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S Raha, C Dosquet, JF Abgrall, P Jolles, AM Fiat and JP Caen
Unite Institut National de la Sante et de la Recherche Medicale 150, Paris,
France.
Short peptides isolated from fibrinogen and K-casein have been shown to
inhibit platelet aggregation and fibrinogen binding to stimulated
platelets. We studied the effects of synthetic peptides occurring in milk
proteins (bovine K-casein, KNQDK, and human lactotransferrin, KRDS) and in
fibrinogen (RGDS and L10) on subsequent binding of monoclonal antibodies
(MoAb) against the glycoprotein (GP) IIb-IIIa complex (AP2 and P2) on
adenosine diphosphate (ADP)-stimulated and unstimulated human platelets and
megakaryocytes (MKs) by using an immunoperoxidase method to visualize
antibody binding. Only KRDS (900 mumol/L) inhibited the binding of AP2 and
P2 on ADP (5 mumol/L)- stimulated platelets, but not on unstimulated
platelets. However, the binding of P2 was considerably more inhibited than
that of AP2 as judged by immunoperoxidase intensity. Radiolabeled AP2
binding was inhibited by 30% with KRDS on ADP-stimulated platelets as
compared with platelets incubated in the absence of ADP. KRDS did not
inhibit the binding of MoAbs against GP IIIa (SZ 21), GP IIb (SZ 22), and
GP Ib (SZ 2) on ADP-stimulated human platelets. Inhibition of P2 binding by
KRDS was also observed in a section of MKs isolated from human bone marrow
and stimulated by 15 or 20 micron ADP. A lower concentration of ADP (5 or
10 mumol/L) failed to produce any inhibition of binding. This indicates
that MKs may not be equally responsive to agonists as platelets. Moreover,
P2 binding inhibition was observed in a larger (P less than .001)
percentage of mature MKs (29%) as compared with younger, maturing MKs
(11%). The observations suggested that a functional ability possessed by
platelets, namely, agonist-induced exposure of the site of interaction of
KRDS, may occur at a late stage of MK development.
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