Adenosine 5 -tetraphosphate (ATPP) was identified and quantified in extracts of rabbit platelets by elution of extracts containing authentic adenosine 5 -tetraphosphate and comparison of retention time with nucleotide standards using high-performance liquid chromatography technique. The amount of adenosine 5 -tetraphosphate was 0.62 nmoles/109 cells which was 62-fold lower than that of ATP but only 10-fold lower than that of ADP. During platelet aggregation induced by thrombin, adenosine 5 -tetraphosphate was released to a relatively high extent. The degradation rates and halflives of adenosine 5 -tetraphosphate were measured during incubation of platelets in whole blood, erythrocyte suspension and plasma, respectively. The results suggest that plasma contributes more than blood cells to the catabolism of adenosine 5 -tetraphosphate. The pattern of degradation indicates that ATPP may be degraded mainly to AMP by soluble enzymes in plasma and very slowly to ADP and/or AMP by ectoenzymes on blood cells such as erythrocyte. The nature of the enzymes responsible fer the degradation of adenosine 5 -tetraphosphate is yet to be identified.
