Determination of the ADP concentration available to participate in energy metabolism in an actin-rich cell, the platelet.

Almost all cells contain actin, which in its polymerized form, F-actin, binds 1 molecule of ADP/monomer. Little is known about the availability to metabolism of this bound ADP. A comparison was therefore made between perchloric acid and EDTA/ethanol extracts of human blood platelets. When the cells were extracted under conditions where the ATPase activity was negligible, the ethanol extracts had a 75% higher ATP/ADP ratio and a higher adenylate energy charge than perchloric acid extracts. The methods differed in that a considerable portion of protein-bound ADP was not extracted by ethanol. This bound ADP behaved as though it were unavailable to energy metabolism and should thus be considered as a compartment separate from the bulk metabolic pool of extragranular platelet adenine nucleotides. These results suggest that the level of ADP obtained with the common acid extraction overestimates the level available to participation in metabolism.