Studies on neuraminidase from influenza virus A(H3N2) obtained by two procedures

Antibody-free digital influenza virus counting based on neuraminidase activity

There is large demand for a quantitative method for rapid and ultra-sensitive detection of the influenza virus. Here, we established a digital influenza virus counting (DIViC) method that can detect a single virion without antibody. In the assay, a virion is stochastically entrapped inside a femtoliter reactor array device for the fluorogenic assay of neuraminidase, and incubated for minutes. By analyzing 600,000 reactors, the practical limit of detection reached the order of 103 (PFU)/mL, only 10-times less sensitive than RT-PCR and more than 1000-times sensitive than commercial rapid test kits (RIDTs). Interestingly, neuraminidase activity differed among virions. The coefficient of variance was 30-40%, evidently broader than that of alkaline phosphatase measured as a model enzyme for comparison, suggesting the heterogeneity in size and integrity among influenza virus particles. Sensitivity to oseltamivir also differed between virions. We also tested DIViC using clinical gargle samples that imposes less burden for sampling while with less virus titre. The comparison with RIDTs showed that DIViC was largely superior to RIDTs in the sensitivity with the clinical samples although a few false-positive signals were observed in some clinical samples that remains as a technical challenge.

Extraction of catalytically active neuraminidase of H5N1 influenza virus using thrombin proteolytic cleavage

The stalk of influenza neuraminidase (NA) has been a target of cleavage by various proteases, resulting in the release of catalytically active globular heads from virus particles. However, despite successful cases in a number of influenza subtypes, this strategy could not be applied to all influenza viruses due to high variation of the NA stalk. In the present study, reverse genetics was employed to construct non-pathogenic recombinant influenza A viruses, termed rgH1N1(LVPR) and rgH1N1(LVPR-GS), that harbor the NA of H5N1 virus engineered to contain a specific thrombin cleavage site at the stalk region. By using thrombin to cleave NA at its stalk, a productive extraction of NA globular heads could be obtained from purified rgH1N1(LVPR). Furthermore, it was found that the NA of rgH1N1(LVPR-GS) could be cleaved by endogenous thrombin present in embryonated chicken eggs, resulting in the release of NA globular heads into allantoic fluids. These data highlight the use of thrombin cleavage as an effective strategy for extraction of active NA heads directly from live viral particles not only of H5N1 but, theoretically, of any subtype of influenza A viruses.

Dynamic properties of Newcastle Disease Virus envelope and their relations with viral hemagglutinin-neuraminidase membrane glycoprotein

The lipid composition of Newcastle Disease Virus (NDV) Clone-30 strain shows a low lipid/protein ratio, a high cholesterol/phospholipid molar ratio, and major phospholipids being qualitatively different to other NDV strains. The major fatty acyl constituents are palmitic, stearic, oleic, and linoleic acids; cerebrosides, sulfatides and two kinds of gangliosides are also found in the NDV membrane. It is reported for the first time in NDV that phospholipid classes are asymmetrically distributed over the two leaflets of the membrane: 60 +/- 4.5% of the phosphatidylcholine and 70 +/- 5.0% of the sphingomyelin are in the outer monolayer. Intact viral membranes and reconstituted NDV envelopes showed similar dynamic properties. Hemagglutinin-neuraminidase (HN) and fusion (F) proteins of NDV membrane affect the lipid thermotropic behaviour in reconstituted proteoliposomes made up of a single class of phospholipids. It is shown that the lipid composition is more important than the bulk membrane fluidity/order for both sialidase (neuraminidase) and hemagglutinating HN activities. Sialidase and hemagglutinating activities requires the presence of definite phospholipids (phosphatidylethanolamine) in its environment.

