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Thallous ion activation of p-nitrophenylphosphatase of rat myometrium plasma membrane
Abstract:
Addition of thallous ion (Tl+) inhibited the spontaneous mechanical activity of rat myometrium in K-free Krebs solutions with an I50 value of 30 microM. The corresponding value for I50 for similar inhibition by addition of K+ was 150 microM. Tl+ as well as K+-activated p-nitrophenylphosphatase (PNPPase) of isolated rat myometrium plasma membrane vesicles. Half maximal activation was caused by 0.47 mM Tl+ or 1.6 mM K+. Maximal enzyme activities obtained using Tl+ and K+ were comparable. The Km values for the substance p-nitrophenylphosphatase using Tl+ (1.24 mM) and K+ (1.46 mM) were also similar. Activation by either ion was inhibited by ouabain, Na+, inorganic phosphate, and vanadate (V +5). The results suggest that Tl+ can substitute for K+ for activation of the Na-K pump of rat myometrium plasma membrane.
Insights
Thallous ion (Tl+) inhibits rat myometrium activity and activates the Na-K pump, similar to potassium (K+). This suggests Tl+ can substitute for K+ in activating this crucial membrane protein.
Area of Science:
- Physiology
- Biochemistry
- Pharmacology
Background:
- The Na-K pump is vital for maintaining cellular ion gradients.
- Potassium (K+) is a key activator of the Na-K pump.
- Understanding alternative activators can reveal pump mechanisms.
Purpose of the Study:
- To investigate the effects of thallous ion (Tl+) on rat myometrium.
- To determine if Tl+ can substitute for K+ in activating the Na-K pump.
Main Methods:
- Measurement of spontaneous mechanical activity in rat myometrium.
- Assay of p-nitrophenylphosphatase (PNPPase) activity in isolated plasma membrane vesicles.
- Determination of inhibition (I50) and activation (Km) constants.
Main Results:
- Thallous ion (Tl+) inhibited myometrium activity with an I50 of 30 microM, while K+ had an I50 of 150 microM.
- Tl+ activated plasma membrane PNPPase with a half-maximal activation at 0.47 mM, comparable to K+ (1.6 mM).
- Km values for Tl+ and K+ were similar, and activation by both was inhibited by ouabain, Na+, phosphate, and vanadate.
Conclusions:
- Thallous ion (Tl+) effectively inhibits rat myometrium mechanical activity.
- Tl+ can substitute for K+ in activating the Na-K pump in rat myometrium plasma membranes.
- These findings highlight the functional similarity between Tl+ and K+ for Na-K pump activation.