optimum of 5.6 as well as aryl beta-galactosides and aryl beta-glucosides at pH 6.2. The corresponding fraction from the lactosyl ceramidosis liver contained only 1--4% of the normal activity towards artificial substrates and lactosyl ceramide. Cross-reacting material identical to the normal was demonstrated in this fraction with antiserum raised against purified neutral beta-galactosidase, but no activity was observed in the precipitin line when stained with naphthol AS-LC-beta-galactoside or naphthol AS-LC-beta-glucoside. A similar deficiency of neutral beta-galactosidase activity was demonstrated in cultivated fibroblasts of the patient with lactosyl ceramidosis. Following adsorption on Concanavalin A-Sepharose and anti-GM1 beta-galactosidase antibody-Sepharose conjugates and chromatography on DEAE cellulose, fibroblast lysates from the patient exhibited 3% of normal activity towards 4-methyl-umbelliferyl beta-glucoside at pH 6.2 and 12% of normal activity towards lactosyl ceramide at pH 5.6. These data suggest that neutral beta-galactosidase may have an in vivo role in the cleavage of lactosyl ceramide and that a deficiency of this activity may be related to the lactosyl ceramide accumulation observed in the patient with lactosyl ceramidosis.

Substrate specificities of the two genetically distinct human brain beta-galactosidases

The two human brain beta-galactosidases were solubilized and fractionated by Sephadex G-200 gel filtration, free from each other. Substrate specificities of the two enzymes were examined for galactosylceramide, lactosyl-[N-stearoyl]ceramide, lactosyl-[N-lignoceroyl]ceramide, galactosyl-N-acetylgalactosaminyl-[N-stearoyl]ceramide, lactosyl-[N-lignoceroyl]ceramide, galactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]galactosyl-glucosylceramide (GMI-ganglioside), galactosyl-N-acetylgalactosaminyl-galactosyl-glucosylceramide (asialo GM1-ganglioside), and 4-methylumbelliferyl beta-galactoside. Under appropriately optimized conditions, either of the two beta-galactosidases could hydrolyze all of the substrates, although with widely varying rates. Relative specific activities of galactosylceramide beta-galactosidase toward galactosylceramide, lactosyl-[N-steroyl]ceramide, lactosyl-[N-lignoceroyl]ceramide. GM1-ganglioside, asialo GM1-ganglioside, and 4-methylumbelliferyl beta-galactoside were 100, 510, 250, 39, 41 and 120, respectively. Relative specific activities of GM1-ganglioside beta-galactosidase toward the same series of the substrates were 0.3, 78, 19, 100, 150 and 240; However, the optimal assay conditions for any given natural substrate were sufficiently different for each beta-galactosidase so that diagnostic assays for the two genetic diseases due to beta-galactosidase deficiencies could be carried out in whole tissues. Since the relative distribution of the two enzymes vary greatly in different tissues, contributions by the two enzymes to degradation of the natural glycosphingolipids in vivo may well vary in different organs. These findings may have an important bearing on the biochemical pathogenesis of these genetic disorders.

Correlation between structural variation and activity of murine kidney beta-galactosidase: implications for genetic control

Two closely linked regulatory genes have been reported to control activity levels of beta-galactosidases in murine tissues. The specific effects of these genes on murine glycolipid metabolism have not been elucidated. A/HeJ kidney 4-methylumbelliferyl-beta-galactosidase exhibited lower thermostability than the corresponding C57BL/6J and SWR/J enzymes. This altered response to heat segregated with the Bgsh allele among progeny derived from backcrosses of F1 (A/HeJ; SWR/J) mice to the respective parental strains. Restriction of the heat-sensitive A/HeJ beta-galactosidase to kidney tissue suggests that it is not determined by the Bgs locus, since the latter appears to be expressed in all tissues. More likely, the Bgs region of chromosome 9 contains a gene cluster consisting of a number of regulatory and structural loci. The proposed structural genes share affinity for the artificial substrates commonly employed for their assay but may differ in their relative affinities for glycosphingolipid substrates. Presence of the Bgsh allele results in an increase of kidney GM1-ganglioside-beta-galactosidase; however, galactosylceramide-beta-galactosidase appears unaffected by this allele.

