Dissociation of thrombin from platelets by hirudin. Evidence for receptor processing.
Hirudin inhibited the binding of human 125I-alpha-thrombin to the saturable, but not the nonsaturable, sites on washed human platelets. When hirudin was added to a thrombin-platelet mixture, it caused a biphasic dissociation of bound thrombin. A partial dissociation was too rapid to measure and was followed by complete dissociation with a first order rate constant of about 10(-2) s-1. The fraction of bound thrombin in the more slowly dissociable form increased from essentially none after a 5-s preincubation of thrombin and platelets to as much as 75% of saturable binding after a 4-min preincubation. Transition to the slowly dissociable state was not accompanied by an increase in the amount bound and was not observed with active site serine-derivatized thrombin. This is the first evidence with intact platelets of a binding characteristic that depends, as does platelet stimulation, on catalytically active thrombin, suggesting that it may represent physiologically significant receptor processing.
D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D006629:Hirudins / Q000494:pharmacology*; D006801:Humans; D007700:Kinetics; D011955:Receptors, Drug / Q000187:drug effects / Q000378:metabolism*; D013917:Thrombin / Q000378:metabolism*
D006629:Hirudins; D011955:Receptors, Drug; D013917:Thrombin