Partial purification and characterization of beta-galactosidase from rat brain hydrolyzing glycosphingolipids

Mass spectrometric analysis of mutant mice

Mass spectrometry (MS) has proven to be the preeminent tool for the rapid, high-sensitivity analysis of the primary structure of glycans derived from diverse biological sources including cells, fluids, secretions, tissues, and organs. These analyses are anchored by matrix-assisted laser desorption ionization time of flight (MALDI-TOF) analysis of permethylated derivatives of glycan pools released from the samples, to produce glycomic mass fingerprints. The application of complimentary techniques, such as chemical and enzymatic digestions, GC-MS linkage analysis, and tandem mass spectrometry (MS/MS) utilizing both electrospray (ES) and MALDI-TOF/TOF, together with bioinformatic tools allows the elucidation of incrementally more detailed structural information from the sample(s) of interest. The mouse as a model organism offers many advantages in the study of human biology, health, and disease; it is a mammal, shares 99% genetic homology with humans and its genome supports targeted mutagenesis in specific genes to produce knockouts efficiently and precisely. Glycomic analyses of tissues and organs from mice genetically deficient in one or more glycosylation gene and comparison with data collected from wild-type samples enables the facile identification of changes and perturbations within the glycome. The Consortium for Functional Glycomics (CFG) has been applying such MS-based glycomic analyses to a range of murine tissues from both wild-type and glycosylation-knockout mice in order to provide a repository of structural data for the glycobiology community. In this chapter, we describe in detail the methodologies used to prepare, derivatize, purify, and analyze glycan pools from mouse organs and tissues by MS. We also present a summary of data produced from the CFG systematic structural analysis of wild-type and knockout mouse tissues, together with a detailed example of a glycomic analysis of the Mgat4a knockout mouse.

Biochemical aspects of globoid and metachromatic leukodystrophies

Galactosylceramides and sulfogalactosylceramides are characteristic lipids of the myelin sheath. Two genetically determined leukodystrophies are caused by an inability to enzymically hydrolyze these glycolipids. Thus, a deficiency of galactocerebroside beta-galactosidase results in globoid cell leukodystrophy, whereas a reduced activity of arylsulfatase A is responsible for metachromatic leukodystrophy. Besides these disorders, deficiencies of arylsulfatases A, B, C, and other sulfatases have been shown in a distinct condition called "multiple sulfatase deficiency." All of these disorders are fatal and are characterized by marked demyelination and severe mental retardation. The cause of this demyelination is not known. However, cytotoxic galactosylsphingosine and sulfogalactosylsphingosine have been suggested as the agents responsible for this demyelination. Recent immunological studies have also shown that patients with globoid and metachromatic leukodystrophies contain a mutant galactocerebroside beta-galactosidase and arylsulfatase A, respectively. The mutant enzymes have different kinetic properties compared to the enzymes from normal subjects. However, they can cross-react with antibodies to these enzymes. Since partially purified preparations of galactocerebroside beta-galactosidase and homogeneous arylsulfatase A are now available, the possibility of enzyme replacement therapy in globoid and metachromatic leukodystrophies is discussed.

Comparison of synthetic and natural glucosylceramides as substrate for glucosylceramidase assay

Commercially available [3H]glucosylceramide is derived from spleen tissue of patients with Gaucher's disease. When such tritiated glucosylceramide was diluted with unlabelled glucosylceramide from different sources and used as the substrate for assays of glucosylceramidase, the apparent activities obtained differed drastically. When diluted with synthetic N-stearoyl- or N-lignoceroyl-glucosyldihydrosphingosine, the release of [3H]glucose was 4-5 times greater than when diluted with unlabelled glucosylceramide from Gaucher spleen, with either brain or fibroblast homogenate as the enzyme source. The Ki values of the synthetic substrates were 15-30 times greater than the Km for the natural mixture, indicating much lower affinity of the synthetic substrates for the enzyme. Although the reason for the reduction in affinity could not be identified, caution is required when the commercial [3H]glucosylceramide is to be diluted with unlabelled glucosylceramide.

Monomer-micelle transition of the ganglioside GM1 and the hydrolysis by Clostridium perfringens neuraminidase

The action of Clostridium perfringens neuraminidase on the ganglioside Gm1 tritiated in the ceramide moiety was studied. The rates of hydrolysis of the Gm1 ganglioside were determined from radioactivity in the neutral glycolipid product, which was separated from the substrate on DEAE-Sephadex columns. In order to study the physical state of the substrate in the conditions used in the neuraminidase treatment, the critical micelle concentrations of the Gm1 ganglioside were determined using formation of the triiodide anion in aqueous iodine solution as an indicator. The critical micelle concentrations were also obtained by determining the non-sedimenting radioactivity at different concentrations of the labeled ganglioside per total volume used in ultracentrifugation experiments. In addition, the concentrations of the monomeric ganglioside were concluded from the results of the ultra-centrifugation studies. The increase in the reaction rate of the Gm1 hydrolysis as the function of the substrate concentration was leveled off at 25-28 microM ganglioside. The abrupt change at this concentration is interpreted as reflecting the monomer-micelle transition of the ganglioside in the conditions used (50mM sodium acetate buffer, pH 4.6). The critical micelle concentration was 29 microM on the basis of the triiodide test, and ultracentrifugation revealed the critical micelle concentration 28 microM. The reaction velocity of the hydrolysis was decreased immediately above the critical micelle concentration, and became constant at higher concentrations of the ganglioside. A close correlation to these changes in the reaction rate is suggested to exist in the concentrations of the monomeric Gm1 ganglioside. Saturation of the buffer used in the neuraminidase assays with butanol effected a striking change in the plot of reaction rate versus ganglioside concentration. The reaction rate increased up to 100-110 microM Gm1 ganglioside. The shift of the inflexion point in the rate plot from 25-28 microM to 100-110 microM ganglioside concentration is suggested to be due to a respective change in the critical micelle concentration effected by butanol. N-Acetylneuraminyllactosyl ceramide, lactosyl ceramide and asialo-Gm1 ganglioside had an inhibitory effect on the reaction. In contrast, N-acetylneuraminyllactose, lactose and some other free saccharides were not inhibitory. The results demonstrate that factors other than the saccharide structure must be taken into account when substrate specificity of a glycosidase is studied using competition experiments. It is suggested that the inhibition effected by the glycolipids is due to an increase in the micellar state of the Gm1 ganglioside.

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.