The ability of purified human gamma-thrombin to stimulate platelet function was related
to its capacity to degrade GP V. Compared to alpha-thrombin, much greater amounts
of gamma- thrombin were required to induce platelet aggregation; and this also applied
to secretion from dense bodies, alpha-granules and lysosomal granules. Platelet stimulation
by gamma-thrombin was additionally characterized by the presence of a lag-phase. Platelets
with 3H-labelled surface glycoproteins showed the same functional response to both alpha-
and gamma-thrombin as unlabelled platelets. But while threshold levels of alpha-thrombin
induced little GP V hydrolysis confirming McGowan et al. (1), amounts of gamma-thrombin
which induced substantial degradation (e. g. 8.3 nM degraded 36% of platelet GP V
in 3 min) were unable to sustain either platelet aggregation or secretion. These results
suggest that protein-binding regions remote from the catalytic site of alpha-thrombin
are more important for platelet activation than GP V hydrolysis. They also provide
further support to the argument that GP V hydrolysis may not be the essential trigger
of platelet activation by thrombin.
Keywords
Platelet aggregation - Platelet secretion - GP V hydrolysis - Human alpha-thrombin
- Human gamma-thrombin
