Studies on the neuraminidases of influenza virus. 3. Stimulation of activity by bivalent cations

Influenza viral neuraminidase: the forgotten antigen

Influenza is the most common cause of vaccine-preventable morbidity and mortality despite the availability of the conventional trivalent inactivated vaccine and the live-attenuated influenza vaccine. These vaccines induce an immunity dominated by the response to hemagglutinin (HA) and are most effective when there is sufficient antigenic relatedness between the vaccine strain and the HA of the circulating wild-type virus. Vaccine strategies against influenza may benefit from inclusion of other viral antigens in addition to HA. Epidemiologic evidence and studies in animals and humans indicate that anti-neuraminidase (NA) immunity will provide protection against severe illness or death in the event of a significant antigenic change in the HA component of the vaccine. However, there is little NA immunity induced by trivalent inactivated vaccine and live-attenuated influenza vaccine. The quantity of NA in influenza vaccines is not standardized and varies significantly among manufacturers, production lots and tested strains. The activity and stability of the NA enzyme is influenced by concentration of divalent cations. If immunity against NA is desirable, a better understanding of how the enzymatic properties affect the immunogenicity is needed.

Combining crystallographic information and an aspherical-atom data bank in the evaluation of the electrostatic interaction energy in an enzyme-substrate complex: influenza neuraminidase inhibition

Although electrostatic interactions contribute only a part of the interaction energies between macromolecules, unlike dispersion forces they are highly directional and therefore dominate the nature of molecular packing in crystals and in biological complexes and contribute significantly to differences in inhibition strength among related enzyme inhibitors. In the reported study, a wide range of complexes of influenza neuraminidases with inhibitor molecules (sialic acid derivatives and others) have been analyzed using charge densities from a transferable aspherical-atom data bank. The strongest interactions of the residues are with the acidic group at the C2 position of the inhibitor ( approximately -300 kJ mol(-1) for -COO(-) in non-aromatic inhibitors, approximately -120-210 kJ mol(-1) for -COO(-) in aromatic inhibitors and approximately -450 kJ mol(-1) for -PO(3)(2-)) and with the amino and guanidine groups at C4 ( approximately -250 kJ mol(-1)). Other groups contribute less than approximately 100 kJ mol(-1). Residues Glu119, Asp151, Glu227, Glu276 and Arg371 show the largest variation in electrostatic energies of interaction with different groups of inhibitors, which points to their important role in the inhibitor recognition. The Arg292-->Lys mutation reduces the electrostatic interactions of the enzyme with the acidic group at C2 for all inhibitors that have been studied (SIA, DAN, 4AM, ZMR, G20, G28, G39 and BCZ), but enhances the interactions with the glycerol group at C6 for inhibitors that contain it. This is in agreement with the lower level of resistance of the mutated virus to glycerol-containing inhibitors compared with the more hydrophobic derivatives.

Variation in the divalent cation requirements of influenza A virus N1 neuraminidases

Enzymatic kinetic parameters of influenza A virus N1 neuraminidases (NA) chromatographically purified from several vaccine candidate strains were tested. With ionic strength held constant, Ca2+ or Mg2+ increased the initial rate of enzymatic activity. The 1934 and 1943 strains had statistically significant highest initial velocities, V(max)/K(m) and V(max). There were no significant differences among the influenza virus strains from 1947 to 1991. Measured K(m) for the 1943 strain (6.2 x 10(-5) M) was significantly higher than other strains (3.1-4.7 x 10(-5) M). V(max)/K(m) varied from 0.78 M(-1) s(-1) to 0.91 M(-1) s(-1) and V(max) varied from 3.0 s(-1) to 5.5 s(-1) before the addition of a divalent cation and increased approximately 2-fold for each of these kinetic parameters for each strain after the addition of exogenous Ca2+ or Mg2+. Dialysis reduced the initial velocity and immunogenicity of each strain with significant differences found among strains. Enzymatic activity and immunogenicity were partially restored by the addition of exogenous Ca2+. Nucleic acid sequence analysis could not predict these differences. Selection of vaccine strains must include analysis of antigenic changes, but also enzymatic studies and determination of the requirement of divalent cations to maintain immunogenicity and activity during production.

Glycocalyx of lung epithelial cells

Due to their diversity and external location on cell membranes, glycans, as glycocalyx components, are key elements in eukaryotic cell, tissue, and organ homeostasis. Although information on the lung glycocalyx is scarce, this article aims to review, discuss, and summarize what is known about bronchoalveolar glycocalyx composition, mainly the sialic acids. It was deemed relevant, however, to make a brief introductory overview of the cell glycocalyx and its particular development in epithelial cells. After that, follows a summary of the evolution of the knowledge regarding the bronchoalveolar glycocalyx composition throughout the years, particularly its morphological features. Since sialic acids are located terminally on the bronchoalveolar lining cells' glycocalyx and play crucial roles, we focused mainly on the existing lung histochemical and biochemical data of these sugar residues, as well as their evolution throughout lung development. The functions of the lung glycocalyx sialic acids are discussed and interpretations of their roles analyzed, including those related to the negative overall superficial shield provided by these molecules. The increasing presence of these sugar residues throughout postnatal lung development should be regarded as pivotal in the development and maintenance of a dynamic bronchoalveolar architecture, supporting the normal histophysiology of the respiratory system. The case for a profound knowledge of lung glycocalyx--given its potential to provide answers to serious clinical problems--is made with particular reference to cystic fibrosis. Finally, concluding remarks and perspectives for future research in this field are put forth.