A rapid method for the estimation of beta-galactocerebrosidase, beta-glucocerebrosidase and sphingomyelinase activities in leukocytes

A method for the assay of leukocyte beta-galactocerebrosidase, beta-glucocerebrosidase and sphingomyelinase activities has been developed, based on the separation of the tritiated sphingolipid substrates from their corresponding radioactive hydrophobic product (ceramide) by thin-layer chromatography on Silica gel H coated microscope slides. For the determination of beta-galactocerebrosidase and beta-glucocerebrosidase activities the silica gel is impregnanted with sodium tetraborate. Each chromatogram is easily divided into two distinct zones and the radioactivity content of each is determined by liquid scintillation counting. The technique described, is rapid, less costly than conventional methods and provides an accurate assessment of sphingolipid hydrolase activity. It is suggested that it should be of considerable value in those areas which require the rapid analysis of large numbers of samples, such as in screening for the sphingolipidoses or for enzyme purification studies.

Effect of bile salts on lactosylceramide beta-galactosidase activities in human brain, liver and cultured skin fibroblasts

The effect of bile salts on the hydrolysis of lactosylcermide by human beta-galactosidases in vitro was studied using cultured skin fibroblasts, liver and brain tissue. The evidence for two distinct enzymes that can catalyze the hydrolysis of lactosylceramide was observed when the bile salt was changed from pure sodium taurocholate to either crude taurocholate, or pure glycodeoxycholate, taurodeoxycholate or taurochenodeoxycholate. Tissues from patients with Krabbe's disease were found to be deficient in lactosylceramide beta-galactosidase activity (lactosylceramidase I) when pure taurocholate was used in the assay. When crude taurocholate was used in the assay, the Krabbe patients appeared to have normal activity for this enzyme. In place of crude taurocholate the pure salts of glycodeoxycholate, taurodeoxycholate and taurochenodeoxycholate worked even better to stimulate the second lactosylceramide beta-galactosidase activity and GM1 gangliosidosis patients exhibiting little if any activity. Therefore, lactosylcermidase I is stimulated by crude taurocholate or pure glycodeoxycholate, taurodeoxycholate and taurochenodeoxycholate. The use of pure bile salts to assay lactosylceramidase I and II will result in better reproducibility for these enzyme activities between laboratories.

Chemical pathology of krabbe's disease. IV. Studies of galactosylceramide and lactosylceramide BETA-galactosidases in brain, white blood cells and aminotic fluid cells

Galactosylceramide beta-galactosidase and lactosylceramide beta-galactosidase activities were investigated in normal human brain, leu-kocytes and amniotic fluid cells. The enzymatic assays were performed on brains from 11 patients with Krabbe's disease, on leukocytes from 16 patients and 18 obligate heterozygotes, and on amniotic fluid cells from 9 foetuses at risk. The brain enzyme was solubilized from a 900 g-100000 g pellet. With this enzyme preparation a profound deficiency of galactosylceramide beta-galactosidase activity in brain, approximately 1% of that in age-matched controls was shown. The lactosylceramide beta-galactosidase activity of brain was also strongly reduced, but not to the same extent as the other beta-galactosidase. Galactosylceramide beta-galactosidase activity in leukocytes from patients with Krabbe's disease was generally less than 5% of that in age-matched controls and there was no overlap between the patients and the obligate heterozygotes. Carrier detection by the leukocyte enzyme was, however, not possible because of considerable overlap between heterozygotes and normal controls. The lactosylceramide beta-galactosidase activity was only moderately reduced in leukocytes, but strongly reduced in cerebral tissue from patients with Krabbe's disease. The changes in the glycolipid pattern of cerebral tissue, recently described by us in patients with Krabbe's disease, offers an explanation to the serious glycolipid beta-galactosidase deficiency in CNS.