Abstract
ATP, CTP, and ITP supported a rapid initial rate of [3H]ouabain binding to rat brain (Na+ + K+)-ATPase, up to 4 times that supported by ADP. These nucleotides also prevented the rapid initial incorporation of the [γ-32P]phosphate of [γ-32P]ATP into the enzyme. [γ-32P]Cytidine triphosphate labeled the enzyme to the same extent as [γ-32P]ATP, suggesting that nucleotides other than ATP phosphorylate this enzyme. In support of this, the phosphorylation from [γ-32P]CTP required Mg2+, was stimulated by Na+, and was not observed in the presence of K+, and the label incorporated into the enzyme from [γ-32P]CTP turned over at the same rate as that from [γ-32P]ATP. In other experiments the initial rates of hydrolysis of ATP, ITP, UTP, and ADP in the absence of added K+ matched the initial rates of [3H]ouabain binding supported by these substrates. The results suggest that these substrates give rise to sufficient phosphoenzyme to account for the initial rates of [3H]ouabain binding supported by them. Concentrations of ADP sufficient to inhibit the phosphorylation of this enzyme by ATP also inhibited the initial rate of the ATP-supported binding of [3H]ouabain, suggesting that ADP can bind to this enzyme without stimulating [3H]ouabain binding.
ACKNOWLEDGMENTS The authors wish to thank Mrs. Annie Han and Mr. Roxy So for excellent technical assistance.
- Copyright ©, 1972, by Academic Press, Inc.
MolPharm articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|