Purification of human hexosaminidases A and B by affinity chromatography

Sperm N-acetylglucosaminidase is involved in primary binding to the zona pellucida

The glycosidase-recognizing N-acetylglucosamine terminal residue, N-acetylglucosaminidase (NAG), has been repetitively implicated in fertilization. Nevertheless, its role in the multiple steps comprising this process is a matter of debate because it has been involved in zona pellucida (ZP) binding and penetration and polyspermy block. In this study, the involvement of NAG during sperm interaction with the ZP was analysed. Soluble ZP was able to inhibit sperm NAG activity, suggesting that it can be recognized as a ligand by this enzyme. Sperm-ZP binding assays were carried out under conditions where acrosome reaction (AR) could not take place (salt-stored oocytes and a modified medium where Ca(2+) was replaced by Sr(2+)). Different NAG-specific reagents-an inhibitor (2-acetamido-2-deoxy-D-glucono-1,5-lactone), a substrate (p-nitrophenyl-N-acetylglucosaminide) and an anti-NAG antibody-were able to impair sperm binding to the ZP when present during these assays. The lactone was also able to inhibit oocyte penetration during IVF assays, although not when present after primary binding had taken place. This result was not related to the interference of lactone with AR or zona penetrability. Exogenous NAG also inhibited sperm-oocyte interaction when present during binding and IVF assays or used for oocyte pre-incubation. These results suggest the participation of NAG in sperm primary binding to the ZP.

Evidence for the participation of beta-hexosaminidase in human sperm-zona pellucida interaction in vitro

Mammalian sperm-zona pellucida (ZP) interaction is mediated by sperm lectin-like proteins and ZP glycoproteins. We have previously reported the participation of binding sites for N-acetylglucosamine (GlcNAc) residues in human sperm function, including sperm interaction with the ZP. Additionally, previous results from our laboratory suggested that some of these events may be mediated by the glycosidase N-acetylglucosaminidase (beta-hexosaminidase, Hex, in mammals). In this study, we report the possible participation of Hex in human sperm-ZP interaction. Human recombinant Hex (hrHex) was obtained by expression in a stable transfected CHO cell line. When the recombinant enzyme was present during hemizona (HZ) assays, the number of sperm bound per HZ was significantly reduced. The same result was obtained when HZ were preincubated with hrHex. Additionally, the presence of a Hex-specific substrate during the HZ assay produced the same inhibitory effect. These results suggest the participation of a sperm Hex in the interaction with human ZP in vitro.

The GM2 activator protein, its roles as a co-factor in GM2 hydrolysis and as a general glycolipid transport protein

Although there is only one documented function carried out by the GM2 activator protein in the lysosome, new information suggests that other less obvious roles may also be played by this protein in vivo. This information includes data demonstrating that the GM2 activator is a secretory, as well as a lysosomal protein, and that cells possess a carbohydrate-independent mechanism to re-capture the activator, with or without bound lipid, from the extracellular fluid. Additionally the GM2 activator has been shown to bind, solubilize and transport a broad spectrum of lipid molecules, such as glycolipids, gangliosides and at least one phosphoacylglycerol, between liposomes. At pH 7 the GM2 activator's rate of lipid transport is reduced by only 50% from its maximum rate which is achieved at approx. pH 5, suggesting that the GM2 activator may serve as a general intra- and/or inter-cellular lipid transport protein in vivo. Since the late 1970s the lysosomal form of the GM2 activator has been known to act as a substrate-specific co-factor for the hydrolysis of GM2 ganglioside by beta-hexosaminidase A. Gangliosides are a class of negatively charged glycolipids particularly abundant in neuronal cells which have been linked to numerous in vivo functions, such as memory formation and signal transduction events. Deficiency of the GM2 activator protein results in the storage of GM2 ganglioside and severe neurological disease, the AB-variant form of GM2 gangliosidosis, usually culminating in death before the age of 4 years. The exact mode-of-action of the GM2 activator in its role as a co-factor, and its specificity for various glycolipids are currently matters of debate in the literature.

Isolation of N-acetyl-beta-hexosaminidase from Acanthamoeba castellanii

The lysosomal enzyme N-acetyl-beta-hexosaminidase (beta hex) has been purified from Acanthamoeba castellanii growth medium by a three step procedure. The enzyme was precipitated with ammonium sulfate, partially purified on a DE52 column and purified to homogeneity on an affinity column. The purified beta hex appeared to be a monomer with a molecular mass of 58 kDa and a pI of approximately 5.8. The enzyme activity in growth medium at RT was stable for several months. The purified beta hex was enzymatically deglycosylated and injected into two rabbits to make polyclonal antibodies. One antiserum was specific for beta hex, but the other stained many bands on immunoblots of whole cell preparations. Using fluorescently labelled secondary antibodies we have determined that both antisera stain digestive vacuoles in the Acanthamoeba cytoplasm, and do not stain the contractile vacuole. The multi-specific antiserum had high avidity for beta hex, but also stained the carbohydrate portion of other molecules. These other molecules may be lysosomal enzymes as well, since the activity of several other lysosomal enzymes was partially immunoprecipitable with the antiserum. We plan to use these antibodies to study traffic patterns among the variety of vacuolar structures in Acanthamoeba cytoplasm.

