Dimethyl sulfoxide: a possible effect on the interconversion of phosphorylated forms of Na+,K(+)-ATPase

Purified Na+,K(+)-ATPase from kidney outer medulla was phosphorylated by Pi in a reaction synergistically stimulated by Mg2+, when 40% (v/v) dimethyl sulfoxide was added to the assay medium. The phosphoenzyme formed at this solvent concentration was able to synthesize ATP even in the presence of Mg2+, because hydrolysis was impaired. ATP in equilibrium 32P]Pi exchange was also inhibited, indicating that partial reactions in the forward direction were blocked by the solvent. In 40% (v/v) dimethyl sulfoxide the enzyme's affinity for ADP decreased, in comparison with the values observed in purely aqueous medium. Addition of K+, which accelerated dephosphorylation of Na+,K(+)-ATPase in a totally water medium, partially reversed the inhibition of hydrolysis that was observed in the presence of dimethyl sulfoxide.