Molecular Pharmacology, Vol 13, 142-149, Copyright © 1977 by the American Society for Pharmacology and Experimental Therapeutics

Solvent Effects on Hydrolysis of the Phosphoenzyme Intermediate in Sodium- and Potassium-Dependent Adenosine Triphosphatase: Correlations with Stimulation of Potassium-Dependent p-Nitrophenyl Phosphatase

¹ Toxicology Research Laboratory, Minneapolis Veterans Hospital, Minneapolis, Minnesota 55417, and Department of Laboratory Medicine and Pathology, University of Minnesota, Minneapolis, Minnesota 55455

Studies were undertaken to examine the effects of the solvents deuterated water (²H₂O) and dimethyl sulfoxide (DMSO) on the breakdown of the phosphorylated intermediate and on the K⁺-dependent p-nitrophenyl phosphatase activity of rat brain (Na⁺ + K⁺)-ATPase. At 4° phosphoenzyme hydrolysis in the presence of K⁺ (0.02-0.33 mM), NH₄⁺ (0.33 mM), or Li⁺ (0.33 mM) followed a biphasic time course, indicating the presence of comparatively rapid (sensitive) and slow (insensitive) components. Phosphoenzyme breakdown in the presence of 5 mM ADP exhibited similar components. However, when both ADP and K⁺ were added together, essentially all of the phosphoenzyme was rapidly hydrolyzed, suggesting that the K⁺-sensitive and ADP-sensitive components predominated. ²H₂O and DMSO stimulated the rate of breakdown of the slower, insensitive component in the presence of K⁺ and NH₄⁺, but not Li⁺. No effect of these solvents was observed in the presence of ADP. The rate of hydrolysis of the rapid component could not be determined, but the solvents appeared to have no significant effect on the amount of phosphoenzyme hydrolyzed within 1 sec. Both ²H₂O and DMSO stimulated the p-nitrophenyl phosphatase activity at 4° in the presence of K⁺ (0.02-0.33 mM) or NH₄⁺ (0.33 mM), but not of Li⁺ (0.33 mM). The stimulatory effects of ²H₂O and DMSO on K⁺-dependent phosphoenzyme breakdown and K⁺-dependent p-nitrophenyl phosphatase activity appear to be related and are interpreted in terms of a solvent role of H₂O in the conformation of the phosphoenzyme which modulates its interaction with K⁺.

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ACKNOWLEDGMENTS The valuable help of Gregory Quarfoth and Alan Davis in this work is gratefully acknowledged.