Localization of the pro-sequence within the total deduced primary structure of human beta-hexosaminidase B

The beta subunit of beta-hexosaminidase (beta-N-acetylhexosaminidase, EC 3.2.1.52) is synthesized in the rough endoplasmic reticulum as a prepropolypeptide. After the loss of the signal peptide and formation of an enzymatically active dimer, the pro-enzyme is either secreted from the cell or transported into the lysosome for processing to its mature form. In order to characterize the early posttranslational events we have purified nearly 1 mg of pro-hexosaminidase B from the NH4Cl containing medium of fibroblasts derived from a patient with the infantile form of Tay-Sachs disease. The partial N-terminal sequence was mapped to a position 42 residues C-terminal to the first in-frame ATG (Met residue) and 79 residues N-terminal to the known mature N-terminus. This position corresponds to that predicted for the cleavage of a 17 amino acid signal peptide generated through the use of the third rather than the first in-frame ATG as the initiation site for protein synthesis.

Altered molecular size of N-acetylglucosamine 1-phosphotransferase in I-cell disease and pseudo-Hurler polydystrophy

The size of the mutant N-acetylglucosamine 1-phosphotransferase in Golgi membranes from fibroblasts of patients with I-cell disease and classical pseudo-Hurler polydystrophy, which comprised one complementation group characterized by deficiency towards both artificial and natural acceptor substrates, was significantly smaller than the normal enzyme, 151-174 kDa compared with 225-278 kDa. The size of the mutant enzyme from cell lines of patients with variant forms of pseudo-Hurler polydystrophy, which comprised another complementation group characterized by normal activity towards mono- and oligo-saccharide substrates, was significantly larger than the normal enzyme, ranging from 321 to 356 kDa in two families and from 528 to 547 kDa in a third family. These findings suggest that the mutations in I-cell disease and classical pseudo-Hurler polydystrophy result in a missing enzyme component, which renders the enzyme catalytically inefficient toward any type of acceptor substrate. In contrast, the mutations in the variant forms of pseudo-Hurler polydystrophy produce a larger enzyme molecule which is active toward small substrates but is incapable of binding natural lysosomal glycoprotein substrates.

Human biochemical genetics of enzyme proteins in the new age of molecular genetics

Advances in protein biochemistry and immunology have had a major impact on the biochemical and genetical analysis of human proteins and have had applications in the analysis of the primary defects in metabolic disorders, as well as in cDNA cloning. The development and expansion of somatic cell genetic techniques has complemented conventional population and family study genetic methods. A large number of mammalian proteins undergo complex processing to achieve the synthesis of the biologically active protein. Much of this processing is under genetic control. Elucidation of these complexities requires a combination of biochemical, immunological and genetical approaches to determine the nature of the events involved.

Purification and properties of bovine mammary gland N-acetyl-beta-D-glucosaminidase

Two forms of N-acetyl-beta-D-glucosaminidase were purified from bovine mammary gland by DEAE-cellulose chromatography, Sephadex G-200 gel filtration, affinity chromatography on Con A-Sepharose and preparative isoelectric focusing. The two forms, designated A and B on the basis of their binding to DEAE-cellulose at pH 7, were glycoproteins with different molecular weights as determined by gel filtration and sedimentation equilibrium analysis. The A form had a molecular weight of 118 000, while the B form had a molecular weight of 234 000. Both A and B forms of the purified enzyme showed the presence of two distinct subunits, having apparent molecular weights of 55 000 and 25 000 as determined by sodium dodecyl sulphate-electrophoresis. Amino acid composition of the purified forms showed that a high degree of similarity existed between the two forms. However, the B form had slightly higher levels of serine and threonine than the A form. The structure and possible interrelationship of these two forms in the bovine mammary gland are discussed in relation to the structure of N-acetyl-beta-D-glucosaminidase from other sources.

High-capacity method for purification of human liver hexosaminidase B using hydrophobic chromatography

Hydrophobic chromatography was investigated as a purification procedure for human liver hexosaminidases. Both phenyl-Sepharose and valine-Sepharose have a high binding capacity of hexosaminidases. A degree of resolution between the A and B ioszymes is achieved with phenyl-Sepharose. Both hydrophobic supports must be used close to their capacity in order to recover the applied enzyme. Two purification procedures for human liver hexosaminidase B were employed, which resulted in recoveries of approximately 48 and 24% with final specific activities of 33,400 and 4840 nmole/min.mg, respectively.

