Human liver neuraminidase

Comparative enzymology, biochemistry and pathophysiology of human exo-alpha-sialidases (neuraminidases)

This review summarizes the current research on human exo-alpha-sialidase (sialidase, neuraminidase). Where appropriate, the properties of viral, bacterial, and human sialidases have been compared. Sialic acids are implicated in diverse physiological processes. Sialidases, as enzymes acting upon sialic acids, assume importance as well. Sialidases hydrolyze the terminal, non-reducing, sialic acid linkage in glycoproteins, glycolipids, gangliosides, polysaccharides, and synthetic molecules. Therefore, a variety of assays are available to measure sialidase activity. Human sialidase is present in several organs and cells. Its cellular distribution could be cytosolic, lysosomal, or in the membrane. Human sialidase occurs in a high molecular-mass complex with several other proteins, including cathepsin A and beta-galactosidase. Multi-protein complexation is important for the in vivo integrity and catalytic activity of the sialidase. However, multi-protein complexation, the occurrence of isoenzymes, diverse subcellular localization, thermal instability, and membrane association have all contributed to difficulties in purifying and characterizing human sialidases. Human sialidase isoenzymes have recently been cloned and sequenced. Even though crystal structures for the human sialidases are not available, the highly conserved regions of the sialidase from various organisms have facilitated molecular modeling of the human enzyme and raise interesting evolutionary questions. While the molecular mechanisms vary, genetic defects leading to human sialidase deficiency are closely associated with at least two well-known human diseases, namely sialidosis and galactosialidosis. No therapy is currently available for either disease. A thorough investigation of human sialidases is therefore crucial to human health.

A sialidase from the hepatopancreas of the shrimp Penaeus japonicus (Crustacea: Decapoda): reversible binding with the acidic beta-galactosidase

1. The sialidase purified from the hepatopancreas of Penaeus japonicus is able to bind the acidic beta-galactosidase in vitro. No protective protein, Mr 32,000, was detected in either purified enzyme preparation. 2. The specific activity of the isolated sialidase is 55.0 mU/mg of protein. After polyacrylamide gel electrophoresis under denaturing conditions, the purified shrimp enzyme was found to consist of monomers of Mr 32,000. 3. The sialidase from shrimp has an isoelectric point (pI) of 4.6 +/- 0.1. 4. The shrimp enzyme has the pH optimum at 5.0 and its Km was 5.5 microM with 2'-(4-methylumbelliferyl)-alpha-D-N-acetylneuraminic acid as substrate. The enzyme activity was inhibited by either Hg2+ or Cu2+ ions.

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.

Renal neuraminidase. Characterization in normal rat kidney and measurement in experimentally induced nephrotic syndrome

Several lines of evidence suggest that increased neuraminidase activity may be responsible for the loss of glomerular N-acetylneuraminic acid (AcNeu) observed in various glomerular diseases. However, virtually no information is available on the activity of neuraminidase in glomeruli or the potential role of this enzyme in glomerular pathophysiology. Utilizing 2'-(4-methylumbelliferyl)-alpha-D-N-acetylneuraminic acid (4MU-AcNeu) as substrate, we defined optimal assay conditions and characterized neuraminidase activity in glomeruli and, for comparison, in other renal fractions and liver. Neuraminidase activity in glomeruli, cortex and tubules was maximal at pH 4.4. The Km for 4MU-AcNeu was estimated to be 195 microM for glomeruli and 226 microM for cortex. Glomerular neuraminidase was inhibited by AcNeu (90% at 25 mM) and high concentrations of Triton X-100 (26% at 0.5%), but unaffected by CaCl2, EDTA or N-ethylmaleimide (each 1 mM). Neuraminidase activity (nmol/h per mg of protein; mean +/- S.E.M.) in normal rat kidney was: cortex, 14.47 +/- 0.76; medulla, 7.85 +/- 0.64; papilla, 2.64 +/- 0.11; tubules, 13.79 +/- 0.70; glomeruli, 5.57 +/- 0.28. In comparison, neuraminidase activity in rat liver was 2.58 +/- 0.14. Puromycin aminonucleoside (PAN)-induced nephrotic syndrome is a model of glomerular disease in which the loss of glomerular AcNeu is well documented. In two separate studies, we observed no change in the specific activity of neuraminidase in either glomeruli or cortex isolated from rats treated with PAN (15 mg/100 g, intraperitoneally) and killed at either the onset or the peak of proteinuria. Results were similar whether neuraminidase activity was expressed per mg of protein or per microgram of DNA.

