KRDS--a tetrapeptide derived from lactotransferrin--inhibits binding of monoclonal antibody against glycoprotein IIb-IIIa on ADP-stimulated platelets and megakaryocytes.

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.