Enzymatic hydrolysis of sphingolipids. VI. Hydrolysis of ceramide glycosides by calf brain beta-N-acetylhexosaminidase

Beta-N-acetylhexosaminidase: a target for the design of antifungal agents

This review provides biochemical, analytical, and biological background information relating to beta-N-acetylhexosaminidase (HexNAc'ase; EC 3.2.1.52) as an emerging target for the design of low-molecular-weight antifungals. The article includes the following: (1) a biochemical description of HexNAc'ase (reaction catalyzed, nomenclature, and mechanism of action) that sets it apart from other, similar enzymes; (2) an overview and a critical evaluation of methods to assay the enzyme, including in crude extracts (photo- and fluorometric procedures with model substrates; HPLC/pulsed amperometric detection of N-acetylglucosamine and chito-oligomers; end-point vs. rate measurements); (3) a summary of some general characteristics of HexNAc'ases from fungi and organisms of other types (Km values, substrate preference, and glycoconjugation); (4) an hypothesis of a specific target function of wall-associated HexNAc'ase (a component of the assembly of surface-located enzymes effecting a continuous turnover and remodelling of the wall fabric through its combined hydrolytic and transglycosylating activities, and a mediator enzyme acting in concert with chitinase and chitin synthase to provide for the controlled lysis and synthesis of chitin during growth); (5) a tabulation of the structural formulae of reaction-based HexNAc'ase inhibitors with Ki values < or = 100 microM (some of them representing transition state mimics that could serve as leads for the development of new antifungals); and (6) an outline of approaches towards the establishment of a three-dimensional model of HexNAc'ase suitable for a truly rational design of antimycotics as well as agricultural fungicides.

Purification and properties of beta-N-acetylhexosaminidase A from pig brain

1. Adult pig brain beta-N-acetylhexosaminidase was separated into four different forms by ion exchange chromatography on diethylaminoethyl-cellulose. 2. Form A was purified 1300-1500 fold by an unusual procedure, the technique of ampholyte displacement, followed by chromatography on concanavalin A Sepharose. 3. The enzyme catalyses the hydrolysis of both beta-N-acetylglucosaminides and beta-N-acetylgalactosaminides. 4. The kinetic studies support the evidence of the association of both activities to a single protein, and at the same active site. 5. A natural substrate, N,N'-diacetylchitobiose, is also hydrolyzed, but not ovalbumin. 6. This enzyme may be considered as an exoglycosidase.

Separation and characterization of four forms of beta-N-acetylhexosaminidase from chicken brain

Chicken brain beta-N-acetylhexosaminidases from embryos (16 and 21 days old), newborns (1 and 4 days old), and adults (3 1/2 months and 2 years old) were separated into four different forms by ion exchange chromatography on diethylaminoethyl-cellulose. Three of these forms were "acid" hexosaminidases (I, IIA, and IIB), and the fourth was a "neutral" form. Throughout development of the chicken, forms IIA and III maintained the same activity ratio, whereas that for form I decreased and that for form IIB showed an increase.

Kinetic studies of four enzyme forms of beta-N-acetylhexosaminidase from embryonic chicken brain

The separation of four different hexosaminidase forms from embryonic chicken brain (16-day-old) has been performed by ion-exchange chromatography. Two different DEAE-cellulose columns have been used: a first one at pH 7.2 and a second one at pH 6.0. Km and Vmax values were estimated from the Lineweaver-Burk or Dixon plots and ki from the Dixon plots, using N-acetyl-D-glucosamine or N-acetyl-D-galactosamine as inhibitors. In both cases we found a kind of competitive inhibition in which Lineweaver-Burk and Dixon plots curve downwards.

Properties of lysosomal beta-hexosaminidase accumulated in Niemann-Pick mouse liver

