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(Arteriosclerosis, Thrombosis, and Vascular Biology. 1995;15:511-514.)
© 1995 American Heart Association, Inc.

Articles

Participation of Calpain in Protein-Tyrosine Phosphorylation and Dephosphorylation in Human Blood Platelets

Hideo Ariyoshi; Atsushi Oda; Edwin W. Salzman

From the Department of Surgery, Beth Israel Hospital, Harvard Medical School, Boston, Mass.

Correspondence to Edwin W. Salzman, MD, Beth Israel Hospital, 330 Brookline Ave, Boston, MA 02215.

Abstract The possible role of calpains in protein-tyrosine phosphorylation in platelets was examined by the use of the cell-permeant calpain inhibitor calpeptin. In platelets stimulated by 1 U/mL thrombin, protein-tyrosine phosphorylation was maximal after 2 minutes and was followed by protein-tyrosine dephosphorylation. Calpeptin (30 µmol/L) or vanadate (2 mmol/L) enhanced protein-tyrosine phosphorylation and delayed protein-tyrosine dephosphorylation. The effects of these two compounds were not additive. We also observed proteolysis of pp60src and autoproteolysis of µ-calpain. Cleavage of the former was significantly slower than that of the latter and slower than protein-tyrosine dephosphorylation. The activity of protein-tyrosine phosphatase in the platelet lysate was transiently increased to 190% by addition of Ca²⁺. Ca²⁺-dependent activation of protein-tyrosine phosphatase was not observed in the presence of leupeptin. Those observations suggest that platelet calpains may be involved in modulation of protein-tyrosine phosphorylation through activation of protein-tyrosine phosphatase rather than through the inactivation of pp60src, a mechanism that was previously suggested.

Key Words: calpain • tyrosine phosphorylation • tyrosine dephosphorylation • platelets

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