A differential scanning calorimetric study of Newcastle disease virus: identification of proteins involved in thermal transitions

The irreversible thermal denaturation of Newcastle disease virus was investigated using different techniques including high-sensitivity differential scanning calorimetry, thermal gel analysis intrinsic fluorescence, and neuraminidase activity assays. Application of a successive annealing procedure to the scanning calorimetric endotherm of Newcastle disease virus furnished four elementary thermal transitions below the overall endotherm; these were further identified as coming from the denaturation of each viral protein. The shape of these transitions, as well as their scanrate dependence, was explained by assuming that thermal denaturation takes place according to the kinetic scheme N-->(k)D, where k is a first-order kinetic constant that changes with temperature, as given by the Arrhenius equation; N is the native state; and D is the denatured state. On the basis of this model, activation energy values were calculated. The data obtained with the other methods used in this work support the proposed two-state kinetic model.

Fusion between Newcastle disease virus and erythrocyte ghosts using octadecyl Rhodamine B fluorescence assay produces dequenching curves that fit the sum of two exponentials

The kinetics of fusion between Newcastle disease virus and erythrocyte ghosts has been investigated with the octadecyl Rhodamine B chloride assay [Hoekstra, De Boer, Klappe, and Wilschut (1984) Biochemistry 23, 5675-5681], and the data from the dequenching curves were fitted by non-linear regression to currently used kinetic models. We used direct computer-assisted fitting of the dequenching curves to the mathematical equations. Discrimination between models was performed by statistical analysis of different fits. The experimental data fit the exponential model previously published [Nir, Klappe, and Hoekstra (1986) Biochemistry 25, 2155-2161] but we describe for the first time that the best fit was achieved for the sum of two exponential terms: A1[1-exp(-k1t)]+A2[1-exp(-k2t)]. The first exponential term represents a fast reaction and the second a slow dequenching reaction. These findings reveal the existence of two independent, but simultaneous, processes during the fusion assay. In order to challenge the model and to understand the meaning of both equation, fusion experiments were carried out under different conditions well known to affect viral fusion (changes in pH, temperature and ghost concentration, and the presence of disulphide-reducing agents or inhibitors of viral neuraminidase activity), and the same computer fitting scheme was followed. The first exponential equation represents the viral protein-dependent fusion process itself, because it is affected by the assay conditions. The second exponential equation accounts for a nonspecific reaction, because it is completely independent of the assay conditions and hence of the viral proteins. An interpretation of this second process is discussed in terms of probe transfer between vesicles.

Increased influenza A virus sialidase activity with N-acetyl-9-O-acetylneuraminic acid-containing substrates resulting from influenza C virus O-acetylesterase action

Influenza virus type C (Johannesburg/1/66) was used as a source for the enzyme O-acetylesterase (EC 3.1.1.53) with several natural sialoglycoconjugates as substrates. The resulting products were immediately employed as substrates using influenza virus type A [(Singapore/6/86) (H1N1) or Shanghai/11/87 (H3N2)] as a source for sialidase (neuraminidase, EC 3.2.1.18). A significant increase in the percentage of sialic acid released was found when the O-acetyl group was cleaved by O-acetylesterase activity from certain substrates (bovine submandibular gland mucin, rat serum glycoproteins, human saliva glycoproteins, mouse erythrocyte stroma, chick embryonic brain gangliosides and bovine brain gangliosides). A common feature of all these substrates is that they contain N-acetyl-9-O-acetylneuraminic acid residues. By contrast, no significant increase in the release of sialic acid was detected when certain other substrates could not be de-O-acetylated by the action of influenza C esterase, either because they lacked O-acetylsialic acid (human glycophorin A, alpha 1-acid glycoprotein from human serum, fetuin and porcine submandibular gland mucin) or because the 4-O-acetyl group was scarcely cleaved by the viral O-acetylesterase (equine submandibular gland mucin). The biological significance of these facts is discussed, relative to the infective capacity of influenza C virus.

