Studies on brain cytosol neuraminidase. II. Extractability, solubility and intraneuronal distribution of the enzyme in pig brain

Cellular localization and substrate specificity of isoelectric forms of human liver neuraminidase activity

The four major isoelectric forms of human liver neuraminidase (with pI values between 3.4 and 4.8) have been isolated by preparative isoelectric focusing and characterized with regard to their substrate specificity using glycoprotein, glycopeptide, oligosaccharide and ganglioside natural substrates. All forms exhibited a rather broad linkage specificity and were capable of hydrolyzing sialic acid glycosidically linked alpha 2-3, alpha 2-6 and alpha 2-8, although differential rates of hydrolysis of the substrates were found for each form. The most acidic form 1 (pI 3.4) was most active on sialyl-lactose, whereas form 2 (pI 3.9) and 3 (pI 4.4) were most active on the more hydrophobic ganglioside substrates. Form 4 (pI 4.8) was most active on the low-Mr hydrophilic substrates (fetuin glycopeptide, sialyl-lactose). Each form was less active on the glycoprotein fetuin than on a glycopeptide derived from fetuin. Organelle-enriched fractions were prepared from fresh human liver tissue and neuraminidase activity on 2'-(4-methylumbelliferyl)-alpha-D-N-acetylneuraminic acid was recovered in plasma membrane, microsomal, lysosomal and cytosolic preparations. Isoelectric focusing of the neuraminidase activity recovered in each of these preparations resulted in significantly different isoelectric profiles (number, relative amounts and pI values of forms) for each preparation. The differential substrate specificity of the isoelectric forms and the different isoelectric focusing profiles of neuraminidase activity recovered in subcellular-enriched fractions suggest that specific isoelectric forms with broad but defined substrate specificity are enriched at separate sites within the cell.

A new chemical procedure for the preparation of gangliosides carrying fluorescent or paramagnetic probes on the lipid moiety

A new chemical procedure is described for preparing labelled GM1 molecular species, carrying as acyl moiety pyrene-decanoic acid, 5-doxyl-stearic acid and 16-doxyl-stearic acid. It makes use of a mixed anhydride formed by ethylchloroformate and the labelled acyl chain, as the reagent for N-acylation of a deacetylated, deacylated GM1 ganglioside, which is prepared by alkaline hydrolysis of natural GM1. The reaction performed with a unitary GM1 derivative/mixed anhydride molar ratio, occurs with a yield of above 40%. The labelled deacetylated GM1 molecular species are then N-acetylated by means of acetic anhydride with quantitative yield. The chemical process of insertion of labelled fatty acid and reconstitution of GM1 ganglioside has been confirmed by GLC-MS and NMR analyses. Fluorescence and electron spin resonance experiments indicate that the labelled gangliosides behave similarly to natural GM1, in both the aggregation properties and the capability to be transferred from micelles to vesicular dispersions of phospholipids.

Interactions of pig brain cytosolic sialidase with gangliosides. Formation of catalytically inactive enzyme-ganglioside complexes

Cytosolic sialidase A was extracted from pig brain and purified about 2000-fold with respect to the starting homogenate (about 550-fold relative to the cytosolic fraction). The enzyme preparation provided a single peak on Ultrogel AcA-34 column chromatography and had an apparent molecular weight of 4 x 10(4). On incubation with micellar ganglioside GT1b, (molecular weight of the micelle, 3.5 x 10(5)) under the conditions used for the enzyme assay, brain cytosolic sialidase A formed two ganglioside-enzyme complexes, I and II, which were isolated and characterized. Complex II had a molecular weight of 4.2 X 10(5), and a ganglioside/protein ratio (w/w) of 4:1. This is consistent with a stoichiometric combination of one ganglioside micelle and two enzyme molecules. Complex I was probably a dimer of complex II. In both complexes I and II cytosolic sialidase was completely inactive. Inactivation of cytosolic sialidase by formation of the corresponding complexes was also obtained with gangliosides GD1a and GD1b, which, like GT1b, are potential substrates for the enzyme and GM1, which is resistant to the enzyme action. Therefore, the enzyme becomes inactive after interacting with ganglioside micelles. GT1b-sialidase complexes acted as excellent substrates for free cytosolic sialidase, as did the complexes with GD1a and GD1b.