title: PPACK-thrombin inhibits thrombin-induced platelet aggregation and cytoplasmic acidification but does not inhibit platelet shape change.
abstract: We have re-evaluated the previously reported ability of TLCK-thrombin (N alpha-tosyl-L-lysine chloromethyl ketone-treated alpha-thrombin) and PPACK-thrombin (D-phenylalanyl-L-prolyl-L-arginine chloromethyl ketone-treated alpha-thrombin) to inhibit alpha-thrombin-induced platelet activation (Harmon JT, Jamieson GA: J Biol Chem 261:15928, 1986; and Harmon JT, Jamieson GA: Biochemistry 27:2151, 1988). Despite several cycles of derivatization with TLCK (10,000-fold molar excess), preparations of TLCK-thrombin have been found to contain about 4% residual alpha-thrombin activity, suggesting that these preparations are an equilibrium mixture of TLCK-thrombin and alpha-thrombin and cannot be used for evaluating competition between these two agents. In contrast, alpha-thrombin activity was completely inhibited by PPACK at 15-fold molar excess. PPACK-thrombin, free of unreacted PPACK and devoid of residual alpha-thrombin activity, did not markedly affect platelet shape change at concentrations as high as 1 mumol/L, but inhibited aggregation and secretion in intact platelets activated with the minimal concentration of alpha-thrombin causing a full response (0.3 to 0.5 nmol/L) and yielded a 50% inhibition constant (IC50) for inhibition of aggregation by PPACK-thrombin of 110 nmol/L. This inhibition was specific for alpha-thrombin-induced platelet activation, and no inhibition was seen with activation induced by ADP, collagen, epinephrine, ristocetin, or arachidonate. At these low alpha-thrombin concentrations (approximately 0.4 nmol/L), a persistent cytoplasmic acidification was observed of -0.062 +/- 0.016 pH units, although alkalinization was observed at higher alpha-thrombin concentrations (greater than 1 nmol/L). While inhibition of aggregation and secretion occurred when alpha-thrombin and PPACK-thrombin were added simultaneously, inhibition of cytoplasmic acidification and of the elevation of cytoplasmic [Ca2+] induced by low concentrations of alpha-thrombin (0.4 nmol/L) occurred only if platelets were preincubated with PPACK-thrombin for 5 minutes before the addition of alpha-thrombin. In platelets treated with Serratia marcescens protease to remove glycoprotein lb (GPlb), alpha-thrombin-induced shape change was attenuated but persisted in the presence of a high concentration (2 mumol/L) of PPACK-thrombin, although aggregation and secretion were inhibited, as seen in intact platelets. The IC50 value for inhibition of aggregation by PPACK-thrombin was approximately 1 mumol/L at the higher alpha-thrombin concentrations (5 nmol/L) required for full activation in this case. These results suggest that PPACK-thrombin may be a useful probe of platelet function since it specifically blocks platelet aggregation and secretion induced by alpha-thrombin.(ABSTRACT TRUNCATED AT 400 WORDS)
authors.affiliation: Cell Biology Laboratory, American Red Cross, Rockville, MD 20855.
authors.forename: N J
authors.initials: NJ
authors.lastname: Greco
authors.forename: T E
authors.initials: TE
authors.lastname: Tenner
authors.forename: N N
authors.initials: NN
authors.lastname: Tandon
authors.forename: G A
authors.initials: GA
authors.lastname: Jamieson
mesh_terms: D000143:Acids / Q000378:metabolism*; D000590:Amino Acid Chloromethyl Ketones / Q000494:pharmacology*; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism / Q000648:ultrastructure*; D002118:Calcium / Q000378:metabolism; D003593:Cytoplasm / Q000187:drug effects / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D015539:Platelet Activation / Q000187:drug effects; D010974:Platelet Aggregation / Q000187:drug effects*; D013917:Thrombin / Q000494:pharmacology*
chemical_list: D000143:Acids; D000590:Amino Acid Chloromethyl Ketones; D013917:Thrombin; C050815:phenylalanyl-prolyl-arginine chloromethyl ketone-thrombin; D002118:Calcium