Nucleotide exchange between cytosolic ATP and F-actin-bound ADP may be a major energy-utilizing process in unstimulated platelets

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Abstract
About 40% of the cytosolic ADP of human platelets is tightly bound to protein and the complex is precipitated from the cells by 43% ethanol. We show that this ADP is bound to F-actin by three criteria (a) copurification with F-actin, (b) specific extraction with water and (c) by specific interaction with DNase I. Platelets contain 0.3 .mu.mol/1011 cells of this F-actin.sbd.ADP complex compared to the total actin content of 0.8 .mu.mol/1011 cells, which is consistent with the view that actin is maintained in different pools (F-actin.sbd.ADP, profilactin, G-actin). In intact platelets the F-actin-bound ADP turns over rapidly and we have determined a turnover rate at 37.degree. C of 0.1 .+-. 0.025 s-1 by using a double-labelling procedure. This rapid turnover indicates that F-actin in intact platelets is in a very dynamic state, which may be necessary for rapid responses to stimuli. If it is assumed that the source of the ADP bound to F-actin is cytosolic ATP, the turnover of F-actin ADP measured represents an ATP-consuming process that would account for up to 50% of total ATP consumption in resting platelets.