Solubilization, stabilization and isoelectric focusing of human liver neuraminidase activity

Homogenate preparations of human liver have been prepared and over 75% of the particulate neuraminidase activity (which comprises approx. 90% of the total activity) has been solubilized using 0.85% (w/v) Triton X-100 in 25 mM phosphate buffer (pH 6.8). The solubilized neuraminidase activity is extremely labile, but can be stabilized for at least 4 weeks at 2-4 degrees C, using 10 mM N-acetylneuraminic acid. Kinetic characterization of homogenate and solubilized supernatant fluid neuraminidase activities indicated comparable pH optimum curves (maximum activity at pH 4.5-4.7) and apparent Km values (0.2-0.4 mM) for the synthetic fluorometric substrate 4-methylumbelliferyl-alpha-D-N-acetylneuraminic acid. Isoelectric focusing has been performed on human liver homogenates and Triton X-100-solubilized neuraminidase activities, and the presence of several forms (4-6) with isoelectric points (pI values) between 4.4 and 5.2 has been demonstrated in both preparations. The similar kinetic and isoelectric focusing properties of the two preparations suggest that the solubilized enzyme activity is representative of the homogenate activity and that the solubilized enzyme is suitable for purification purposes.

Methylumbelliferyl-N-acetylneuraminic acid sialidase in human liver

Human liver sialidase was measured using methylumbelliferyl-N-acetylneuraminic acid as a substrate. The enzyme activity was linear for only 20 min and linearity was not improved by adding albumin, CaCl2, dithiothreitol, or Ep-459. The optimal pH was 4.5 and the apparent Km value, approximately 0.090 mM. Without substrate addition, the enzyme was unstable at temperatures between 0 and 37 degrees C, retaining only 35 and 5% of its activity, respectively, after 81/2 hr, but was protected by albumin at 5 mg/ml. The enzyme was more stable when either total liver or liver homogenate was kept frozen at -20 degrees C. Liver sialidase also retained about 70% of its activity after mechanical homogenization for 5 min. Potential inhibitors, notably, p- aminooxanilic acid, fetuin III, Triton X-100, mucin, sialyllactose, colominic acid, sodium taurocholate, N-acetylneuraminic acid, and methoxyphenyl-N-acetylneuraminic acid, were tested. Sialyllactose, methoxyphenyl-N-acetylneuraminic acid, fetuin, N-acetylneuraminic acid, and colominic acid were competitive inhibitors with Ki values of 1.12, 0.37, 0.20, 0.78, and 0.22 mM, respectively. The 0.11 M solutions of NaCl, LiCl, and KCl inhibited 20-30%, and CaCl2 about 60%, of the enzyme activity.

Neuraminidase activity in middle ear effusions

Analyses of Streptococcus pneumoniae culture filtrates and middle ear effusions (MEE) containing S pneumoniae for various hydrolytic enzymes have demonstrated substantial levels of neuraminidase activity when measured employing a sensitive fluorometric assay. S pneumoniae neuraminidase exhibits optimum activity near neutral pH (6.0 to 6.5), and catalyzes the cleavage of sialic acid residues from glycoproteins, gangliosides and mucopolysaccharides. S pneumoniae begins secreting large amounts of neutral neuraminidase (mean [means] = 43.3 units/mL culture filtrate) when cells enter the stationary phase. Nearly all (96%) human chronic MEEs yielding positive cultures for S pneumoniae contain neuraminidase activity (means = 0.200 units/mg protein), while only 21.1% to 45.5% of all other effusions contain the enzyme. Middle ear effusions obtained from S pneumoniae infected-chinchillas contained large amounts of neuraminidase activity (approximately 200 units/mL), which decayed exponentially in vivo with an apparent half-life of 8 1/2 days. Three neuraminidase isoenzymes (designated I-III) were identified in S pneumoniae culture filtrates, as well as in MEEs from chinchillas infected with the organism, using a combination of ion-exchange and gel filtration chromatography. With 4-methylumbelliferyl-N-acetylneuraminic acid serving as substrate, preparation I from both culture filtrates and MEEs was characterized by a high Michaelis constant (Km), while forms II and III had low Km values. Preferred substrates were orosomucoid and neuramin-lactose; gangliosides, thyroglobulin, and bovine submaxillary mucin were poorer substrates.

Solubilization and affinity chromatography of a sialidase from human liver

A sialidase, acting on gangliosides and mucus glycoproteins (pH optimum 4.0-4.5) was solubilized by 1% Triton X-100 and short ultrasonication from a crude mitochondrial-lysosomal fraction isolated from human liver. The enzyme was enriched over 1000-fold with the aid of affinity chromatography on equine submandibular gland mucin bound to Sepharose and 20mM Tris/HCl buffer, pH 7.5, as eluent. The sialidase exhibited a molecular mass of about 200 000 Da on Sephadex G-200. On analytical gel electrophoresis, an enzymically active protein band of about 70 000 Da was observed. A sialidase acting on sialyllactose remained in the membrane fraction and could not be solubilized.