Heat inactivation of the neuraminidase and haemagglutinin estimated in the agglutination-separation reactions using red blood cells sensitized with Newcastle disease virus.

The agglutination-separation (AS) reactions estimate the effects of heat on the release of altered Newcastle disease virus (NDV) and HN glycoprotein spikes from red blood cells (RBC) sensitized with NDV (SRBC), the inactivations of the neuraminidase (NA), then the haemagglutinin (HA) in a direct assay. Heating SRBC for 1.5 min at 56 degrees C inactivated the NA by 50%; after 4.5 min no separation occurred indicating 100% inactivation of the NA. Heating a suspension of NDV for 78 min inactivated the NA 50% as assayed by cleavage of fetuin. Comparatively, the AS test was up to 52-fold (78 min/1.5 min = 52) more efficient in detecting NA inactivation than was the basic reference test where cleavage of fetuin was assayed. The HA was 50% inactivated after 18 min of heating and 100% inactivated after 36 min as no agglutination was seen. Free HA on SRBC was agglutinated by and thus was titrated with the sialic acid on NRBC. The large area of RBC increased the efficiency of the AS test when compared with tests using suspensions of NDV. At 51-60 degrees C all NA and HA inactivations were sequential, and invariably the NA was more heat labile than the HA. The release of altered NDV and HN spikes was inhibited with mild heat although the separation of SRBC and NRBC continued. Biological purifications showed that the heat stability of the HA and the lability of the NA were genetically stable.