Various lysosomal acid hydrolases from tissues of Niemann-Pick mice, a mutant strain of C57BL/KsJ mice (spm/spm), were examined and compared to those from control mice. Activities of beta-hexosaminidase, beta-galactosidase, acid phosphatase, and cathepsin L were elevated in the liver and spleen of the affected mice, whereas no significant changes in beta-glucosidase and acid alpha-glucosidase were observed. Alpha-Mannosidase and neutral alpha-glucosidase activities were rather decreased in the affected mouse liver. The level of beta-hexosaminidase in the Niemann-Pick mice was raised sixfold in the liver and two- to threefold in the spleen and brain, whereas its total activity was decreased in the kidney. Sixty to ninety percent of total activity of lysosomal hydrolases was solubilized with 0.1% Triton X-100 in control mice, but most of the beta-hexosaminidase activity of the Niemann-Pick mice remained associated with the membrane fraction of liver lysosomes. The beta-hexosaminidase of the Niemann-Pick mice was appreciably stable when heated at 55 degrees C, while hydrolases of the affected mice and all of the enzymes tested in control mice were heat labile. The relative content of two beta-hexosaminidase fractions separated by DEAE-cellulose column chromatography was 8% for beta-hexosaminidase I and 92% for beta-hexosaminidase II in the case of the control mouse liver. The isozyme pattern of hexosaminidases in Niemann-Pick mice was similar to that of control enzymes. However, the beta-hexosaminidase II accumulated in Niemann-Pick mouse liver was different from that of the control in optimum pH, Km values and thermostability.

Significance of glycosidases and phosphatases in Cercopithecus and Macaca cells

When selected ratios of different glycosidases and phosphatases from primary monkey kidney cells or from monkey kidney cell lines are presented graphically, characteristic patterns do evolve. Three different subtypes of Vero cells show similar glycosidase patterns. The Vero subtypes tested show glycosidase patterns that are closely similar to those of primary cells of Cercopithecus aethiops. Glycosidase patterns of BS-C-1 and CV-1 cells are less similar to those of primary Cercopithecus cells than are those of Vero cells. Primary kidney cells from Macaca cynomolgus show significantly different glycosidase patterns compared with those of different Cercopithecus cells. The distinct glycosidase patterns can be used to classify the tested cell lines in relation to each other.

Lipids of nervous tissue: composition and metabolism

As indicated in the Introduction, the many significant developments in the recent past in our knowledge of the lipids of the nervous system have been collated in this article. That there is a sustained interest in this field is evident from the rather long bibliography which is itself selective. Obviously, it is not possible to summarize a review in which the chemistry, distribution and metabolism of a great variety of lipids have been discussed. However, from the progress of research, some general conclusions may be drawn. The period of discovery of new lipids in the nervous system appears to be over. All the major lipid components have been discovered and a great deal is now known about their structure and metabolism. Analytical data on the lipid composition of the CNS are available for a number of species and such data on the major areas of the brain are also at hand but information on the various subregions is meagre. Such investigations may yet provide clues to the role of lipids in brain function. Compared to CNS, information on PNS is less adequate. Further research on PNS would be worthwhile as it is amenable for experimental manipulation and complex mechanisms such as myelination can be investigated in this tissue. There are reports correlating lipid constituents with the increased complexity in the organization of the nervous system during evolution. This line of investigation may prove useful. The basic aim of research on the lipids of the nervous tissue is to unravel their functional significance. Most of the hydrophobic moieties of the nervous tissue lipids are comprised of very long chain, highly unsaturated and in some cases hydroxylated residues, and recent studies have shown that each lipid class contains characteristic molecular species. Their contribution to the properties of neural membranes such as excitability remains to be elucidated. Similarly, a large proportion of the phospholipid molecules in the myelin membrane are ethanolamine plasmalogens and their importance in this membrane is not known. It is firmly established that phosphatidylinositol and possibly polyphosphoinositides are involved with events at the synapse during impulse propagation, but their precise role in molecular terms is not clear. Gangliosides, with their structural complexity and amphipathic nature, have been implicated in a number of biological events which include cellular recognition and acting as adjuncts at receptor sites. More recently, growth promoting and neuritogenic functions have been ascribed to gangliosides. These interesting properties of gangliosides wIll undoubtedly attract greater attention in the future.(ABSTRACT TRUNCATED AT 400 WORDS)

Purification and properties of the hexosaminidase A-activating protein from human liver

