(1) Dimethyl sulfoxide (DMSO) markedly inhibited the V(max) of multisite ATPase activity in Escherichia coli F1-ATPase at concentrations > 30% (v/v). V(max)/K(M) was reduced by 2 orders of magnitude in 40% (v/v) DMSO at pH 7.5, primarily due to reduction of V(max). The inhibition was rapidly reversed on dilution into aqueous buffer. (2) K(d)ATP at the first, high-affinity catalytic site was increased 1500-fold from 2.3 x 10(-10) to 3.4 x 10(-7) M in 40% DMSO at pH 7.5, whereas K(d)ADP was increased 3.2-fold from 8.8 to 28-mu-M. This suggests that the high-affinity catalytic site presents a hydrophobic environment for ATP binding in native enzyme, that there is a significant difference between the conformation for ADP binding as opposed to ATP binding, and that the ADP-binding conformation is more hydrophilic. (3) Rate constants for hydrolysis and resynthesis of bound ATP in unisite catalysis were slowed approximately 10-fold by 40% DMSO; however, the equilibrium between bound P(i)/bound ATP was little changed. The reduction in catalysis rates may well be related to the large increase in K(d)ATP (less constrained site). (4) Significant P(i) binding to E. coli F1 could not be detected either in 40% DMSO or in aqueous buffer using a centrifuge column procedure. (5) We infer, on the basis of the measured constants K(a)ATP, K2 (hydrolysis/resynthesis of ATP), k+3 (P(i) release), and K(d)ADP and from estimates of k-3 (P(i) binding) that DELTA-G for ATP hydrolysis in 40% DMSO-containing pH 7.5 buffer is between -9.2 and -16.8 kJ/mol.
