Functional Characterization of Membrane Sialyltransferase Activity of Human Platelets

To evaluate the role of membrane sialyltransferase in the initiation of platelet aggregation, we studied the stimulatory effect of epinephrine and adenosine diphosphate and the inhibitory effect of aspirin on the platelet surface sialyl transferase activity. The enzyme activity was assayed under optimal conditions as determined previously. The assay mixture consisted of intact washed human platelets, CMP-¹⁴C-sialic acid, desialated fetuin, Mn²⁺ and buffer to a final volume of 1 ml. The enzyme activity was enhanced to 172% of control by 1μH, 152% by 5μM and 146% by 10μM epinephrine. Adenosine diphosphate enhanced the enzyme activity to a lesser extent:103% at 1μM and 113% at 5μM. In contrast, aspirin inhibited the enzyme activity to 46% of control when 10μg/ml of aspirin was used. Higher concentrations of aspirin failed to cause further inhibition. In the in-vivo experiment, 600 mg aspirin was given to normal subjects and the surface enzyme activity was determined 12 hours later. The enzyme activity reduced to 43% following aspirin administration. Furthermore, we studied the enzyme activity in a patient with “aspirin-like” release disorder. While the mean surface enzyme activity of 10 normal subjects was 1.56 + 0.21 (S. D.) pmole-hr^-1 per 10⁸ platelets, the enzyme activity of the patient was only 0.91 pmole.hr^-l. The results strongly suggest that the membrane sialyltransferase plays an important part in the initiation of platelet release reaction.