Human liver extracts contain an activating protein which is required for hexosaminidase A-catalysed hydrolysis of the N-acetylgalactosaminyl linkage of G(M2) ganglioside [N-acetylgalactosaminyl-(N-acetylneuraminyl) galactosylglucosylceramide]. A partially purified preparation of human liver hexosaminidase A that is substantially free of G(M2) ganglioside hydrolase activity is used to assay the activating protein. The proceudres of heat and alcohol denaturation, ion-exchange chromatography and gel filtration were used to purify the activating protein over 100-fold from crude human liver extracts. When the purified activating protein is analysed by polyacrylamide-gel disc electrophoresis, two closely migrating protein bands are seen. When purified activating protein is used to reconstitute the G(M2) ganglioside hydrolase activity, the rate of reaction is proportional to the amount of hexosaminidase A used. The activation is specific for G(M2) ganglioside and and hexosaminidase A. The activating protein did not stimulate hydrolysis of asialo-G(M2) ganglioside by either hexosaminidase A or B. Hexosaminidase B did not catalyse hydrolysis of G(M2) ganglioside with or without the activator. Kinetic experiments suggest the presence of an enzyme-activator complex. The dissociation constant of this complex is decreased when higher concentrations of substrate are used, suggesting the formation of a ternary complex between enzyme, activator and substrate. Determination of the molecular weight of the activating protein by gel-filtration and sedimentation-velocity methods gave values of 36000 and 39000 respectively.

The tissue distribution of hexosaminidase S and hexosaminidase C

The proportion of hex S to hex C in normal and Sandhoff's fibroblasts was determined to be between 1:1 and 1:2 by differential staining of hex S at pH 4.4 with 4-methylumbelliferyl-beta-N-acetylgalactosaminide and of hex C at pH 7.0 with 4-methylumbelliferyl-beta-N-acetylglucosaminide. Hex S and hex C were also semi-quantitated in various normal tissues--brain, liver, spleen, heart, kidney, intestine, placenta, skeletal muscle and fibroblasts. Hex C was most prominent in brain and, somewhat less so, in liver, skeletal muscle and fibroblasts. The greatest amount of hex S activity was found in fibroblast, but it was also observed in lesser amounts in liver, kidney, intestine and placenta.

The separation of bovine brain beta-N-acetyl-D-hexosaminidases. Abnormal gel-filtration behaviour of beta-N-acetyl-D-glucosaminidase C

Bovine brain tissue was extracted and the 50 000g supernatant was separated by electrophoresis, DEAE-Sephadex chromatography and gel filtration on Sephadex G-200 and Bio-Gel P-200. The electrophoretic separation showed that the beta-N-acetyl-D-hexosaminidases (hexosaminidases) of bovine brain tissue were composed of four different fractions. Two fractions (A and B) exerted both glucosaminidase and galactosaminidase activity, a third fraction (C) showed only glucosaminidase activity, whereas a fourth form (D) with specificity towards the galactosaminide moiety was found to be present. DEAE-Sephadex chromatography at pH 7.0 showed that the B form was eluted with the void volume, whereas the A and D forms could be eluted in one peak by raising that salt concentration. The C form could not be detected in the eluate. Gel filtration on Sephadex G-200 showed that the B, A and D forms had almost equal molecular weights. In this case also the C form could not be detected in the column eluates. Gel filtration on Bio-Gel P-200 revealed that the C form was eluted with the void volume.

The metabolism of Tay-Sachs ganglioside: catabolic studies with lysosomal enzymes from normal and Tay-Sachs brain tissue

The catabolism of Tay-Sachs ganglioside, N-acetylgalactosaminyl- (N-acetylneuraminosyl) -galactosylglucosylceramide, has been studied in lysosomal preparations from normal human brain and brain obtained at biopsy from Tay-Sachs patients. Utilizing Tay-Sachs ganglioside labeled with (14)C in the N-acetylgalactosaminyl portion or (3)H in the N-acetylneuraminosyl portion, the catabolism of Tay-Sachs ganglioside may be initiated by either the removal of the molecule of N-acetylgalactosamine or N-acetylneuraminic acid. The activity of the N-acetylgalactosamine-cleaving enzyme (hexosaminidase) is drastically diminished in such preparations from Tay-Sachs brain whereas the activity of the N-acetylneuraminic acid-cleaving enzyme (neuraminidase) is at a normal level. Total hexosaminidase activity as measured with an artificial fluorogenic substrate is increased in tissues obtained from patients with the B variant form of Tay-Sachs disease and it is virtually absent in the O-variant patients. The addition of purified neuraminidase and various purified hexosaminidases exerted only a minimal synergistic effect on the hydrolysis of Tay-Sachs ganglioside in the lysosomal preparations from the control or patient with the O variant of Tay-Sachs disease.