Influenza virus sialidase: effect of calcium on steady-state kinetic parameters

Ca2+ increases the initial rate of activity of sialidase from influenza virus (A/Tokyo/3/67). Increasing ionic strength also activates influenza virus sialidase. When ionic strength is controlled, smaller but still significant Ca2+ effects are observed, with Vmax/Km increased from 0.8.10(5) to 1.4.10(5) M-1 s-1 and Vmax increased from 6.3 to 9.5 s-1 by saturating Ca2+. The Ki of the competitive inhibitor 2,3-dehydro-2-deoxy-N-acetylneuraminic acid was decreased from 2.7.10(-6) to 1.15.10(-6) M after the addition of saturating Ca2+. The data show that Ca2+ exerts a specific effect on Vmax/Km, leading to an increased rate of interaction of substrate with the enzyme. The Kd-app for the Ca2(+)-sialidase complex is 2 mM. Except for Mg2+ which behaves similarly to Ca2+, other mono- and divalent cations have little specific effect on sialidase kinetics. Sequence analysis of a range of subtypes of sialidases from influenza virus supports the proposal that Ca2+ binds at the subunit interface transmitting a conformational change to the enzyme active site. Ca2+ activation may have a physiological role in switching on sialidase activity during the release of newly synthesised virions from the host cell surface.

Enzymological heterogeneity of influenza B virus neuraminidase demonstrated by the fluorometric assay method

The neuraminidase activity of 26 strains of influenza B virus isolated from all over the world was investigated colorimetrically, using fetuin as a substrate, and fluorometrically, using 4-methylumbelliferyl(4-MU)-N-Ac-alpha-D-neuraminide as a substrate, with special reference to enzymological heterogeneity. The activity of influenza A viral neuraminidases and of a commercially available pure viral one was strongly inactivated by either ethylenediaminetetraacetic acid or glycoletherdiaminetetraacetic acid, when measured by the fluorometric assay method, whereas that of influenza B ones was not at all. However, the viral neuraminidases of both influenza A and B viruses were found to be calcium ion-dependent by the colorimetric assay method. A difference in the catalytic rate between the two assay methods was observed with influenza B viral neuraminidase to a much greater extent as compared with that of influenza A. A difference in substrate specificity of these enzymes was demonstrated to be due to a difference in the degree of competitive inhibition by N-acetylneuraminic acid. These findings strongly suggest that enzymological heterogeneity in influenza B viral neuraminidase may be attributed to delicate structural differences, between the enzymes of influenza A and B viruses demonstratable only by the fluorometric neuraminidase assay method using 4-MU-N-Ac-alpha-D-neuraminide as a substrate.

Complete metal ion requirement of influenza virus N1 neuraminidases. Brief report

Influenza virus neuraminidases of the N1 serotype exhibit a complete dependence on Ca++ ions for activity. It is recommended that viral neuraminidase activity routinely be assessed in buffers containing 5 mM CaCl2.

Effect of Ca++ on the stability of influenza virus neuraminidase

The neuraminidases of different strains of influenza virus varied in their stability at 37 degrees C. The enzymes of the strains with N1 neuraminidases were found to be unstable during incubation at 37 degrees C whereas the enzymes of the strains with the N2 neuraminidases were stable. Among the strains with N2 neuraminidases, the enzymes of some strains were inactivated during dialysis at 37 degrees C whereas the enzymes of others were stable. This observed loss of enzyme activity during dialysis at 37 degrees C was not restricted to a single substrate as the same loss of enzyme activity was observed irrespective of the size of the substrate used in the assay. The enzymically inactive neuraminidase was found to be non-antigenic and non-immunogenic. The inactivation of the enzyme could be prevented by the addition of Ca++ but not Mg++. Out results suggest that Ca++ is essential for the stability of the enzyme at 37 degrees C. The results would also suggest that the enzymic, antigenic and immunogenic sites are either the same or very closely situated on the surface of the neuraminidase molecule.

Sialidase-like enzymes produced by group A, B, C, G, and L streptococci and by Streptococcus sanguis

A group of enzymes were prepared from the culture fluids of streptococci belonging to groups A, B, C, G, and L, and from a strain of Streptococcus sanguis. These streptococcal enzymes (designated St-sialidases) released a substance shown to belong to the sialic acid group from the specific substrate BSM-St, a sialomucoid prepared from bovine submaxillary gland. They were inactive on N-acetylneuramin lactose prepared from bovine colustrum and on a sialomucoid prepared from bovine submaxillary mucin, whereas these substances are susceptible to sialidases produced by group K streptococci and by Vibrio cholerae. Some of the St-sialidases were markedly activated by divalent cations, but others showed little response. The heat stability of the enzymes produced by the different strains varied. The optimal pH was between 5.5 and 6.5 with acetate buffer and was about 7 with phosphate buffer. K(m) values were determined for the St-sialidases with BSM-St as substrate.