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 V(max)/K(m) increased from 0.8 . 10(5) to 1.4 . 10(5) M-1 s-1 and V(max) increased from 6.3 to 9.5 s-1 by saturating Ca2+. The K(i) 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 V(max)/K(m), leading to an increased rate of interaction of substrate with the enzyme. The K(d-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.