Beta-thymosins sequester G-actin and preserve a pool of monomers of actin which constitute an important prerequisite for cellular function of the microfilament system. To study the influence of paraquat binding to G-actin on the interaction of G-actin with thymosin beta4 we determined the apparent dissociation constant of the G-actin-thymosin beta4 complex in the absence or presence of paraquat using an ultrafiltration assay. Paraquat (1,1'-dimethyl-4,4'-dipyridylium dichloride) attenuates this interaction in a concentration- and time-dependent manner. When exposed to 10 mM paraquat, the apparent dissociation constant increased 10-85-fold within 15 min to 24 h. After incubation for 24 h even a paraquat concentration as low as 100 microM increased the dissociation constant of the G-actin-thymosin beta4 complex from 0.66 microM to 0.82 microM (P < 0.05). Diquat (1,1'-ethylene-2,2'-dipyridylium dibromide) similarly weakens the interaction of G-actin and beta-thymosins. In none of the experiments was oxidation of the methionine residue or any other modification of thymosin beta4 detected. Therefore we conclude that the dipyridyls paraquat and diquat directly interact with G-actin and thereby impede the interaction between G-actin and thymosin beta4.