MECHANISTIC STUDIES ON HUMAN PLATELET ISOPRENYLATED PROTEIN METHYLTRANSFERASE - FARNESYLCYSTEINE ANALOGS BLOCK PLATELET-AGGREGATION WITHOUTINHIBITING THE METHYLTRANSFERASE

The kinetic mechanism of the human platelet S-adenosyl-L-methionine (A doMet)-linked isoprenylated protein methyltransferase was studied and determined to be ordered bibi. AdoMet binds first, and S-adenosyl-L-ho mocysteine (AdoHcy) departs last. Simple N-acetylated farnesylated cys teine analogs, such as N-acetyl-S-farnesyl-L-cysteine (AFC), are excel lent substrates for the enzyme. Although many N-acetylated farnesylate d cysteine analogs are excellent substrates for the enzyme, analogs wi th bulky moieties adjacent to the farnesylcysteine are neither substra tes nor inhibitors of the enzyme. Two molecules of this class, N-benzo yl-S-farnesyl-L-cysteine (BzFC) and N-pivaloyl-S-farnesyl-L-cysteine ( PFC) are useful in sorting out the putative physiological role of the methyltransferase in mediating human platelet aggregation because thei r pharmacological activities are unlinked to methyltransferase inhibit ion. When studied as inhibitors of platelet aggregation, the analogs a re as active, or more active, than bona fide methyltransferase inhibit ors of similar structure. Therefore, although it is possible that meth yltransferase inhibitors, such as AFC, inhibit the enzyme when applied to cells, the observed pharmacological effects appear to be unrelated to this blockade. The new FC analogs described here have revealed a n ew signal transduction target which, will be of some interest to explo re.