Biochemistry and genetics of gangliosidoses

The gangliosidoses comprise an-ever increasing number of biochemically and phenotypically variant diseases. In most of them an autosomal recessive inherited deficiency of a lysosomal hydrolase results in the fatal accumulation of glucolipids (predominantly in the nervous tissue) and of oligosaccharides. The structure, substrate specificity, immunological properties of and genetic studies on the relevant glycosidases, ganglioside GM1 beta-galactosidase and beta-hexosaminidase isoenzymes, are reviewed in this paper. Contrary to general expectation, only a poor correlation is observed between the severity of the disease and residual activity of the defective enzyme when measured with synthetic or natural substrates in the presence of detergents. For the understanding of variant diseases and for their pre- and postnatal diagnosis, the necessity of studying the substrate specificity of normal and mutated enzymes under conditions similar to the in vivo situation, e.g., with natural substrates in the presence of appropriate activator proteins, is stressed. The possibility that detergents may have adverse affects on the substrate specificity of the enzymes is discussed for the beta-hexosaminidases. The significance of activator proteins for the proper interaction of lipid substrates and water-soluble hydrolases is illustrated by the fatal glycolipid storage resulting from an activator protein deficiency in the AB variant of GM2-gangliosidosis. Recent somatic complementation studies have revealed the existence of a presumably post-translational modification factor necessary for the expression of ganglioside GM1 beta-galactosidase activity. This factor is deficient in a group of variants of GM1-glangliosidosis. Among the possible reasons for the variability of enzyme activity levels in heterozygotes and patients, allelic mutations, formation of hybrid enzymes, and the existence of patients as compound heterozygotes are discussed. All these may result in the production of mutant enzymes with an altered specificity for a variety of natural substrates.

Stabilization of human beta-D-N-acetylhexosaminidase A towards proteolytic inactivation by coupling it to poly(N-vinylpyrrolidone)

Human hexosaminidase A was covalently bound to soluble poly(N-vinylpyrrolidone), and the effect of this binding on the enzyme inactivation by various procedures was investigated. Whereas the polymer-bound hexosaminidase underwent inactivation to the same extent as the free enzyme, when exposed to heat or acidic pH, the conjugation to polymer appeared to protect the enzyme towards proteolysis. Thus, the polymer-bound enzyme exhibited considerably higher resistance to treatment of both pronase and macrophage cathepsins. The clearance rate from rabbit blood, of the polymer-bound enzyme (expressed as enzyme activity), was shown to be significantly slower than that of the free enzyme.

The purification and properties of a beta-N-acetylhexosaminidase from Trichomonas foetus

A beta-N-acetylhexosaminidase was purified 800-fold from extracts of Trichomonas foetus by affinity chromatography on a column of N-(epsilon-aminohexanoyl)-2-acetamido-2-deoxy-beta-D-glucopyranosylamine bound to CNBr-activated Sepharose. The enzyme has a dual specificity for the p-nitrophenyl beta-D-glycosides of N-acetylglucosamine and N-acetyl-galactosamine. The parent sugars are both competitive inhibitors. The enzyme has a mol. wt. approx. 150000 and a pH optimum of 6.2. It is suggested that the same active site catalyses both activities and that no part is played by the 4-hydroxyl group in substrate binding, but it is involved in determining the catalytic rate.

Specific determination of N-acetyl-beta-D-hexosaminidase isozymes A and B by radioimmunoassay and radial immunodiffusion

The two major isozymes of N-acetylhexosaminidase, namely hexosaminidases A and B were quantitatively determined in tissues and biological fluids of both normal individuals and Tay-Sachs patients. The determination was carried out by two sensitive immunoassays:radial immunodiffusion, using chromogenic substrate, and radioimmunoassay, which were developed in this study. For this purpose [corrected] we used either a cross-reactive antiserum which reacts to a similar extent with both isozymes, or an antiserum reacting exclusively with hexosaminidase A (obtained by selective immunoadsorption). This enabled the quantitisation of the two isozymes separately, or in the presence of each other, in purified enzyme preparations or in tissue homogenates, affording a direct positive determination of hexosaminidase A. The results demonstrated that normal tissues contain the two isozymes in comparable amounts, whereas tissues of Tay-Sachs patients lack hexosaminidase A or any material which carries the A-specific antigenic determinants. The possible applications of these assays and their potential use in diagnosis are discussed.

Immunoaffinity chromatography of human beta-hexosaminidase A

A highly specific method for the purification of human beta-hexosaminidase A employing immunoaffinity chromatography is described. Using an antiserum against the unique antigenic determinant, alpha, of beta-hexosaminidase A, and elution with 8.0M urea, a 283-or 417-fold purification of the enzyme was obtained in a single step from crude human placental homogenate.