Enzyme and hormonal markers in the differential diagnosis of human azoospermia

The serum concentrations of FSH, LH, prolactin, testosterone, and estradiol and the enzymatic activities of hyaluronidase, glucosidases (alpha-glucosidase, beta-glucosidase, alpha-mannosidase, N-acetyl-beta-D-glucosaminidase, beta-glucuronidase, and beta-galactosidase), lactate dehydrogenase and its isoenzymes (LDH1, LDH2, LDH3, LDH-X, LDH4), and total proteins were measured in the semen of 69 subjects (8 normozoospermic controls, 7 secretory, and 54 excretory azoospermic subjects). FSH levels rose with the deterioration in spermatogenesis and served to differentiate the secretory from the excretory azoospermias. The only source of hyaluronidase and LDH-X in the ejaculate is the spermatozoa. alpha-Glucosidase activity essentially originates in the epididymis. The seminal determination of alpha-glucosidase and, to a lesser extent, alpha-mannosidase and N-acetyl-beta-D-glucosaminidase helps rapidly, sensitivity, reliably, and noninvasively to differentiate secretory azoospermias (with higher enzymatic activity) from the excretory type (less enzymatic activity) and may be of use in identifying with a certain degree of reliability the site of obstruction in the male genital tract.

Kinetic studies on the sialidase of three influenza B and three influenza A virus strains

Sialidase of influenza virus type A has been extensively studied through structural and kinetic approaches. However, sialidase of influenza virus type B has been less investigated. In this work, we have studied the activity and some properties (optimal pH, KM, Vmax, thermal stability) of sialidase in three influenza virus strains of type B (circulating in the period 1983-86) and also the activity and properties of sialidase from three virus strains of type A circulating at the same period of time. The results show that the activity and the Vmax was always higher for sialidase of type A viruses relative to those values of type B. Differences were also found for optimal pH and, in some cases, for thermal stability of the sialidase between strains belonging to the influenza viruses type A and B. However, the behaviour for the sialidase in all strains was very similar towards two competitive inhibitors. Thus, it could be suggested that the evolution pattern of the sialidase of both types of influenza viruses determines some modifications which result in a higher efficiency for sialidase of some strains of influenza virus type A, but maintaining in the two types of viruses a similar behaviour towards competitive inhibitors.

Action of influenza virus neuraminidase on gangliosides. Haemagglutinin inhibits viral neuraminidase

The action of partly purified neuraminidase (NA) of influenza A virus, a mixture of detergent solubilized NA and haemagglutinin (HA) and of intact virions on gangliosides GT1b, GD1a, GD1b, GM1 was studied. The viral NA transformed GT1b mainly into GD1b with formation of only minor amounts of GM1. HA was found to inhibit the hydrolysis activity of viral NA. At the same time viral NA transformed GD1a quantitatively into GM1 which was not hydrolyzed by the enzyme. These results suggest that the function of NA is to transfer the 'primary' receptor (such as GT1b) into the proper carbohydrate sequence (GD1b-like) which is proposed to serve as the minimal structure required for influenza virus reception.

A fluorometric procedure for measuring the neuraminidase activity: its application to the determination of this activity in influenza and parainfluenza viruses

A fluorometric procedure for quantitating the amount of N-acetylneuraminic acid enzymatically released by the neuraminidase activity from N-acetylneuraminyl-lactose (sialyl-lactose) has been developed. The liberated lactose is hydrolyzed with beta-galactosidase, and the released galactose is oxidized with galactose dehydrogenase and NAD+; finally, the NADH produced is measured by fluorometry (excitation at 340 nm and analysis of emitted light at 465 nm). The fluorometric assay is about 10-fold more sensitive than the spectrophotometric procedure that measures NADH at 340 nm. It readily measures amounts as little as 2 nmol of sialic acid, and does not require the use of radioactive isotopes. Interferences due to sucrose or other substances, which cause errors in some cases with the use of the periodate-thiobarbiturate method for neuraminidase activity determination, are avoided. The procedure reported here provides a sensitive, rapid, and relatively simple method (feasible with commercialized reagents) for measuring the neuraminidase activity not only in purified samples from different sources but also directly in biological materials such as viruses. The technique has been tested with some viruses recently isolated belonging to Orthomyxoviridae or Paramyxoviridae families, known to be rich in neuraminidase. Reciprocally, this method can also be employed for determining the sialic acid concentration in acylneuraminyl-lactose-containing compounds when using purified neuraminidase for hydrolysis.