Abstract
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 Km of platelet TPST for EAY was 3.7 microM and Vmax 0.09 pmol/min. For the cofactor 3' phosphoadenosine 5' phosphosulfate (PAPS) the Km was 1.7 microM and Vmax 0.11 pmol/min. The PAPS analogue 3',5'-adenosine diphosphate inhibited platelet TPST with an IC50 of 15.4 microM. 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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