Amphiphilic and hydrophilic nature of sheep and human platelet phosphotyrosine phosphatase forms

To date, although at least 75 different PTPases (protein-tyrosine-phosphate -phosphohydrolase, EC 3.1.3.48) have been identified, those detected in pla telets are rather scarce. Based on previous results from our laboratory, we investigated the existence of new PTPases in platelets. Triton X-114 phase partitioning of Triton X-100-solubilized human and sheep platelet membrane s allowed PTPase to be recovered in the detergent-rich (40-35%, respectivel y) and -poor phases (60-65%, respectively). Sedimentation analyses of both phases from the sheep species revealed hydrophilic 6S and 3.7S, and amphiph ilic 7.5S and 10.3S PTPase forms. Sedimentation analyses of human platelet membrane-associated or cytosolic PTPase revealed hydrophilic 6.7S and 4.3S, and amphiphilic 5.5S and 10.8S forms, or hydrophilic 4S, 5.9S and 6.9S for ms, respectively. Western blot analysis using monoclonal antibodies (MoAb a gainst human PTP1B, PTP1C, PTP1D and RPTP alpha (mouse anti-human PTPase Mo Abs) showed that RPTP alpha was not present in platelets and that the PTP1C type and PTP1D type (but probably not the PTP1B type) were expressed in sh eep species. Immunoblots also revealed that all PTPases detected were mainl y membrane-associated, with similar percentages of cellular distribution in both species. All PTPases were mainly recovered in the detergent-poor phas es from the Triton X-114 phase partitioning, although PTP1D from human spec ies was also significantly present (30%) in the detergent-rich phase. Addit ionally, all PTPases sedimented within the same PTPase peak in sucrose grad ients (sedimentation coefficients around 4S). These findings indicate that amphiphilic and hydrophilic PTPases different from PTP1B, PTP1C, PTP1D or R PTP alpha, with higher sedimentation coefficients and with higher activity when O-phosphotyrosine or a synthetic peptide phosphorylated on tyrosine we re used as substrates, are present in platelets. (C) 1999 Elsevier Science B.V. All rights reserved.