Human platelets possess tyrosylprotein sulfotransferase (TPST) activity

Area of Science:

• Biochemistry
• Molecular Biology
• Hematology

Background:

• Tyrosine sulfation is a crucial post-translational modification for secreted proteins, including key coagulation factors synthesized in the liver.
• Factor V, a known tyrosine-sulfated protein, is also synthesized in megakaryocytes, the precursors to platelets.

Purpose of the Study:

• To investigate the presence and characteristics of tyrosylprotein sulfotransferase (TPST) activity in human platelets.
• To determine if platelets possess the enzymatic machinery for tyrosine sulfation of proteins like coagulation factor V.

Main Methods:

• Platelet homogenates were used to assay tyrosylprotein sulfotransferase (TPST) activity using the synthetic substrate EAY.
• Kinetic parameters, including Km and Vmax for the substrate (EAY) and cofactor (PAPS), were determined.
• Inhibition studies using a PAPS analogue were conducted to characterize the enzyme's activity.

Main Results:

• Substantial TPST activity was detected in platelet homogenates, quantified as 0.405 +/- 0.049 pmol EAY-SO4 formed min-1 mg-1.
• Kinetic analysis revealed a Km of 3.7 microM for EAY and 1.7 microM for PAPS, with Vmax values of 0.09 pmol/min and 0.11 pmol/min, respectively.
• Platelet TPST activity was confirmed to be intrinsic and not due to contaminating cells or plasma, with an IC50 of 15.4 microM for the PAPS analogue.

Conclusions:

• Human platelets possess significant tyrosylprotein sulfotransferase (TPST) activity.
• This finding supports the hypothesis that tyrosine sulfation of coagulation factor V occurs within megakaryocytes.
• Platelets represent a promising and accessible source for further research into TPST function and regulation.