Human Platelets Possess Tyrosylprotein Sulfotransferase (TPST) Activity

Summary

Tyrosine sulfation is a widespread modification of secreted proteins including several coagulation proteins synthesized in the liver. Since factor V, a tyrosine-sulfated protein, is also synthesized in megakaryocytes, we determined whether platelets posses tyrosylprotein sulfotransferase (TPST) activity. Using the synthetic substrate EAY, substantial TPST activity (0.405 ± 0.049 pmol EAY-SO₄ formed min⁻¹ mg⁻¹) was detected in platelet homogenates. This activity could not be accounted for by contaminating leukocytes, erythrocytes or plasma. The K _m of platelet TPST for EAY was 3.7 μM and V _max 0.09 pmol/min. For the cofactor 3' phosphoadenosine 5' phosphosulfate (PAPS) the K _m was 1.7 μM and V _max 0.11 pmol/min. The PAPS analogue 3', 5'-adenosine diphosphate inhibited platelet TPST with an IC₅₀ of 15.4 μM. These findings suggest that tyrosine sulfation of factor V will occur in megakaryocytes. Platelets may be a useful source for further study of TPST.

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