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

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 here 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 μmol/1011 cells of this F‐actin – ADP complex compared to the total actin content of 0.8 μmol/1011 cells, which is consistent with the view that actin is maintained in different pools (F‐actin—ADP, profilactin, G‐actin). In intact platelets the F‐actin‐bound ADP turns over rapidly and we have determined a turnover rate at 37°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. Copyright © 1986, Wiley Blackwell. All rights reserved