The importance of aspartate aminotransferase for platelet aggregation

In bovine platelets aspartate aminotransferase has a high activity. The enzyme in vitro is inhibited in a dose dependent manner by aminooxyacetate (IC50 = 10(-4) M), hydroxylamine (IC50 = 10(-4) M), and cycloserine (IC50 = 5 X 10(-3). The inhibitory effect of all the three compounds is strongest at low substrate (aspartate) concentration. Blocking of aspartate aminotransferase activity by these compounds in intact platelets is accompanied by the inhibition of ADP and collagen-induced aggregation. Among the three compounds the strongest inhibitor of platelet aggregation was hydroxylamine, which was also the most effective inhibitor of aspartate aminotransferase. Other metabolic blockers, i.e. dinitrophenol (DNP), rotenone and antimycin also inhibited the aggregation of platelets, and a synergism has been demonstrated between DNP, rotenone and antimycin A action on platelet aggregation and blockade of aspartate aminotransferase activity. The results are interpreted to mean that transamination is of importance in the energy production in the activated platelet, probably through its participation in reducing equivalents transport from the cytosol to the mitosol via the malate: oxaloacetate: aspartate shuttle.